Group Object Security for Constrained RESTful Environments (Group OSCORE)

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Readers are expected to be familiar with the terms and concepts described in CoAP [RFC7252], including "endpoint", "client", "server", "sender", and "recipient"; group communication for CoAP [I-D.ietf-core-groupcomm-bis]; Observe [RFC7641]; CBOR [RFC8949]; COSE [RFC9052][RFC9053] and related countersignatures [RFC9338].¶

Readers are also expected to be familiar with the terms and concepts for protection and processing of CoAP messages through OSCORE, such as "Security Context" and "Master Secret", defined in [RFC8613].¶

Terminology for constrained environments, such as "constrained device" and "constrained-node network", is defined in [RFC7228].¶

This document refers also to the following terminology.¶

• Keying material: data that is necessary to establish and maintain secure communication among endpoints. This includes, for instance, keys and IVs [RFC4949].¶
• Authentication credential: information associated with an entity, including that entity's public key and parameters associated with the public key. Examples of authentication credentials are CBOR Web Tokens (CWTs) and CWT Claims Sets [RFC8392], X.509 certificates [RFC5280] and C509 certificates [I-D.ietf-cose-cbor-encoded-cert]. Further details about authentication credentials are provided in Section 2.4.¶
• Group: a set of endpoints that share group keying material and security parameters (Common Context, see Section 2). That is, unless otherwise specified, the term group used in this document refers to a "security group" (see Section 2.1 of [I-D.ietf-core-groupcomm-bis]), not to be confused with "CoAP group" or "application group".¶
• Group Manager: an entity responsible for a group, neither required to be an actual group member nor to take part in the group communication. The responsibilities of the Group Manager are listed in Section 3.4.¶
• Silent server: a member of a group that performs only group mode processing on incoming requests and never sends responses protected with Group OSCORE. For CoAP group communications, requests are normally sent without necessarily expecting a response. A silent server may send unprotected responses, as error responses reporting a Group OSCORE error.¶
• Group Identifier (Gid): identifier assigned to the group, unique within the set of groups of a given Group Manager.¶
• Birth Gid: with respect to a group member, the Gid obtained by that group member upon (re-)joining the group.¶
• Key Generation Number: an integer value identifying the current version of the keying material used in a group.¶
• Source authentication: evidence that a received message in the group originated from a specific identified group member. This also provides assurance that the message was not tampered with by anyone, be it a different legitimate group member or an endpoint which is not a group member.¶
• Group request: a CoAP request message sent by a client in the group to servers in that group.¶

• Long exchange: an exchange of messages associated with a request that is a group request and/or an Observe request [RFC7641].¶

  In either case, multiple responses can follow from the same server to the request associated with the long exchange. The client terminates a long exchange when freeing up the CoAP Token value used for the associated request, for which no further responses will be accepted afterwards.¶

2.1. Common Context

The following sections specify how the Common Context is used and extended compared to [RFC8613]. All algorithms (AEAD, Group Encryption, Signature, Pairwise Key Agreement) are immutable once the Common Context is established. The Common Context may be acquired from the Group Manager (see Section 3).¶

2.2. Sender Context and Recipient Context

The Sender ID SHALL be unique for each endpoint in a group with a certain triplet (Master Secret, Master Salt, Group Identifier), see Section 3.3 of [RFC8613].¶

The maximum length of a Sender ID in bytes equals L minus 6, where L is determined as follows.¶

• If only one among the AEAD Algorithm and the Group Encryption Algorithm is set (see Section 2.1.1 and Section 2.1.6), then L is the AEAD nonce length for the set algorithm.¶
• If both the AEAD Algorithm and the Group Encryption Algorithm are set, then L is the smallest AEAD nonce length among those of the two algorithms.¶

With the exception of the authentication credential of the sender endpoint, a receiver endpoint can derive a complete Security Context from a received Group OSCORE message and the Common Context (see Section 2.3).¶

The authentication credentials in the Recipient Contexts can be retrieved from the Group Manager (see Section 3) upon joining the group. An authentication credential can alternatively be acquired from the Group Manager at a later time, for example the first time a message is received from a particular endpoint in the group (see Section 8.2 and Section 8.4).¶

For severely constrained devices, it may be infeasible to simultaneously handle the ongoing processing of a recently received message in parallel with the retrieval of the sender endpoint's authentication credential. Such devices can be configured to drop a received message for which there is no (complete) Recipient Context, and retrieve the sender endpoint's authentication credential in order to have it available to verify subsequent messages from that endpoint.¶

An endpoint may admit a maximum number of Recipient Contexts for a same Security Context, e.g., due to memory limitations. After reaching that limit, the endpoint has to delete a current Recipient Context to install a new one (see Section 2.6.1.2). It is up to the application to define policies for Recipient Contexts to delete.¶

2.3. Establishment of Security Context Parameters

OSCORE defines the derivation of Sender Context and Recipient Context (specifically, of Sender/Recipient Keys) and of the Common IV, from a set of input parameters (see Section 3.2 of [RFC8613]).¶

The derivation of Sender/Recipient Keys and of the Common IV defined in OSCORE applies also to Group OSCORE, with the following modifications compared to Section 3.2.1 of [RFC8613].¶

• If Group Encryption Algorithm in the Common Context is set (see Section 2.1.6), then the 'alg_aead' element of the 'info' array MUST specify Group Encryption Algorithm from the Common Context as a CBOR integer or text string, consistently with the "Value" field in the "COSE Algorithms" Registry for this algorithm.¶
• If Group Encryption Algorithm in the Common Context is not set, then the 'alg_aead' element of the 'info' array MUST specify AEAD Algorithm from the Common Context (see Section 2.1.1), as per Section 5.4 of [RFC8613].¶
• When deriving the Common IV, the 'L' element of the 'info' array MUST specify the length of the Common IV in bytes, which is determined as defined in Section 2.1.3.¶

2.4. Authentication Credentials

The authentication credentials of the endpoints in a group MUST be encoded according to the format used in the group, as indicated by the Authentication Credential Format parameter in the Common Context (see Section 2.1.4). The authentication credential of the Group Manager SHOULD be encoded according to that same format. The format of authentication credentials MUST provide the public key and a comprehensive set of information related to the public key algorithm, including, e.g., the used elliptic curve (when applicable).¶

If Group Encryption Algorithm in the Common Context is not set (see Section 2.1.6), then the public key algorithm is the Pairwise Key Agreement Algorithm used in the group (see Section 2.1.9), else the Signature Algorithm used in the group (see Section 2.1.7).¶

If the authentication credentials are X.509 certificates [RFC5280] or C509 certificates [I-D.ietf-cose-cbor-encoded-cert], the public key algorithm is fully described by the "algorithm" field of the "SubjectPublicKeyInfo" structure, and by the "subjectPublicKeyAlgorithm" element, respectively.¶

If authentication credentials are CBOR Web Tokens (CWTs) or CWT Claims Sets [RFC8392], the public key algorithm is fully described by a COSE key type and its "kty" and "crv" parameters.¶

Authentication credentials are used to derive pairwise keys (see Section 2.5.1) and are included in the external additional authenticated data when processing messages (see Section 4.4). In both these cases, an endpoint in a group MUST treat authentication credentials as opaque data, i.e., by considering the same binary representation made available to other endpoints in the group, possibly through a designated trusted source (e.g., the Group Manager).¶

For example, an X.509 certificate is provided as its direct binary serialization. If C509 certificates or CWTs are used as authentication credentials, each is provided as the binary serialization of a (possibly tagged) CBOR array. If CWT Claims Sets are used as authentication credentials, each is provided as the binary serialization of a CBOR map.¶

If authentication credentials are CWTs, then the untagged CWT associated with an entity is stored in the Security Context and used as authentication credential for that entity.¶

If authentication credentials are X.509 / C509 certificates or CWTs, and the authentication credential associated with an entity is provided within a chain or a bag, then only the end-entity certificate or end-entity untagged CWT is stored in the Security Context and used as authentication credential for that entity.¶

Storing whole authentication credentials rather than only a subset of those may result in a non-negligible storage overhead. On the other hand, it also ensures that authentication credentials are correctly used in a simple, flexible and non-error-prone way, also taking into account future credential formats as entirely new or extending existing ones. In particular, it is ensured that:¶

• When used to derive pairwise keys and when included in the external additional authenticated data, authentication credentials can also specify possible metadata and parameters related to the included public key. Besides the public key algorithm, these comprise other relevant pieces of information such as key usage, expiration time, issuer, and subject.¶
• All endpoints using another endpoint's authentication credential use exactly the same binary serialization, as obtained and distributed by the credential provider (e.g., the Group Manager), and as originally crafted by the credential issuer. In turn, this does not require to define and maintain canonical subsets of authentication credentials and their corresponding encoding, and spares endpoints from error-prone re-encoding operations.¶

Depending on the particular deployment and the intended group size, limiting the storage overhead of endpoints in a group can be an incentive for system/network administrators to prefer using a compact format of authentication credentials in the first place.¶

2.5. Pairwise Keys

Certain signature schemes, such as EdDSA and ECDSA, support a secure combined signature and encryption scheme. This section specifies the derivation of "pairwise keys", for use in the pairwise mode defined in Section 9.¶

Group OSCORE keys used for both signature and encryption MUST be used only for purposes related to Group OSCORE. These include the processing of messages with Group OSCORE, as well as performing proof-of-possession of private keys, e.g., upon joining a group through the Group Manager (see Section 3).¶

Pairwise Sender Key    = HKDF(Sender Key, IKM-Sender, info, L)
Pairwise Recipient Key = HKDF(Recipient Key, IKM-Recipient, info, L)

with

IKM-Sender    = Sender Auth Cred | Recipient Auth Cred | Shared Secret
IKM-Recipient = Recipient Auth Cred | Sender Auth Cred | Shared Secret

2.6. Update of Security Context

It is RECOMMENDED that the immutable part of the Security Context is stored in non-volatile memory, or that it can otherwise be reliably accessed throughout the operation of the group, e.g., after device reboots. However, also immutable parts of the Security Context may need to be updated, for example due to scheduled key renewal, new or re-joining members in the group, or the fact that the endpoint changes Sender ID (see Section 2.6.3).¶

The mutable parts of the Security Context are updated by the endpoint when executing the security protocol, but may be lost (see Section 2.6.1) or become outdated by exhaustion of Sender Sequence Numbers (see Section 2.6.2).¶

3.1. Set-up of New Endpoints

From the Group Manager, an endpoint acquires group data such as the Gid and OSCORE input parameters including its own Sender ID, with which it can derive the Sender Context.¶

When joining the group or later on as a group member, an endpoint can also retrieve from the Group Manager the authentication credential of the Group Manager as well as the authentication credential and other information associated with other members of the group, with which it can derive the corresponding Recipient Context. An application can configure a group member to asynchronously retrieve information about Recipient Contexts, e.g., by Observing [RFC7641] a resource at the Group Manager to get updates on the group membership.¶

Upon endpoints' joining, the Group Manager collects their authentication credentials and MUST verify proof-of-possession of the respective private key. Together with the requested authentication credentials of other group members, the Group Manager MUST provide the joining endpoints with the Sender ID of the associated group members and the current Key Generation Number in the group (see Section 3.2).¶

An endpoint may join a group, for example, by explicitly interacting with the responsible Group Manager, or by being configured with some tool performing the tasks of the Group Manager. When becoming members of a group, endpoints are not required to know how many and what endpoints are in the same group.¶

Communications that the Group Manager has with joining endpoints and group members MUST be secured. Specific details on how to secure such communications are out of the scope of this document.¶

The Group Manager MUST verify that the joining endpoint is authorized to join the group. To this end, the Group Manager can directly authorize the joining endpoint, or expect it to provide authorization evidence previously obtained from a trusted entity. Further details about the authorization of joining endpoints are out of the scope of this document.¶

In case of successful authorization check, the Group Manager provides the joining endpoint with the keying material to initialize the Security Context. The actual provisioning of keying material and parameters to the joining endpoint is out of the scope of this document.¶

One realization of a Group Manager is specified in [I-D.ietf-ace-key-groupcomm-oscore], where the join process is based on the ACE framework for authentication and authorization in constrained environments [RFC9200].¶

3.2. Management of Group Keying Material

In order to establish a new Security Context for a group, the Group Manager MUST generate and assign to the group a new Group Identifier (Gid) and a new value for the Master Secret parameter. When doing so, a new value for the Master Salt parameter MAY also be generated and assigned to the group. When establishing the new Security Context, the Group Manager should preserve the current value of the Sender ID of each group member.¶

The specific group key management scheme used to distribute new keying material is out of the scope of this document. A simple group key management scheme is defined in [I-D.ietf-ace-key-groupcomm-oscore]. When possible, the delivery of rekeying messages should use a reliable transport, in order to reduce chances of group members missing a rekeying instance.¶

The set of group members should not be assumed as fixed, i.e., the group membership is subject to changes, possibly on a frequent basis.¶

The Group Manager MUST rekey the group without undue delay in case one or more endpoints leave the group. An endpoint may leave the group at own initiative, or may be evicted from the group by the Group Manager, e.g., in case an endpoint is compromised, or is suspected to be compromised. In either case, rekeying the group excludes such endpoints from future communications in the group, and thus preserves forward security. If a network node is compromised or suspected to be compromised, the Group Manager MUST evict from the group all the endpoints hosted by that node that are member of the group and rekey the group accordingly.¶

If required by the application, the Group Manager MUST rekey the group also before one or more new joining endpoints are added to the group, thus preserving backward security.¶

Separately for each group, the value of the Key Generation Number increases by one each time the Group Manager distributes new keying material to that group (see Section 3.2). The first Key Generation Number assigned to every group MUST be 0.¶

The establishment of the new Security Context for the group takes the following steps.¶

1. The Group Manager MUST increment the Key Generation Number for the group by 1.¶

2. The Group Manager MUST build a set of stale Sender IDs including:¶

   • The Sender IDs that, during the current Gid, were both assigned to an endpoint and subsequently relinquished (see Section 2.6.3.1).¶
   • The current Sender IDs of the group members that the upcoming group rekeying aims to exclude from future group communications, if any.¶

3. The Group Manager rekeys the group, by distributing:¶

   • The new keying material, i.e., the new Master Secret, the new Gid and (optionally) the new Master Salt.¶
   • The new Key Generation Number from step 1.¶
   • The set of stale Sender IDs from step 2.¶

   Further information may be distributed, depending on the specific group key management scheme used in the group.¶

When receiving the new group keying material, a group member considers the received stale Sender IDs and performs the following actions.¶

• The group member MUST remove every authentication credential associated with a stale Sender ID from its list of group members' authentication credentials used in the group.¶
• The group member MUST delete each of its Recipient Contexts used in the group whose corresponding Recipient ID is a stale Sender ID.¶

After that, the group member installs the new keying material and derives the corresponding new Security Context.¶

A group member might miss one or more consecutive instances of group rekeying. As a result, the group member will retain old group keying material with Key Generation Number GEN_OLD. Eventually, the group member can notice the discrepancy, e.g., by repeatedly failing to verify incoming messages, or by explicitly querying the Group Manager for the current Key Generation Number. Once the group member gains knowledge of having missed a group rekeying, it MUST delete the old keying material it stores.¶

Then, the group member proceeds according to the following steps.¶

1. The group member retrieves from the Group Manager the current group keying material, together with the current Key Generation Number GEN_NEW. The group member MUST NOT install the obtained group keying material yet.¶
2. The group member asks the Group Manager for the set of stale Sender IDs.¶

3. If no exact indication can be obtained from the Group Manager, the group member MUST remove all the authentication credentials from its list of group members' authentication credentials used in the group and MUST delete all its Recipient Contexts used in the group.¶

   Otherwise, the group member MUST remove every authentication credential associated with a stale Sender ID from its list of group members' authentication credentials used in the group, and MUST delete each of its Recipient Contexts used in the group whose corresponding Recipient ID is a stale Sender ID.¶

4. The group member installs the current group keying material, and derives the corresponding new Security Context.¶

Alternatively, the group member can re-join the group. In such a case, the group member MUST take one of the following two actions.¶

• First, the group member performs steps 2 and 3 above. Then, the group member re-joins the group.¶

• The group member re-joins the group with the same roles it currently has in the group, and, during the re-joining process, it asks the Group Manager for the authentication credentials of all the current group members.¶

  Then, given Z, the set of authentication credentials received from the Group Manager, the group member removes every authentication credential which is not in Z from its list of group members' authentication credentials used in the group, and deletes each of its Recipient Contexts used in the group that does not include any of the authentication credentials in Z.¶

By removing authentication credentials and deleting Recipient Contexts associated with stale Sender IDs, it is ensured that a recipient endpoint storing the latest group keying material does not store the authentication credentials of sender endpoints that are not current group members. This in turn allows group members to rely on stored authentication credentials to confidently assert the group membership of sender endpoints, when receiving incoming messages protected in group mode (see Section 8).¶

Such a strictness in managing the authentication credentials and Recipient Contexts associated with other group members is required for two reasons. First, as further discussed in Section 13.1, it ensures that the group mode can be used securely, even in a group where the Group Encryption Algorithm does not provide integrity protection (see Section 2.1.6) and external signature checkers are used (see Section 8.5). Second, it ensures that the wrong (old) authentication credential associated with a group member A is never used with a sender ID that used to be associated with A and has been later issued to a different group member B (see Section 3.2.1.2), thus preventing the need to recover from an identity mix-up.¶

Components changed in lockstep
    upon a group rekeying
+----------------------------+            * Changing a kid does not
|                            |              need changing the Group ID
| Master               Group |<--> kid1
| Secret <---> o <--->  ID   |            * A kid is not reassigned
|              ^             |<--> kid2     under the ongoing usage of
|              |             |              the current Group ID
|              |             |<--> kid3
|              v             |            * Upon changing the Group ID,
|         Master Salt        | ... ...      every current kid should
|         (optional)         |              be preserved for efficient
|                            |              key rollover
| The Key Generation Number  |
| is incremented by 1        |            * After changing Group ID, an
|                            |              unused kid can be assigned,
+----------------------------+              even if it was used before
                                            the Group ID change

3.3. Support for Signature Checkers

The Group Manager may serve signature checkers, e.g., intermediary gateways, which verify countersignatures of messages protected in group mode (see Section 8.5). These entities do not join a group as members, but can retrieve authentication credentials of group members and other selected group data from the Group Manager.¶

In order to verify countersignatures of messages in a group, a signature checker needs to retrieve the following information about the group:¶

• The current ID Context (Gid) used in the group.¶

• The authentication credentials of the group members and of the Group Manager.¶

  If the signature checker is provided with a CWT for a given entity, then the authentication credential associated with that entity is the untagged CWT.¶

  If the signature checker is provided with a chain or a bag of X.509 / C509 certificates or of CWTs for a given entity, then the authentication credential associated with that entity is the end-entity certificate or end-entity untagged CWT.¶

• The current Signature Encryption Key (see Section 2.1.8).¶
• The identifiers of the algorithms used in the group (see Section 2), i.e.: i) Group Encryption Algorithm and Signature Algorithm; and ii) AEAD Algorithm and Pairwise Key Agreement Algorithm, if such parameters are set in the Common Context (see Section 2.1.1 and Section 2.1.9).¶

A signature checker MUST be authorized before it can retrieve such information, for example with the use of [I-D.ietf-ace-key-groupcomm-oscore].¶

3.4. Responsibilities of the Group Manager

The Group Manager is responsible for performing the following tasks:¶

1. Creating and managing OSCORE groups. This includes the assignment of a Gid to every newly created group, ensuring uniqueness of Gids within the set of its OSCORE groups and, optionally, the secure recycling of Gids.¶
2. Defining policies for authorizing the joining of its OSCORE groups.¶
3. Handling the join process to add new endpoints as group members.¶
4. Establishing the Common Context part of the Security Context, and providing it to authorized group members during the join process, together with the corresponding Sender Context.¶
5. Updating the Key Generation Number and the Gid of its OSCORE groups, upon renewing the respective Security Context.¶

6. Generating and managing Sender IDs within its OSCORE groups, as well as assigning and providing them to new endpoints during the join process, or to current group members upon request of renewal or re-joining. This includes ensuring that:¶

   • Each Sender ID is unique within each of the OSCORE groups;¶
   • Each Sender ID is not reassigned within the same group since the latest time when the current Gid value was assigned to the group. That is, the Sender ID is not reassigned even to a current group member re-joining the same group, without a rekeying happening first.¶
7. Defining communication policies for each of its OSCORE groups, and signaling them to new endpoints during the join process.¶
8. Renewing the Security Context of an OSCORE group upon membership change, by revoking and renewing common security parameters and keying material (rekeying).¶
9. Providing the management keying material that a new endpoint requires to participate in the rekeying process, consistently with the key management scheme used in the group joined by the new endpoint.¶
10. Assisting a group member that has missed a group rekeying instance to understand which authentication credentials and Recipient Contexts to delete, as associated with former group members.¶
11. Acting as key repository, in order to handle the authentication credentials of the members of its OSCORE groups, and providing such authentication credentials to other members of the same group upon request. The actual storage of authentication credentials may be entrusted to a separate secure storage device or service.¶
12. Validating that the format and parameters of authentication credentials of group members are consistent with the public key algorithm and related parameters used in the respective OSCORE group.¶

The Group Manager specified in [I-D.ietf-ace-key-groupcomm-oscore] provides this functionality.¶

4.1. Countersignature

When protecting a message in group mode, the 'unprotected' field MUST additionally include the following parameter:¶

• Countersignature0 version 2: its value is set to the countersignature of the COSE object.¶

  The countersignature is computed by the sender as described in Sections 3.2 and 3.3 of [RFC9338], by using its private key and according to the Signature Algorithm in the Security Context.¶

  In particular, the Countersign_structure contains the context text string "CounterSignature0", the external_aad as defined in Section 4.4 of this document, and the ciphertext of the COSE object as payload.¶

4.2. The 'kid' and 'kid context' parameters

The value of the 'kid' parameter in the 'unprotected' field of response messages MUST be set to the Sender ID of the endpoint transmitting the message, if the request was protected in group mode. That is, unlike in [RFC8613], the 'kid' parameter is always present in responses to a request that was protected in group mode.¶

The value of the 'kid context' parameter in the 'unprotected' field of request messages MUST be set to the ID Context, i.e., the Group Identifier value (Gid) of the group. That is, unlike in [RFC8613], the 'kid context' parameter is always present in requests.¶

4.3. AEAD Nonce

The AEAD nonce is constructed like in OSCORE, with the difference that step 4 in Section 5.2 of [RFC8613] is replaced with:¶

A. and then XOR with the X least significant bits of the Common IV, where X is the length in bits of the AEAD nonce.¶

4.4. external_aad

The external_aad of the Additional Authenticated Data (AAD) is different compared to OSCORE [RFC8613], and is defined in this section.¶

The same external_aad structure is used in group mode and pairwise mode for encryption/decryption (see Section 5.3 of [RFC9052]), as well as in group mode for computing and verifying the countersignature (see Sections 3.2 and 3.3 of [RFC9338]).¶

In particular, the external_aad includes also the Signature Algorithm, the Group Encryption Algorithm, the Pairwise Key Agreement Algorithm, the value of the 'kid context' in the COSE object of the request, the OSCORE option of the protected message, the sender's authentication credential, and the Group Manager's authentication credential.¶

The external_aad SHALL be a CBOR array wrapped in a bstr object as defined below, following the notation of [RFC8610]:¶

external_aad = bstr .cbor aad_array

aad_array = [
   oscore_version : uint,
   algorithms : [alg_aead : int / tstr / null,
                 alg_group_enc : int / tstr / null,
                 alg_signature : int / tstr / null,
                 alg_pairwise_key_agreement : int / tstr / null],
   request_kid : bstr,
   request_piv : bstr,
   options : bstr,
   request_kid_context : bstr,
   OSCORE_option : bstr,
   sender_cred : bstr,
   gm_cred : bstr
]

Compared with Section 5.4 of [RFC8613], the aad_array has the following differences.¶

• The 'algorithms' array is extended as follows.¶

  The parameter 'alg_aead' MUST be set to the CBOR simple value null (0xf6) if the parameter AEAD Algorithm in the Security Context is not set (see Section 2). Otherwise, regardless of whether the endpoint supports the pairwise mode or not, this parameter MUST specify AEAD Algorithm from the Common Context (see Section 2.1.1) as per Section 5.4 of [RFC8613].¶

  Furthermore, the 'algorithms' array additionally includes:¶

  • 'alg_group_enc', which specifies Group Encryption Algorithm from the Common Context (see Section 2.1.6). This parameter MUST be set to the CBOR simple value null (0xf6) if the parameter Group Encryption Algorithm in the Common Context is not set. Otherwise, regardless of whether the endpoint supports the group mode or not, this parameter MUST specify Group Encryption Algorithm as a CBOR integer or text string, consistently with the "Value" field in the "COSE Algorithms" Registry for this algorithm.¶
  • 'alg_signature', which specifies Signature Algorithm from the Common Context (see Section 2.1.7). This parameter MUST be set to the CBOR simple value null (0xf6) if the parameter Signature Algorithm in the Common Context is not set. Otherwise, regardless of whether the endpoint supports the group mode or not, this parameter MUST specify Signature Algorithm as a CBOR integer or text string, consistently with the "Value" field in the "COSE Algorithms" Registry for this algorithm.¶
  • 'alg_pairwise_key_agreement', which specifies Pairwise Key Agreement Algorithm from the Common Context (see Section 2.1.9). This parameter MUST be set to the CBOR simple value null (0xf6) if Pairwise Key Agreement Algorithm in the Common Context is not set. Otherwise, regardless of whether the endpoint supports the pairwise mode or not, this parameter MUST specify Pairwise Key Agreement Algorithm as a CBOR integer or text string, consistently with the "Value" field in the "COSE Algorithms" Registry for this algorithm.¶

• The new element 'request_kid_context' contains the value of the 'kid context' in the COSE object of the request (see Section 4.2).¶

  This enables endpoints to safely keep a long exchange active beyond a possible change of Gid (i.e., of ID Context), following a group rekeying (see Section 3.2). In fact, it ensures that every response within a long exchange cryptographically matches with only one request (i.e., the request associated with that long exchange), rather than with multiple requests that were protected with different keying material but share the same 'request_kid' and 'request_piv' values.¶

• The new element 'OSCORE_option', containing the value of the OSCORE Option present in the protected message, encoded as a binary string. This prevents the attack described in Section 13.7 when using the group mode, as further explained in Section 13.7.2.¶

  Note for implementation: this construction requires the OSCORE option of the message to be generated and finalized before computing the ciphertext of the COSE_Encrypt0 object (when using the group mode or the pairwise mode) and before calculating the countersignature (when using the group mode). Also, the aad_array needs to be large enough to contain the largest possible OSCORE option.¶

• The new element 'sender_cred', containing the sender's authentication credential. This parameter MUST be set to a CBOR byte string, which encodes the sender's authentication credential in its original binary representation made available to other endpoints in the group (see Section 2.4).¶
• The new element 'gm_cred', containing the Group Manager's authentication credential. This parameter MUST be set to a CBOR byte string, which encodes the Group Manager's authentication credential in its original binary representation made available to other endpoints in the group (see Section 2.4). This prevents the attack described in Section 13.8.¶

5.1. Keystream Derivation for Countersignature Encryption

The following defines how an endpoint derives the keystream KEYSTREAM, used to encrypt/decrypt the countersignature of an outgoing/incoming message M protected in group mode.¶

The keystream SHALL be derived as follows, by using the HKDF Algorithm from the Common Context (see Section 3.2 of [RFC8613]), which consists of composing the HKDF-Extract and HKDF-Expand steps [RFC5869].¶

KEYSTREAM = HKDF(salt, IKM, info, L)¶

The input parameters of HKDF are as follows.¶

• salt takes as value the Partial IV (PIV) used to protect M. Note that, if M is a response, salt takes as value either: i) the fresh Partial IV generated by the server and included in the response; or ii) the same Partial IV of the request generated by the client and not included in the response.¶
• IKM is the Signature Encryption Key from the Common Context (see Section 2.1.8).¶
• info is the serialization of a CBOR array consisting of (the notation follows [RFC8610]):¶

   info = [
     id : bstr,
     id_context : bstr,
     type : bool,
     L : uint
   ]

where:¶

• id is the Sender ID of the endpoint that generated PIV.¶

• id_context is the ID Context (Gid) used when protecting M.¶

  Note that, in case of group rekeying, a server might use a different Gid when protecting a response, compared to the Gid that it used to verify (that the client used to protect) the request, see Section 8.3.¶

• type is the CBOR simple value true (0xf5) if M is a request, or the CBOR simple value false (0xf4) otherwise.¶
• L is the size of the countersignature, as per Signature Algorithm from the Common Context (see Section 2.1.7), in bytes.¶

5.2. Examples of Compressed COSE Objects

This section covers a list of OSCORE Header Compression examples of Group OSCORE used in group mode (see Section 5.2.1) or in pairwise mode (see Section 5.2.2).¶

The examples assume that the COSE_Encrypt0 object is set (which means the CoAP message and cryptographic material is known). Note that the examples do not include the full CoAP unprotected message or the full Security Context, but only the input necessary to the compression mechanism, i.e., the COSE_Encrypt0 object. The output is the compressed COSE object as defined in Section 5 and divided into two parts, since the object is transported in two CoAP fields: OSCORE option and payload.¶

The examples assume that the plaintext (see Section 5.3 of [RFC8613]) is 6 bytes long, and that the AEAD tag is 8 bytes long, hence resulting in a ciphertext which is 14 bytes long. When using the group mode, the COSE_Countersignature0 byte string as described in Section 4 is assumed to be 64 bytes long.¶

   [
     / protected / h'',
     / unprotected / {
       / kid /                           4 : h'25',
       / Partial IV /                    6 : h'05',
       / kid context /                  10 : h'44616c',
       / Countersignature0 version 2 /  12 : h'66e6d9b0
       db009f3e105a673f8855611726caed57f530f8cae9d0b168
       513ab949fedc3e80a96ebe94ba08d3f8d3bf83487458e2ab
       4c2f936ff78b50e33c885e35'
     },
     / ciphertext / h'aea0155667924dff8a24e4cb35b9'
   ]

   Flag byte: 0b00111001 = 0x39 (1 byte)

   Option Value: 0x39 05 03 44 61 6c 25 (7 bytes)

   Payload: 0xaea0155667924dff8a24e4cb35b9 de9e ... f1
   (14 bytes + size of the encrypted countersignature)

   [
     / protected / h'',
     / unprotected / {
       / kid /                           4 : h'52',
       / Countersignature0 version 2 /  12 : h'f5b659b8
       24487eb349c5c5c8a3fe401784cade2892725438e8be0fab
       daa2867ee6d29f68edb0818e50ebf98c28b923d0205f5162
       e73662e27c1a3ec562a49b80'
     },
     / ciphertext / h'60b035059d9ef5667c5a0710823b'
   ]

   Flag byte: 0b00101000 = 0x28 (1 byte)

   Option Value: 0x28 52 (2 bytes)

   Payload: 0x60b035059d9ef5667c5a0710823b ca1e ... b3
   (14 bytes + size of the encrypted countersignature)

   [
     / protected / h'',
     / unprotected / {
       / kid /           4 : h'25',
       / Partial IV /    6 : h'05',
       / kid context /  10 : h'44616c'
     },
     / ciphertext / h'aea0155667924dff8a24e4cb35b9'
   ]

   Flag byte: 0b00011001 = 0x19 (1 byte)

   Option Value: 0x19 05 03 44 61 6c 25 (7 bytes)

   Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)

   [
     / protected / h'',
     / unprotected / {},
     / ciphertext / h'60b035059d9ef5667c5a0710823b'
   ]

   Flag byte: 0b00000000 = 0x00 (1 byte)

   Option Value: 0x (0 bytes)

   Payload: 0x60b035059d9ef5667c5a0710823b (14 bytes)

6.1. Supporting Multiple Responses in Long Exchanges

For each of its ongoing long exchange, a client maintains one Response Number for each different server. Then, separately for each server, the client uses the associated Response Number to perform ordering and replay protection of responses received from that server within that long exchange (see Section 6.3.1).¶

That is, the Response Number has the same purpose that the Notification Number has in OSCORE (see Section 4.1.3.5.2 of [RFC8613]), but a client uses it for handling any response from the associated server within a long exchange.¶

Group OSCORE allows to preserve a long exchange active indefinitely, even in case the group is rekeyed, with consequent change of ID Context, or in case the client obtains a new Sender ID.¶

As defined in Section 8, this is achieved by the client and server(s) storing the 'kid' and 'kid context' used in the original request, throughout the whole duration of the long exchange.¶

Upon leaving the group or before re-joining the group, a group member MUST terminate all the ongoing long exchanges that it has started in the group as a client, and hence frees up the CoAP Token associated with the corresponding request.¶

6.2. Synchronization and Freshness

If the application requires freshness, e.g., according to time- or event-based policies (see Section 2.5.1 of [RFC9175]), a server can use the approach in Section 10 as a variant of the procedure in Appendix B.1.2 of [RFC8613], before delivering request messages from a client to the application. This also makes the server (re-)synchronize with the client's Sender Sequence Number.¶

Like in OSCORE [RFC8613], assuming an honest server, the message binding guarantees that a response is not older than the request it replies to. Therefore, building on Section 7.3 of [RFC8613], the following properties hold for Group OSCORE.¶

• The freshness of a response can be assessed if it is received soon after the request.¶

  For responses within a long exchange, this assessment gets weaker with time. If such responses are Observe notifications [RFC7641], it is RECOMMENDED that the client regularly re-register the observation.¶

  If the request was neither a group request nor an Observe request, there is at most a single valid response and only from one, individually targeted server in the group. Thus, freshness can be assessed depending on when the request was sent.¶

• It is not guaranteed that a misbehaving server did not create the response before receiving the request, i.e., Group OSCORE does not verify server aliveness.¶
• For requests and responses, the received Partial IV allows a recipient to determine the relative order of requests or responses.¶

6.3. Replay Protection

Like in OSCORE [RFC8613], the replay protection relies on the Partial IV of incoming messages. A server updates the Replay Window of its Recipient Contexts based on the Partial IV values in received request messages, which correspond to the Sender Sequence Numbers of the clients. Note that there can be large jumps in these Sender Sequence Number values, for example when a client exchanges unicast messages with other servers. The operation of validating the Partial IV and performing replay protection MUST be atomic. The update of Replay Windows is described in Section 2.6.1.¶

The protection from replay of requests is performed as per Section 7.4 of [RFC8613], separately for each client and by leveraging the Replay Window in the corresponding Recipient Context. The protection from replay of responses in a long exchange is performed as defined in Section 6.3.1.¶

8.1. Protecting the Request

When using the group mode to protect a request, a client SHALL proceed as described in Section 8.1 of [RFC8613], with the following modifications.¶

• In step 2, the Additional Authenticated Data is modified as described in Section 4 of this document.¶
• In step 4, the encryption of the COSE object is modified as described in Section 4 of this document. The encoding of the compressed COSE object is modified as described in Section 5 of this document. In particular, the Group Flag MUST be set to 1. The Group Encryption Algorithm from the Common Context MUST be used.¶
• In step 5, the countersignature is computed and the format of the OSCORE message is modified as described in Section 4 and Section 5 of this document. In particular, the payload of the Group OSCORE message includes also the encrypted countersignature.¶

In addition, the following applies when sending a request that establishes a long exchange.¶

• If the client intends to keep the long exchange active beyond a possible change of Sender ID, the client MUST store the value of the 'kid' parameter from the request, and retain it until the long exchange is terminated. Even in case the client is individually rekeyed and receives a new Sender ID from the Group Manager (see Section 2.6.3.1), the client MUST NOT update the stored 'kid' parameter value associated with the long exchange and the corresponding request.¶

• If the client intends to keep the long exchange active beyond a possible change of ID Context following a group rekeying (see Section 3.2), then the following applies.¶

  • The client MUST store the value of the 'kid context' parameter from the request, and retain it until the long exchange is terminated. Upon establishing a new Security Context with a new Gid as ID Context (see Section 2.6.3.2), the client MUST NOT update the stored 'kid context' parameter value associated with the long exchange and the corresponding request.¶

  • The client MUST store an invariant identifier of the group, which is immutable even in case the Security Context of the group is re-established. For example, this invariant identifier can be the "group name" in [I-D.ietf-ace-key-groupcomm-oscore], where it is used for joining the group and retrieving the current group keying material from the Group Manager.¶

    After a group rekeying, such an invariant information makes it simpler for the client to retrieve the current group keying material from the Group Manager, in case the client has missed both the rekeying messages and the first response protected with the new Security Context (see Section 8.3).¶

8.2. Verifying the Request

Upon receiving a protected request with the Group Flag set to 1, following the procedure in Section 7, a server SHALL proceed as described in Section 8.2 of [RFC8613], with the following modifications.¶

• In step 2, the decoding of the compressed COSE object follows Section 5 of this document. In particular:¶

  • If the server discards the request due to not retrieving a Security Context associated with the OSCORE group, the server MAY respond with a 4.01 (Unauthorized) error message. When doing so, the server MAY set an Outer Max-Age option with value zero, and MAY include a descriptive string as diagnostic payload.¶
  • If the received 'kid context' matches an existing ID Context (Gid) but the received 'kid' does not match any Recipient ID in this Security Context, then the server MAY create a new Recipient Context for this Recipient ID and initialize it according to Section 3 of [RFC8613], and also retrieve the authentication credential associated with the Recipient ID to be stored in the new Recipient Context. Such a configuration is application specific. If the application does not specify dynamic derivation of new Recipient Contexts, then the server SHALL stop processing the request.¶
• In step 4, the Additional Authenticated Data is modified as described in Section 4 of this document.¶

• In step 6, the server also verifies the countersignature, by using the public key from the client's authentication credential stored in the associated Recipient Context. In particular:¶

  • If the server does not have the public key of the client yet, the server MUST stop processing the request and MAY respond with a 5.03 (Service Unavailable) response. The response MAY include a Max-Age Option, indicating to the client the number of seconds after which to retry. If the Max-Age Option is not present, a retry time of 60 seconds will be assumed by the client, as default value defined in Section 5.10.5 of [RFC7252].¶
  • The server MUST perform signature verification before decrypting the COSE object, as defined below. Implementations that cannot perform the two steps in this order MUST ensure that no access to the plaintext is possible before a successful signature verification and MUST prevent any possible leak of time-related information that can yield side-channel attacks.¶

  • The server retrieves the encrypted countersignature ENC_SIGNATURE from the message payload, and computes the original countersignature SIGNATURE as¶

    SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM¶

    where KEYSTREAM is derived as per Section 5.1.¶

  • The server verifies the original countersignature SIGNATURE as described in Sections 3.2 and 3.3 of [RFC9338], by using the client's public key and according to the Signature Algorithm in the Security Context.¶

    In particular, the Countersign_structure contains the context text string "CounterSignature0", the external_aad as defined in Section 4.4 of this document, and the ciphertext of the COSE object as payload.¶

  • If the signature verification fails, the server SHALL stop processing the request, SHALL NOT update the Replay Window, and MAY respond with a 4.00 (Bad Request) response. Such a response MAY include an Outer Max-Age option with value zero, and its diagnostic payload MAY contain a string, which, if present, MUST be "Decryption failed" as if the decryption of the COSE object had failed.¶
  • When decrypting the COSE object using the Recipient Key, the Group Encryption Algorithm from the Common Context MUST be used.¶
• Additionally, if the used Recipient Context was created upon receiving this request and the message is not verified successfully, the server MAY delete that Recipient Context. Such a configuration, which is specified by the application, mitigates attacks that aim at overloading the server's storage.¶

A server SHOULD NOT process a request if the received Recipient ID ('kid') is equal to its own Sender ID in its own Sender Context. For an example where this is not fulfilled, see Section 9.2.1 of [I-D.ietf-core-observe-multicast-notifications].¶

In addition, the following applies if the request establishes a long exchange and the server intends to reply with multiple responses.¶

• The server MUST store the value of the 'kid' parameter from the request, and retain it until the last response has been sent. The server MUST NOT update the stored value of the 'kid' parameter associated with the request, even in case the client is individually rekeyed and starts using a new Sender ID received from the Group Manager (see Section 2.6.3.1).¶
• The server MUST store the value of the 'kid context' parameter from the request, and retain it until the last response has been sent, i.e., beyond a possible change of ID Context following a group rekeying (see Section 3.2). That is, upon establishing a new Security Context with a new Gid as ID Context (see Section 2.6.3.2), the server MUST NOT update the stored value of a 'kid context' parameter associated with the request.¶

8.3. Protecting the Response

When using the group mode to protect a response, a server SHALL proceed as described in Section 8.3 of [RFC8613], with the following modifications.¶

Note that the server always protects a response with the Sender Context from its latest Security Context, and that establishing a new Security Context resets the Sender Sequence Number to 0 (see Section 3.2).¶

• In step 2, the Additional Authenticated Data is modified as described in Section 4 of this document.¶

  In addition, the following applies if the server intends to reply with multiple responses, within the long exchange established by the corresponding request.¶

  • The server MUST use the stored value of the 'kid' parameter from the request (see Section 8.2), as value for the 'request_kid' parameter in the external_aad structure (see Section 4.4).¶
  • The server MUST use the stored value of the 'kid context' parameter from the request (see Section 8.2), as value for the 'request_kid_context' parameter in the external_aad structure (see Section 4.4).¶

• In step 3, if any of the following two conditions holds, the server MUST include its Sender Sequence Number as Partial IV in the response and use it to build the AEAD nonce to protect the response. This prevents the server from reusing the AEAD nonce from the request together with the same encryption key.¶

  • The response is not the first response that the server sends to the request.¶
  • The server is using a different Security Context for the response compared to what was used to verify the request (see Section 3.2).¶

• In step 4, the encryption of the COSE object is modified as described in Section 4 of this document. The encoding of the compressed COSE object is modified as described in Section 5 of this document. In particular, the Group Flag MUST be set to 1. The Group Encryption Algorithm from the Common Context MUST be used.¶

  In addition, the following applies.¶

  • If the server is using a different ID Context (Gid) for the response compared to what was used to verify the request (see Section 3.2) and this is the first response from the server to that request, then the new ID Context MUST be included in the 'kid context' parameter of the response.¶

  • The server may be replying to a request that was protected with an old Security Context. After completing the establishment of a new Security Context, the server MUST protect all the responses to that request with the Sender Context of the new Security Context.¶

    For each ongoing long exchange, the server can help the client to synchronize, by including also the 'kid context' parameter in responses following a group rekeying, with value set to the ID Context (Gid) of the new Security Context.¶

    If there is a known upper limit to the duration of a group rekeying, the server SHOULD include the 'kid context' parameter during that time. Otherwise, the server SHOULD include it until the Max-Age has expired for the last response sent before the installation of the new Security Context.¶

  • The server can obtain a new Sender ID from the Group Manager, when individually rekeyed (see Section 2.6.3.1) or when re-joining the group. In such a case, the server can help the client to synchronize, by including the 'kid' parameter in a response protected in group mode, even when the request was protected in pairwise mode (see Section 9.3).¶

    That is, when responding to a request protected in pairwise mode, the server SHOULD include the 'kid' parameter in a response protected in group mode, if it is replying to that client for the first time since the assignment of its new Sender ID.¶

• In step 5, the countersignature is computed and the format of the OSCORE message is modified as described in Section 4 and Section 5 of this document. In particular the payload of the Group OSCORE message includes also the encrypted countersignature (see Section 5).¶

8.4. Verifying the Response

Upon receiving a protected response with the Group Flag set to 1, following the procedure in Section 7, a client SHALL proceed as described in Section 8.4 of [RFC8613], with the modifications described in this section.¶

Note that a client may receive a response protected with a Security Context different from the one used to protect the corresponding request, and that, upon the establishment of a new Security Context, the client re-initializes its Replay Windows in its Recipient Contexts (see Section 3.2).¶

• In step 2, the decoding of the compressed COSE object is modified as described in Section 5 of this document. In particular, a 'kid' may not be present, if the response is a reply to a request protected in pairwise mode. In such a case, the client assumes the response 'kid' to be the Recipient ID for the server to which the request protected in pairwise mode was intended for.¶

  If the response 'kid context' matches an existing ID Context (Gid) but the received/assumed 'kid' does not match any Recipient ID in this Security Context, then the client MAY create a new Recipient Context for this Recipient ID and initialize it according to Section 3 of [RFC8613], and also retrieve the authentication credential associated with the Recipient ID to be stored in the new Recipient Context. If the application does not specify dynamic derivation of new Recipient Contexts, then the client SHALL stop processing the response.¶

• In step 3, the Additional Authenticated Data is modified as described in Section 4 of this document.¶

  In addition, the following applies if the client processes a response to a request within a long exchange.¶

  • The client MUST use the stored value of the 'kid' parameter from the request (see Section 8.1), as value for the 'request_kid' parameter in the external_aad structure (see Section 4.4).¶
  • The client MUST use the stored value of the 'kid context' parameter from the request (see Section 8.1), as value for the 'request_kid_context' parameter in the external_aad structure (see Section 4.4).¶

  This ensures that, throughout a long exchange, the client can correctly verify the received responses, even in case the client is individually rekeyed and starts using a new Sender ID received from the Group Manager (see Section 2.6.3.1), as well as when it installs a new Security Context with a new ID Context (Gid) following a group rekeying (see Section 3.2).¶

• In step 5, the client also verifies the countersignature, by using the public key from the server's authentication credential stored in the associated Recipient Context. In particular:¶

  • The client MUST perform signature verification as defined below, before decrypting the COSE object. Implementations that cannot perform the two steps in this order MUST ensure that no access to the plaintext is possible before a successful signature verification and MUST prevent any possible leak of time-related information that can yield side-channel attacks.¶

  • The client retrieves the encrypted countersignature ENC_SIGNATURE from the message payload, and computes the original countersignature SIGNATURE as¶

    SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM¶

    where KEYSTREAM is derived as per Section 5.1.¶

    The client verifies the original countersignature SIGNATURE.¶

  • If the verification of the countersignature fails, the client: i) SHALL stop processing the response; and ii) SHALL NOT update the Response Number associated with the server.¶

  • After a successful verification of the countersignature, the client performs also the following actions in case the request was protected in pairwise mode (see Section 9.3).¶

    • If the 'kid' parameter is present in the response, the client checks whether this received 'kid' is equal to the expected 'kid', i.e., the known Recipient ID for the server to which the request was intended for.¶
    • If the 'kid' parameter is not present in the response, the client checks whether the server that has sent the response is the same one to which the request was intended for. This can be done by checking that the public key used to verify the countersignature of the response is equal to the public key included in the authentication credential Recipient Auth Cred, which was taken as input to derive the Pairwise Sender Key used for protecting the request (see Section 2.5.1).¶

    In either case, if the client determines that the response has come from a different server than the expected one, then the client: i) SHALL discard the response and SHALL NOT deliver it to the application; ii) SHALL NOT update the Response Number associated with the server.¶

    Otherwise, the client hereafter considers the received 'kid' as the current Recipient ID for the server.¶

• In step 5, when decrypting the COSE object using the Recipient Key, the Group Encryption Algorithm from the Common Context MUST be used.¶

  In addition, the client performs the following actions if the response is received within a long exchange.¶

  • The ordering and the replay protection of responses received from the server during the long exchange are performed as per Section 6.3.1 of this document, by using the Response Number associated with that server within that long exchange. In case of unsuccessful decryption and verification of a response, the client SHALL NOT update the Response Number associated with the server.¶
  • When receiving the first valid response from the server within the long exchange, the client MUST store the current kid "kid1" of that server for that long exchange. If the 'kid' field is included in the OSCORE option of the response, its value specifies "kid1". If the request was protected in pairwise mode (see Section 9.3), the 'kid' field may not be present in the OSCORE option of the response (see Section 4.2). In this case, the client assumes "kid1" to be the Recipient ID for the server to which the request was intended for.¶

  • When receiving another valid response to the same request from the same server - which can be identified and recognized through the same public key used to verify the countersignature and included in the server's authentication credential - the client determines the current kid "kid2" of the server as above for "kid1", and MUST check whether "kid2" is equal to the stored "kid1".¶

    If "kid1" and "kid2" are different, the client SHOULD NOT accept the response as valid to be delivered to the application, and SHOULD NOT update the Response Number associated with the server. Exceptions can apply if the client has a means of preserving the order of responses in a more elaborate way out of the scope of this document, or the client application does not require response ordering altogether.¶

  Note that, if "kid2" is different from "kid1" and the 'kid' field is omitted from the response - which is possible if the request was protected in pairwise mode - then the client will compute a wrong keystream to decrypt the countersignature (i.e., by using "kid1" rather than "kid2" in the 'id' field of the 'info' array in Section 5.1), thus subsequently failing to verify the countersignature and discarding the response.¶

  This ensures that the client remains able to correctly perform the ordering and replay protection of responses within a long exchange, even in case the server legitimately starts using a new Sender ID, as received from the Group Manager when individually rekeyed (see Section 2.6.3.1) or when re-joining the group.¶

• In step 8, if the used Recipient Context was created upon receiving this response and the message is not verified successfully, the client MAY delete that Recipient Context. Such a configuration, which is specified by the application, mitigates attacks that aim at overloading the client's storage.¶

8.5. External Signature Checkers

When receiving a message protected in group mode, a signature checker (see Section 3.3) proceeds as follows.¶

• The signature checker retrieves the encrypted countersignature ENC_SIGNATURE from the message payload, and computes the original countersignature SIGNATURE as¶

  SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM¶

  where KEYSTREAM is derived as per Section 5.1.¶

• The signature checker verifies the original countersignature SIGNATURE, by using the public key of the sender endpoint as included in that endpoint's authentication credential. The signature checker determines the right authentication credential based on the ID Context (Gid) and the Sender ID of the sender endpoint.¶

Note that the following applies when attempting to verify the countersignature of a response message.¶

• The response may not include a Partial IV and/or an ID Context. In such a case, the signature checker considers the same values from the corresponding request, i.e., the request matching with the response by CoAP Token value.¶
• The response may not include a Sender ID. This can happen when the response protected in group mode matches a request protected in pairwise mode (see Section 9.1), with a case in point provided by [I-D.amsuess-core-cachable-oscore]. In such a case, the signature checker needs to use other means (e.g., source addressing information of the server endpoint) to identify the correct authentication credential including the public key to use for verifying the countersignature of the response.¶

The particular actions following a successful or unsuccessful verification of the countersignature are application specific and out of the scope of this document.¶

9.1. Pre-Conditions

In order to protect an outgoing message in pairwise mode, the sender needs to know the authentication credential and the Recipient ID for the recipient endpoint, as stored in the Recipient Context associated with that endpoint (see Section 2.5.4).¶

Furthermore, the sender needs to know the individual address of the recipient endpoint. This information may not be known at any given point in time. For instance, right after having joined the group, a client may know the authentication credential and Recipient ID for a given server, but not the addressing information required to reach it with an individual, one-to-one request.¶

In order to make addressing information of individual endpoints available, servers in the group MAY expose a resource to which a client can send a request targeting a set of servers, identified by their 'kid' values specified in the request payload, or implicitly if the request is sent in pairwise mode. Further details of such an interface are out of scope for this document.¶

9.2. Main Differences from OSCORE

The pairwise mode protects messages between two members of a group, essentially following [RFC8613], but with the following notable differences.¶

• The 'kid' and 'kid context' parameters of the COSE object are used as defined in Section 4.2 of this document.¶
• The external_aad defined in Section 4.4 of this document is used for the encryption and decryption process.¶
• The Pairwise Sender/Recipient Keys used as Sender/Recipient keys are derived as defined in Section 2.5 of this document.¶

9.3. Protecting the Request

When using the pairwise mode to protect a request, a client SHALL proceed as described in Section 8.1 of [RFC8613], with the differences summarized in Section 9.2 of this document.¶

Furthermore, when sending a request that establishes a long exchange, what is specified in Section 8.1 of this document holds, with respect to storing the value of the 'kid' and 'kid context' parameters, and to storing an invariant identifier of the group.¶

9.4. Verifying the Request

Upon receiving a protected request with the Group Flag set to 0, following the procedure in Section 7, a server SHALL proceed as described in Section 8.2 of [RFC8613], with the differences summarized in Section 9.2 of this document. The following differences also apply.¶

• If the server discards the request due to not retrieving a Security Context associated with the OSCORE group or to not supporting the pairwise mode, the server MAY respond with a 4.01 (Unauthorized) error message or a 4.02 (Bad Option) error message, respectively. When doing so, the server MAY set an Outer Max-Age option with value zero, and MAY include a descriptive string as diagnostic payload.¶
• If a new Recipient Context is created for this Recipient ID, new Pairwise Sender/Recipient Keys are also derived (see Section 2.5.1). The new Pairwise Sender/Recipient Keys are deleted if the Recipient Context is deleted as a result of the message not being successfully verified.¶

• What is specified in Section 8.2 of this document holds with respect to the following points.¶

  • The possible, dynamic creation and configuration of a Recipient Context upon receiving the request.¶
  • The possible deletion of a Recipient Context created upon receiving the request, in case the request is not verified successfully.¶
  • The rule about processing the request where the received Recipient ID ('kid') is equal to the server's Sender ID.¶
  • The storing of the value of the 'kid' and 'kid context' parameters from the request, if the server intends to reply with multiple responses within the long exchange established by the request.¶

9.5. Protecting the Response

When using the pairwise mode to protect a response, a server SHALL proceed as described in Section 8.3 of [RFC8613], with the differences summarized in Section 9.2 of this document. The following differences also apply.¶

• What is specified in Section 8.3 of this document holds with respect to the following points.¶

  • The protection of a response when using a different Security Context than the one used to protect the corresponding request. That is, the server always protects a response with the Sender Context from its latest Security Context, and establishing a new Security Context resets the Sender Sequence Number to 0 (see Section 3.2).¶
  • The use of the stored value of the 'kid' and 'kid context' parameters, if the server intends to reply with multiple responses within the long exchange established by the request.¶
  • The rules for the inclusion of the server's Sender Sequence Number as Partial IV in a response, as used to build the AEAD nonce to protect the response.¶
  • The rules for the inclusion of the ID Context (Gid) in the 'kid context' parameter of a response, if the ID Context used for the response differs from the one used to verify the request (see Section 3.2), also for helping the client to synchronize.¶
  • The rules for the inclusion of the Sender ID in the 'kid' parameter of a response to a request that was protected in pairwise mode, if the server has obtained a new Sender ID from the Group Manager when individually rekeyed (see Section 2.6.3.1), thus helping the client to synchronize.¶

9.6. Verifying the Response

Upon receiving a protected response with the Group Flag set to 0, following the procedure in Section 7, a client SHALL proceed as described in Section 8.4 of [RFC8613], with the differences summarized in Section 9.2 of this document. The following differences also apply.¶

• The client may receive a response protected with a Security Context different from the one used to protect the corresponding request. Also, upon the establishment of a new Security Context, the client re-initializes its Replay Windows in its Recipient Contexts (see Section 2.2).¶
• The same as described in Section 8.4 holds with respect to handling the 'kid' parameter of the response, when received as a reply to a request protected in pairwise mode. The client can also in this case check whether the replying server is the expected one, by relying on the server's public key. However, since the response is protected in pairwise mode, the public key is not used for verifying a countersignature as in Section 8.4. Instead, the expected server's authentication credential - namely Recipient Auth Cred and including the server's public key - was taken as input to derive the Pairwise Recipient Key used to decrypt and verify the response (see Section 2.5.1).¶
• If a new Recipient Context is created for this Recipient ID, new Pairwise Sender/Recipient Keys are also derived (see Section 2.5.1). The new Pairwise Sender/Recipient Keys are deleted if the Recipient Context is deleted as a result of the message not being successfully verified.¶

• What is specified in Section 8.4 of this document holds with respect to the following points.¶

  • The possible, dynamic creation and configuration of a Recipient Context upon receiving the response.¶
  • The use of the stored value of the 'kid' and 'kid context' parameters, when processing a response received within a long exchange.¶
  • The performing of ordering and replay protection for responses received within a long exchange.¶
  • The possible deletion of a Recipient Context created upon receiving the response, in case the response is not verified successfully.¶

13.1. Security of the Group Mode

The group mode defined in Section 8 relies on commonly shared group keying material to protect communication within a group. Using the group mode has the implications discussed below. The following refers to group members as the endpoints in the group storing the latest version of the group keying material.¶

• Source authentication of messages sent to a group is ensured through a countersignature, which is computed by the sender using its own private key and then appended to the message payload. Also, the countersignature is encrypted by using a keystream derived from the group keying material (see Section 5). This ensures group privacy, i.e., an attacker cannot track an endpoint over two groups by linking messages between the two groups, unless being also a member of those groups.¶
• Messages are encrypted at a group level (group-level data confidentiality), i.e., they can be decrypted by any member of the group, but not by an external adversary or other external entities.¶

• If the used Group Encryption Algorithm provides integrity protection, then it also ensures group authentication and proof of group membership, but not source authentication. That is, it ensures that a message sent to a group has been sent by a member of that group, but not necessarily by the alleged sender. In fact, any group member is able to derive the Sender Key used by the actual sender endpoint, and thus can compute a valid authentication tag. Therefore, the message content could originate from any of the current group members.¶

  Furthermore, if the used Group Encryption Algorithm does not provide integrity protection, then it does not ensure any level of message authentication or proof of group membership.¶

  On the other hand, proof of group membership is always ensured by construction through the strict management of the group keying material (see Section 3.2). That is, the group is rekeyed in case of members' leaving, and the current group members are informed of former group members. Thus, a current group member storing the latest group keying material does not store the authentication credential of any former group member.¶

  This allows a recipient endpoint to rely on the stored authentication credentials and public keys included therein, in order to always confidently assert the group membership of a sender endpoint when processing an incoming message, i.e., to assert that the sender endpoint was a group member when it signed the message. In turn, this prevents a former group member to possibly re-sign and inject in the group a stored message that was protected with old keying material.¶

  A case in point is a group where the Group Encryption Algorithm does not provide integrity protection; a group member leaves the group; and, after the group rekeying, associates with the group as external signature checker (see Section 8.5). When doing so, it obtains from the Group Manager the new Signature Encryption Key, from which it can derive keystreams for encrypting and decrypting the countersignatures of messages protected in group mode.¶

  If, when participating in the group rekeying, the current group members had not deleted the Recipient Context and authentication credential of the former group member, then the signature checker would be able to successfully inject messages protected in group mode, as encrypted with the old group keying material, signed with its own private key, and with the countersignature encrypted by means of the latest Signature Encryption Key. Then, the group members, as still retaining the authentication credential of the signature checker, will verify and accept the message, even though the sender was not a group member when signing the message.¶

The security properties of the group mode are summarized below.¶

1. Asymmetric source authentication, by means of a countersignature.¶
2. Symmetric group authentication, by means of an authentication tag (only for Group Encryption Algorithms providing integrity protection).¶
3. Symmetric group confidentiality, by means of symmetric encryption.¶
4. Proof of group membership, by strictly managing the group keying material, as well as by means of integrity tags when using a Group Encryption Algorithm that provides also integrity protection.¶
5. Group privacy, by encrypting the countersignature.¶

The group mode fulfills the security properties above while also displaying the following benefits. First, the use of a Group Encryption Algorithm that does not provide integrity protection results in a minimal communication overhead, by limiting the message payload to the ciphertext without integrity tag together with the encrypted countersignature. Second, it is possible to deploy semi-trusted entities such as signature checkers (see Section 3.3), which can break property 5, but cannot break properties 1, 2, 3, and 4.¶

13.2. Security of the Pairwise Mode

The pairwise mode defined in Section 9 protects messages by using pairwise symmetric keys, derived from the static-static Diffie-Hellman shared secret computed from the asymmetric keys of the sender and recipient endpoint (see Section 2.5).¶

The used AEAD Algorithm MUST provide integrity protection. Therefore, the pairwise mode ensures both pairwise data-confidentiality and source authentication of messages, without using countersignatures. Furthermore, the recipient endpoint achieves proof of group membership for the sender endpoint, since only current group members have the required keying material to derive a valid Pairwise Sender/Recipient Key.¶

The long-term storing of the Diffie-Hellman shared secret is a potential security issue. In fact, if the shared secret of two group members is leaked, a third group member can exploit it to impersonate any of those two group members, by deriving and using their pairwise key. The possibility of such leakage should be contemplated, as more likely to happen than the leakage of a private key, which could be rather protected at a significantly higher level than generic memory, e.g., by using a Trusted Platform Module. Therefore, there is a trade-off between the maximum amount of time a same shared secret is stored and the frequency of its re-computing.¶

13.3. Uniqueness of (key, nonce)

The proof for uniqueness of (key, nonce) pairs in Appendix D.4 of [RFC8613] is also valid in group communication scenarios. That is, given an OSCORE group:¶

• Uniqueness of Sender IDs within the group is enforced by the Group Manager. In fact, from the moment when a Gid is assigned to a group until the moment when a new Gid is assigned to that same group, the Group Manager does not reassign a Sender ID within the group (see Section 3.2).¶
• The case A in Appendix D.4 of [RFC8613] concerns all requests as well as all responses including a Partial IV (e.g., Observe notifications [RFC7641] or any other subsequent responses after the first one). In this case, same considerations from [RFC8613] apply here as well.¶
• The case B in Appendix D.4 of [RFC8613] concerns responses not including a Partial IV (e.g., a single response to a request). In this case, same considerations from [RFC8613] apply here as well.¶

As a consequence, each message encrypted/decrypted with the same Sender Key is processed by using a different (ID_PIV, PIV) pair. This means that nonces used by any fixed encrypting endpoint are unique. Thus, each message is processed with a different (key, nonce) pair.¶

13.4. Management of Group Keying Material

The protocol described in this document should take into account the risk of compromise of group members. In particular, this document specifies that a key management scheme for secure revocation and renewal of Security Contexts and group keying material MUST be adopted.¶

[I-D.ietf-ace-key-groupcomm-oscore] specifies a simple rekeying scheme for renewing the Security Context in a group.¶

Alternative rekeying schemes that are more scalable with the group size may be needed in dynamic, large groups where endpoints can join and leave at any time, in order to limit the impact on performance due to the Security Context and keying material update.¶

13.5. Update of Security Context and Key Rotation

A group member can receive a message shortly after the group has been rekeyed, and new security parameters and keying material have been distributed by the Group Manager.¶

This may result in a client using an old Security Context to protect a request, and a server using a different new Security Context to protect a corresponding response. As a consequence, clients may receive a response protected with a Security Context different from the one used to protect the corresponding request.¶

In particular, a server may first get a request protected with the old Security Context, then install the new Security Context, and only after that produce a response to send back to the client. In such a case, as specified in Section 8.3, the server MUST protect the potential response using the new Security Context. Specifically, the server MUST include its Sender Sequence Number as Partial IV in the response and use it to build the AEAD nonce to protect the response. This prevents the AEAD nonce from the request from being reused with the new Security Context.¶

The client will process that response using the new Security Context, provided that it has installed the new security parameters and keying material before the message processing.¶

In case block-wise transfer [RFC7959] is used, the same considerations from Section 10.3 of [I-D.ietf-ace-key-groupcomm] hold.¶

Furthermore, as described below, a group rekeying may temporarily result in misaligned Security Contexts between the sender and recipient of a same message.¶

13.6. Collision of Group Identifiers

In case endpoints are deployed in multiple groups managed by different non-synchronized Group Managers, it is possible for Group Identifiers of different groups to coincide.¶

This does not impair the security of the AEAD algorithm. In fact, as long as the Master Secret is different for different groups and this condition holds over time, AEAD keys are different among different groups.¶

In case multiple groups use the same IP multicast address, the entity assigning that address may help limiting the chances to experience such collisions of Group Identifiers. In particular, it may allow the Group Managers of those groups using the same IP multicast address to share their respective list of assigned Group Identifiers currently in use.¶

13.7. Cross-group Message Injection

A same endpoint is allowed to and would likely use the same pair (private key, authentication credential) in multiple OSCORE groups, possibly administered by different Group Managers.¶

When a sender endpoint sends a message protected in pairwise mode to a recipient endpoint in an OSCORE group, a malicious group member may attempt to inject the message to a different OSCORE group also including the same endpoints (see Section 13.7.1).¶

This practically relies on altering the content of the OSCORE option, and having the same MAC in the ciphertext still correctly validating, which has a success probability depending on the size of the MAC.¶

As discussed in Section 13.7.2, the attack is practically infeasible if the message is protected in group mode, thanks to the countersignature also bound to the OSCORE option through the Additional Authenticated Data used in the signing process (see Section 4.4).¶

13.8. Prevention of Group Cloning Attack

Both when using the group mode and the pairwise mode, the message protection covers also the Group Manager's authentication credential. This is included in the Additional Authenticated Data used in the signing process and/or in the integrity-protected encryption process (see Section 4.4).¶

By doing so, an endpoint X member of a group G1 cannot perform the following attack.¶

1. X sets up a group G2 where it acts as Group Manager.¶
2. X makes G2 a "clone" of G1, i.e., G1 and G2 use the same algorithms and have the same Master Secret, Master Salt and ID Context.¶
3. X collects a message M sent to G1 and injects it in G2.¶
4. Members of G2 accept M and believe it to be originated in G2.¶

The attack above is effectively prevented, since message M is protected by including the authentication credential of G1's Group Manager in the Additional Authenticated Data. Therefore, members of G2 do not successfully verify and decrypt M, since they correctly use the authentication credential of X as Group Manager of G2 when attempting to.¶

13.9. Group OSCORE for Unicast Requests

If a request is intended to be sent over unicast as addressed to a single group member, it is NOT RECOMMENDED for the client to protect the request by using the group mode as defined in Section 8.1.¶

This does not include the case where the client sends a request over unicast to a proxy, to be forwarded to multiple intended recipients over multicast [I-D.ietf-core-groupcomm-bis]. In this case, the client typically protects the request with the group mode, even though it is sent to the proxy over unicast (see Section 8).¶

If the client uses the group mode with its own Sender Key to protect a unicast request to a group member, an on-path adversary can, right then or later on, redirect that request to one/many different group member(s) over unicast, or to the whole OSCORE group over multicast. By doing so, the adversary can induce the target group member(s) to perform actions intended for one group member only. Note that the adversary can be external, i.e., they do not need to also be a member of the OSCORE group.¶

This is due to the fact that the client is not able to indicate the single intended recipient in a way which is secure and possible to process for Group OSCORE on the server side. In particular, Group OSCORE does not protect network addressing information such as the IP address of the intended recipient server. It follows that the server(s) receiving the redirected request cannot assert whether that was the original intention of the client, and would thus simply assume so.¶

The impact of such an attack depends especially on the REST method of the request, i.e., the Inner CoAP Code of the OSCORE request message. In particular, safe methods such as GET and FETCH would trigger (several) unintended responses from the targeted server(s), while not resulting in destructive behavior. On the other hand, non safe methods such as PUT, POST, and PATCH/iPATCH would result in the target server(s) taking active actions on their resources and possible cyber-physical environment, with the risk of destructive consequences and possible implications for safety.¶

A client can instead use the pairwise mode as defined in Section 9.3, in order to protect a request sent to a single group member by using pairwise keying material (see Section 2.5). This prevents the attack discussed above by construction, as only the intended server is able to derive the pairwise keying material used by the client to protect the request. In a group where the AEAD Algorithm and Pairwise Key Agreement Algorithm are set in the Security Context, a client supporting the pairwise mode SHOULD use it to protect requests sent to a single group member over unicast. For an example where this is not fulfilled, see Section 9.2.1 of [I-D.ietf-core-observe-multicast-notifications].¶

The use of block-wise transfers [RFC7959] with group communication for CoAP is as discussed in Section 3.8 of [I-D.ietf-core-groupcomm-bis].¶

Additional considerations are discussed in Section 10, with respect to requests including a CoAP Echo Option [RFC9175] that have to be sent over unicast, as a challenge-response method for servers to achieve synchronization of clients' Sender Sequence Number.¶

13.10. End-to-end Protection

The same considerations from Section 12.1 of [RFC8613] hold for Group OSCORE.¶

Additionally, (D)TLS and Group OSCORE can be combined for protecting message exchanges occurring over unicast. However, it is not possible to combine (D)TLS and Group OSCORE for protecting message exchanges where messages are sent over multicast.¶

13.11. Master Secret

Group OSCORE derives the Security Context using the same construction used by OSCORE, and by using the Group Identifier of a group as the related ID Context. Hence, the same required properties of the Security Context parameters discussed in Section 3.3 of [RFC8613] hold for this document.¶

With particular reference to the OSCORE Master Secret, it has to be kept secret among the members of the respective OSCORE group and the Group Manager responsible for that group. Also, the Master Secret must have a good amount of randomness, and the Group Manager can generate it offline using a good random number generator. This includes the case where the Group Manager rekeys the group by generating and distributing a new Master Secret. Randomness requirements for security are described in [RFC4086].¶

13.12. Replay Protection

As in OSCORE [RFC8613], also Group OSCORE relies on Sender Sequence Numbers included in the COSE message field 'Partial IV' and used to build AEAD nonces.¶

Note that the Partial IV of an endpoint does not necessarily grow monotonically. For instance, upon exhaustion of the endpoint's Sender Sequence Number space, the endpoint's Partial IV space also gets exhausted. As discussed in Section 2.6.3, this results either in the endpoint being individually rekeyed and getting a new Sender ID, or in the establishment of a new Security Context in the group. Therefore, uniqueness of (key, nonce) pairs (see Section 13.3) is preserved also when a new Security Context is established.¶

Since one-to-many communication such as multicast usually involves unreliable transports, the simplification of the Replay Window to a size of 1 suggested in Section 7.4 of [RFC8613] is not viable with Group OSCORE, unless exchanges in the group rely only on unicast messages.¶

A server may not be synchronized with a client's Sender Sequence Number, or a Replay Window may be initialized as invalid (see Section 2.6.1). The server can either retrieve a new Group OSCORE Security Context, or synchronize with the clients' Sender Sequence Numbers before resuming to accept incoming messages from other group members.¶

13.13. Message Ordering

Assuming that the other endpoint is honest, Group OSCORE provides relative ordering of received messages. For a given Group OSCORE Security Context, the received Partial IV (when included) allows the recipient endpoint to determine the order in which requests or responses were sent by the other endpoint.¶

In case the Partial IV was omitted in a response, this indicates that it was the oldest response from the sender endpoint to the corresponding request (like notification responses in OSCORE, see Section 7.4.1 of [RFC8613]). A received response is not older than the corresponding request.¶

13.14. Message Freshness

As in OSCORE, Group OSCORE provides only the guarantee that the request is not older than the Group OSCORE Security Context used to protect it. Other aspects of freshness are discussed in Section 6.2.¶

The challenge-response approach described in Section 10 provides an assurance of freshness of the request without depending on the honesty of the client. However, it can result in an impact on performance which is undesirable or unbearable, especially in large groups where many endpoints at the same time might join as new members or lose synchronization.¶

Endpoints configured as silent servers are not able to perform the challenge-response described above, as they do not store a Sender Context to secure the 4.01 (Unauthorized) response to the client. Thus, silent servers should adopt alternative approaches to achieve and maintain synchronization with Sender Sequence Numbers of clients.¶

Since requests including the Echo Option are sent over unicast, a server can be victim of the attack discussed in Section 13.9, in case such requests are protected in group mode. Instead, protecting those requests with the pairwise mode prevents the attack above. In fact, only the very server involved in the challenge-response exchange is able to derive the pairwise key used by the client to protect the request including the Echo Option.¶

In either case, an internal on-path adversary would not be able to mix up the Echo Option value of two different unicast requests, sent by a same client to any two different servers in the group. In fact, even if the group mode was used, this would require the adversary to forge the countersignature of both requests. As a consequence, each of the two servers remains able to selectively accept a request with the Echo Option only if it is waiting for that exact integrity-protected Echo Option value, and is thus the intended recipient.¶

13.15. Client Aliveness

Like in OSCORE (see Section 12.5 of [RFC8613]), a server may verify the aliveness of the client by using the CoAP Echo Option [RFC9175] as described in Section 10.¶

In the interest of avoiding otherwise unnecessary uses of such an approach, the server can exploit the fact that the received request cannot be older than the Security Context used to protect it. This effectively allows the server to verify the client aliveness relative to the installation of the latest group keying material.¶

13.16. Cryptographic Considerations

The same considerations from Section 12.6 of [RFC8613] about the maximum Sender Sequence Number hold for Group OSCORE.¶

As discussed in Section 2.6.2, an endpoint that experiences an exhaustion of its own Sender Sequence Number space MUST NOT protect further messages including a Partial IV, until it has derived a new Sender Context. This prevents the endpoint to reuse the same AEAD nonce with the same Sender Key.¶

In order to renew its own Sender Context, the endpoint SHOULD inform the Group Manager, which can either renew the whole Security Context by means of group rekeying, or provide only that endpoint with a new Sender ID value. In either case, the endpoint derives a new Sender Context, and in particular a new Sender Key.¶

Additionally, the same considerations from Section 12.6 of [RFC8613] hold for Group OSCORE, about building the AEAD nonce and the secrecy of the Security Context parameters.¶

The group mode uses the "encrypt-then-sign" construction, i.e., the countersignature is computed over the COSE_Encrypt0 object (see Section 4.1). This is motivated by enabling signature checkers (see Section 3.3), which do not join a group as members but are allowed to verify countersignatures of messages protected in group mode without being able to decrypt them (see Section 8.5).¶

If the Group Encryption Algorithm used in group mode provides integrity protection, countersignatures of COSE_Encrypt0 with short authentication tags do not provide the security properties associated with the same algorithm used in COSE_Sign (see Section 6 of [RFC9338]). To provide 128-bit security against collision attacks, the tag length MUST be at least 256-bits. A countersignature of a COSE_Encrypt0 with AES-CCM-16-64-128 provides at most 32 bits of integrity protection.¶

The derivation of pairwise keys defined in Section 2.5.1 is compatible with ECDSA and EdDSA asymmetric keys, but is not compatible with RSA asymmetric keys.¶

For the public key translation from Ed25519 (Ed448) to X25519 (X448) specified in Section 2.5.1, variable time methods can be used since the translation operates on public information. Any byte string of appropriate length is accepted as a public key for X25519 (X448) in [RFC7748]. It is therefore not necessary for security to validate the translated public key (assuming the translation was successful).¶

The security of using the same key pair for Diffie-Hellman and for signing (by considering the ECDH procedure in Section 2.5 as a Key Encapsulation Mechanism (KEM)) is demonstrated in [Degabriele] and [Thormarker].¶

Applications using ECDH (except X25519 and X448) based KEM in Section 2.5 are assumed to verify that a peer endpoint's public key is on the expected curve and that the shared secret is not the point at infinity. The KEM in [Degabriele] checks that the shared secret is different from the point at infinity, as does the procedure in Section 5.7.1.2 of [NIST-800-56A] which is referenced in Section 2.5.¶

By extending Theorem 2 of [Degabriele], [Thormarker] shows that the same key pair can be used with X25519 and Ed25519 (X448 and Ed448) for the KEM specified in Section 2.5. By symmetry in the KEM used in this document, both endpoints can consider themselves to have the recipient role in the KEM - as discussed in Section 7 of [Thormarker] - and rely on the mentioned proofs for the security of their key pairs.¶

Theorem 3 in [Degabriele] shows that the same key pair can be used for an ECDH based KEM and ECDSA. The KEM uses a different KDF than in Section 2.5, but the proof only depends on that the KDF has certain required properties, which are the typical assumptions about HKDF, e.g., that output keys are pseudorandom. In order to comply with the assumptions of Theorem 3, received public keys MUST be successfully validated, see Section 5.6.2.3.4 of [NIST-800-56A]. The validation MAY be performed by a trusted Group Manager. For [Degabriele] to apply as it is written, public keys need to be in the expected subgroup. For this, we rely on cofactor Diffie-Hellman as per Section 5.7.1.2 of [NIST-800-56A], which is referenced in Section 2.5.1.¶

HashEdDSA variants of Ed25519 and Ed448 are not used by COSE (see Section 2.2 of [RFC9053]), and are not covered by the analysis in [Thormarker]. Hence, they MUST NOT be used with the public keys used to derive pairwise keys as specified in this document.¶

13.17. Message Segmentation

The same considerations from Section 12.7 of [RFC8613] hold for Group OSCORE.¶

13.18. Privacy Considerations

Group OSCORE ensures end-to-end integrity protection and encryption of the message payload and of all the options that are not used for proxy operations. In particular, options are processed according to the same class U/I/E that they have for OSCORE. Therefore, the same privacy considerations from Section 12.8 of [RFC8613] hold for Group OSCORE, with the following addition.¶

• When protecting a message in group mode, the countersignature is encrypted by using a keystream derived from the group keying material (see Section 5 and Section 5.1). This ensures group privacy. That is, an attacker cannot track an endpoint over two groups by linking messages between the two groups, unless being also a member of those groups.¶

Furthermore, the following privacy considerations hold about the OSCORE option, which may reveal information on the communicating endpoints.¶

• The 'kid' parameter, which is intended to help a recipient endpoint to find the right Recipient Context, may reveal information about the Sender Endpoint. When both a request and the corresponding responses include the 'kid' parameter, this may reveal information about both a client sending a request and all the possibly replying servers sending their own individual response.¶
• The 'kid context' parameter, which is intended to help a recipient endpoint to find the right Security Context, reveals information about the sender endpoint. In particular, it reveals that the sender endpoint is a member of a particular OSCORE group, whose current Group ID is indicated in the 'kid context' parameter.¶

When receiving a group request, each of the recipient endpoints can reply with a response that includes its Sender ID as 'kid' parameter. All these responses will be matchable with the request through the CoAP Token. Thus, even if these responses do not include a 'kid context' parameter, it becomes possible to understand that the responder endpoints are in the same group of the requester endpoint.¶

Furthermore, using the approach described in Section 10 to achieve synchronization with a client's Sender Sequence Number may reveal when a server device goes through a reboot. This can be mitigated by the server device storing the precise state of the Replay Window of each known client on a clean shutdown.¶

Finally, the approach described in Section 13.6 to prevent collisions of Group Identifiers from different Group Managers may reveal information about events in the respective OSCORE groups. In particular, a Group Identifier changes when the corresponding group is rekeyed. Thus, Group Managers might use the shared list of Group Identifiers to infer the rate and patterns of group membership changes triggering a group rekeying, e.g., due to newly joined members or evicted (compromised) members. In order to alleviate this privacy concern, it should be hidden from the Group Managers which exact Group Manager has currently assigned which Group Identifiers in its OSCORE groups.¶

14.1. OSCORE Flag Bits Registry

IANA is asked to add the following entry to the "OSCORE Flag Bits" registry within the "Constrained RESTful Environments (CoRE) Parameters" registry group.¶

+--------------+-------+-----------------------------+------------+
| Bit Position | Name  | Description                 | Reference  |
+--------------+-------+-----------------------------+------------+
|       2      | Group | For using a Group OSCORE    | [RFC-XXXX] |
|              | Flag  | Security Context, set to 1  |            |
|              |       | if the message is protected |            |
|              |       | with the group mode         |            |
+--------------+-------+-----------------------------+------------+

14.2. Target Attributes Registry

IANA is asked to add the following entry to the "Target Attributes" registry within the "Constrained RESTful Environments (CoRE) Parameters" registry group.¶

Attribute Name: gosc
Brief Description: Hint: resource only accessible
                   using Group OSCORE or OSCORE
Change Controller: IETF
Reference: [RFC-XXXX]

A.1. Assumptions

The following points are assumed to be already addressed and are out of the scope of this document.¶

• Multicast communication topology: this document considers both 1-to-N (one sender and multiple recipients) and M-to-N (multiple senders and multiple recipients) communication topologies. The 1-to-N communication topology is the simplest group communication scenario that would serve the needs of a typical Low-power and Lossy Network (LLN). Examples of use cases that benefit from secure group communication are provided in Appendix B.¶

  In a 1-to-N communication model, only a single client transmits data to the CoAP group, in the form of request messages; in an M-to-N communication model (where M and N do not necessarily have the same value), M clients transmit data to the CoAP group. According to [I-D.ietf-core-groupcomm-bis], any possible proxy entity is supposed to know about the clients. Also, every client expects and is able to handle multiple response messages associated with a same request sent to the CoAP group.¶

• Group size: security solutions for group communication should be able to adequately support different and possibly large security groups. The group size is the current number of members in a security group. In the use cases mentioned in this document, the number of clients (normally the controlling devices) is expected to be much smaller than the number of servers (i.e., the controlled devices). A security solution for group communication that supports 1 to 50 clients would be able to properly cover the group sizes required for most use cases that are relevant for this document. The maximum group size is expected to be in the range of hundreds to thousands of devices, with large groups easier to manage if including several silent servers. Security groups larger than that should be divided into smaller independent groups. One should not assume that the set of members of a security group remains fixed. That is, the group membership is subject to changes, possibly on a frequent basis.¶
• Communication with the Group Manager: an endpoint must use a secure dedicated channel when communicating with the Group Manager, also when not registered as a member of the security group.¶
• Provisioning and management of Security Contexts: a Security Context must be established among the members of the security group. A secure mechanism must be used to generate, revoke and (re-)distribute keying material, communication policies and security parameters in the security group. The actual provisioning and management of the Security Context is out of the scope of this document.¶
• Multicast data security cipher suite: all members of a security group must use the same cipher suite to provide authenticity, integrity and confidentiality of messages in the group. The cipher suite is specified as part of the Security Context.¶
• Ensuring backward security: a new device joining the security group should not have access to any old Security Contexts used before its joining. This ensures that a new member of the security group is not able to decrypt confidential data sent before it has joined the security group. The adopted key management scheme should ensure that the Security Context is updated to ensure backward confidentiality. The actual mechanism to update the Security Context and renew the group keying material in the security group upon a new member's joining has to be defined as part of the group key management scheme.¶
• Ensuring forward security: entities that leave the security group should not have access to any future Security Contexts or message exchanged within the security group after their leaving. This ensures that a former member of the security group is not able to decrypt confidential data sent within the security group anymore. Also, it ensures that a former member is not able to send protected messages to the security group anymore. The actual mechanism to update the Security Context and renew the group keying material in the security group upon a member's leaving has to be defined as part of the group key management scheme.¶

A.2. Security Objectives

The protocol described in this document aims at fulfilling the following security objectives:¶

• Data replay protection: request messages or response messages replayed within the security group must be detected.¶
• Data confidentiality: messages sent within the security group shall be encrypted.¶
• Group-level data confidentiality: the group mode provides group-level data confidentiality since messages are encrypted at a group level, i.e., in such a way that they can be decrypted by any member of the security group, but not by an external adversary or other external entities.¶
• Pairwise data confidentiality: the pairwise mode especially provides pairwise data confidentiality, since messages are encrypted using pairwise keying material shared between any two group members, hence they can be decrypted only by the intended single recipient.¶
• Source message authentication: messages sent within the security group shall be authenticated. That is, it is essential to ensure that a message is originated by a member of the security group in the first place, and in particular by a specific, identifiable member of the security group.¶
• Message integrity: messages sent within the security group shall be integrity protected. That is, it is essential to ensure that a message has not been tampered with, either by a group member, or by an external adversary or other external entities which are not members of the security group.¶
• Message ordering: it must be possible to determine the ordering of messages coming from a single sender. Like in OSCORE [RFC8613], a recipient endpoint can determine the relative order of requests or responses from another sender endpoint by means of their Partial IV. It is not required to determine ordering of messages from different senders.¶

D.1. Version -20 to -21

• Updated author list.¶
• Terminology: improved definition of "group request".¶
• Editorial: removed quotation marks when using the CBOR simple values true, false, and null.¶
• Editorial: expanded name of the "CoRE Parameters" registry group.¶

D.2. Version -19 to -20

• Change Controller for the target attribute "gosc" set to "IETF".¶

D.3. Version -18 to -19

• Unified presentation of handling of multiple responses.¶
• Added Rikard Hoeglund as Contributor.¶

D.4. Version -17 to -18

• Changed document title.¶
• Possible use with CoAP-mappable HTTP.¶
• Added Common Context parameter "Authentication Credential Format".¶
• Renamed "Group Encryption Key" to "Signature Encryption Key". Consistent fixes in its derivation.¶
• Renamed "Signature Encryption Algorithm" to "Group Encryption Algorithm".¶
• Ensured a single Common IV, also when the two encryption algorithms have different AEAD nonce sizes.¶
• Guidelines on the Pairwise Key Agreement Algorithm and derivation of the Diffie-Hellman secret.¶
• The possible use of a mode follows from the set parameters.¶
• The Group Manager is always present; 'gm_cred' in the external_aad cannot be null anymore.¶
• The authentication credential of the Group Manager can have a different format than that of the group members'.¶
• Set-up of new endpoints moved to document body.¶
• The encrypted countersignature is a result of the header compression, not of COSE.¶
• Revised examples of compressed and non-compressed COSE object.¶
• Removed excessive requirements on group rekeying scheduling.¶
• More considerations on the strictness of group key management.¶
• Clearer alternatives on retaining an old Security Context.¶
• Revised used of terminology on freshness.¶
• Clarifications, fixes and editorial improvements.¶

D.5. Version -16 to -17

• Definition and registration of the target attribute "gosc".¶
• Reference update and editorial fixes.¶

D.6. Version -15 to -16

• Clients "SHOULD" use the group mode for one-to-many requests.¶
• Handling of multiple non-notification responses.¶
• Revised presentation of security properties.¶
• Improved listing of operations defined for the group mode that are inherited by the pairwise mode.¶
• Editorial improvements.¶

D.7. Version -14 to -15

• Updated references and editorial fixes.¶

D.8. Version -13 to -14

• Replaced "node" with "endpoint" where appropriate.¶
• Replaced "owning" with "storing" (of keying material).¶
• Distinction between "authentication credential" and "public key".¶
• Considerations on storing whole authentication credentials.¶
• Considerations on Denial of Service.¶
• Recycling of Group IDs by tracking the "Birth Gid" of each group member is now optional to support and use for the Group Manager.¶
• Fine-grained suppression of error responses.¶
• Changed section title "Mandatory-to-Implement Compliance Requirements" to "Implementation Compliance".¶
• "Challenge-Response Synchronization" moved to the document body.¶
• RFC 7641 and draft-ietf-core-echo-request-tag as normative references.¶
• Clarifications and editorial improvements.¶

D.9. Version -12 to -13

• Fixes in the derivation of the Group Encryption Key.¶
• Added Mandatory-to-Implement compliance requirements.¶
• Changed UCCS to CCS.¶

D.10. Version -11 to -12

• No mode of operation is mandatory to support.¶
• Revised parameters of the Security Context, COSE object and external_aad.¶
• Revised management of keying material for the Group Manager.¶
• Informing of former members when rekeying the group.¶
• Admit encryption-only algorithms in group mode.¶
• Encrypted countersignature through a keystream.¶
• Added public key of the Group Manager as key material and protected data.¶
• Clarifications about message processing, especially notifications.¶
• Guidance for message processing of external signature checkers.¶
• Updated derivation of pairwise keys, with more security considerations.¶
• Termination of ongoing observations as client, upon leaving or before re-joining the group.¶
• Recycling Group IDs by tracking the "Birth Gid" of each group member.¶
• Expanded security and privacy considerations about the group mode.¶
• Removed appendices on skipping signature verification and on COSE capabilities.¶
• Fixes and editorial improvements.¶

D.11. Version -10 to -11

• Loss of Recipient Contexts due to their overflow.¶
• Added diagram on keying material components and their relation.¶
• Distinction between anti-replay and freshness.¶
• Preservation of Sender IDs over rekeying.¶
• Clearer cause-effect about reset of SSN.¶
• The GM provides public keys of group members with associated Sender IDs.¶
• Removed 'par_countersign_key' from the external_aad.¶
• One single format for the external_aad, both for encryption and signing.¶
• Presence of 'kid' in responses to requests protected in pairwise mode.¶
• Inclusion of 'kid_context' in notifications following a group rekeying.¶
• Pairwise mode presented with OSCORE as baseline.¶
• Revised examples with signature values.¶
• Decoupled growth of clients' Sender Sequence Numbers and loss of synchronization for server.¶
• Sender IDs not recycled in the group under the same Gid.¶
• Processing and description of the Group Flag bit in the OSCORE option.¶
• Usage of the pairwise mode for multicast requests.¶
• Clarifications on synchronization using the Echo option.¶
• General format of context parameters and external_aad elements, supporting future registered COSE algorithms (new Appendix).¶
• Fixes and editorial improvements.¶

D.12. Version -09 to -10

• Removed 'Counter Signature Key Parameters' from the Common Context.¶
• New parameters in the Common Context covering the DH secret derivation.¶
• New countersignature header parameter from draft-ietf-cose-countersign.¶
• Stronger policies non non-recycling of Sender IDs and Gid.¶
• The Sender Sequence Number is reset when establishing a new Security Context.¶
• Added 'request_kid_context' in the aad_array.¶
• The server can respond with 5.03 if the client's public key is not available.¶
• The observer client stores an invariant identifier of the group.¶
• Relaxed storing of original 'kid' for observer clients.¶
• Both client and server store the 'kid_context' of the original observation request.¶
• The server uses a fresh PIV if protecting the response with a Security Context different from the one used to protect the request.¶
• Clarifications on MTI algorithms and curves.¶
• Removed optimized requests.¶
• Overall clarifications and editorial revision.¶

D.13. Version -08 to -09

• Pairwise keys are discarded after group rekeying.¶
• Signature mode renamed to group mode.¶
• The parameters for countersignatures use the updated COSE registries. Newly defined IANA registries have been removed.¶
• Pairwise Flag bit renamed as Group Flag bit, set to 1 in group mode and set to 0 in pairwise mode.¶
• Dedicated section on updating the Security Context.¶
• By default, sender sequence numbers and replay windows are not reset upon group rekeying.¶
• An endpoint implementing only a silent server does not support the pairwise mode.¶
• Separate section on general message reception.¶
• Pairwise mode moved to the document body.¶
• Considerations on using the pairwise mode in non-multicast settings.¶
• Optimized requests are moved as an appendix.¶
• Normative support for the signature and pairwise mode.¶
• Revised methods for synchronization with clients' sender sequence number.¶
• Appendix with example values of parameters for countersignatures.¶
• Clarifications and editorial improvements.¶

D.14. Version -07 to -08

• Clarified relation between pairwise mode and group communication (Section 1).¶
• Improved definition of "silent server" (Section 1.1).¶
• Clarified when a Recipient Context is needed (Section 2).¶
• Signature checkers as entities supported by the Group Manager (Section 2.3).¶
• Clarified that the Group Manager is under exclusive control of Gid and Sender ID values in a group, with Sender ID values under each Gid value (Section 2.3).¶
• Mitigation policies in case of recycled 'kid' values (Section 2.4).¶
• More generic exhaustion (not necessarily wrap-around) of sender sequence numbers (Sections 2.5 and 10.11).¶
• Pairwise key considerations, as to group rekeying and Sender Sequence Numbers (Section 3).¶
• Added reference to static-static Diffie-Hellman shared secret (Section 3).¶
• Note for implementation about the external_aad for signing (Sectino 4.3.2).¶
• Retransmission by the application for group requests over multicast as Non-confirmable (Section 7).¶
• A server MUST use its own Partial IV in a response, if protecting it with a different context than the one used for the request (Section 7.3).¶
• Security considerations: encryption of pairwise mode as alternative to group-level security (Section 10.1).¶
• Security considerations: added approach to reduce the chance of global collisions of Gid values from different Group Managers (Section 10.5).¶
• Security considerations: added implications for block-wise transfers when using the signature mode for requests over unicast (Section 10.7).¶
• Security considerations: (multiple) supported signature algorithms (Section 10.13).¶
• Security considerations: added privacy considerations on the approach for reducing global collisions of Gid values (Section 10.15).¶
• Updates to the methods for synchronizing with clients' sequence number (Appendix E).¶
• Simplified text on discovery services supporting the pairwise mode (Appendix G.1).¶
• Editorial improvements.¶

D.15. Version -06 to -07

• Updated abstract and introduction.¶
• Clarifications of what pertains a group rekeying.¶
• Derivation of pairwise keying material.¶
• Content re-organization for COSE Object and OSCORE header compression.¶
• Defined the Pairwise Flag bit for the OSCORE option.¶
• Supporting CoAP Observe for group requests and responses.¶
• Considerations on message protection across switching to new keying material.¶
• New optimized mode based on pairwise keying material.¶
• More considerations on replay protection and Security Contexts upon key renewal.¶
• Security considerations on Group OSCORE for unicast requests, also as affecting the usage of the Echo option.¶
• Clarification on different types of groups considered (application/security/CoAP).¶
• New pairwise mode, using pairwise keying material for both requests and responses.¶

D.16. Version -05 to -06

• Group IDs mandated to be unique under the same Group Manager.¶
• Clarifications on parameter update upon group rekeying.¶
• Updated external_aad structures.¶
• Dynamic derivation of Recipient Contexts made optional and application specific.¶
• Optional 4.00 response for failed signature verification on the server.¶
• Removed client handling of duplicated responses to multicast requests.¶
• Additional considerations on public key retrieval and group rekeying.¶
• Added Group Manager responsibility on validating public keys.¶
• Updates IANA registries.¶
• Reference to RFC 8613.¶
• Editorial improvements.¶

D.17. Version -04 to -05

• Added references to draft-dijk-core-groupcomm-bis.¶
• New parameter Counter Signature Key Parameters (Section 2).¶
• Clarification about Recipient Contexts (Section 2).¶
• Two different external_aad for encrypting and signing (Section 3.1).¶
• Updated response verification to handle Observe notifications (Section 6.4).¶
• Extended Security Considerations (Section 8).¶
• New "Counter Signature Key Parameters" IANA Registry (Section 9.2).¶

D.18. Version -03 to -04

• Added the new "Counter Signature Parameters" in the Common Context (see Section 2).¶
• Added recommendation on using "deterministic ECDSA" if ECDSA is used as countersignature algorithm (see Section 2).¶
• Clarified possible asynchronous retrieval of keying material from the Group Manager, in order to process incoming messages (see Section 2).¶
• Structured Section 3 into subsections.¶
• Added the new 'par_countersign' to the aad_array of the external_aad (see Section 3.1).¶
• Clarified non reliability of 'kid' as identity identifier for a group member (see Section 2.1).¶
• Described possible provisioning of new Sender ID in case of Partial IV wrap-around (see Section 2.2).¶
• The former signature bit in the Flag Byte of the OSCORE option value is reverted to reserved (see Section 4.1).¶
• Updated examples of compressed COSE object, now with the sixth less significant bit in the Flag Byte of the OSCORE option value set to 0 (see Section 4.3).¶
• Relaxed statements on sending error messages (see Section 6).¶
• Added explicit step on computing the countersignature for outgoing messages (see Sections 6.1 and 6.3).¶
• Handling of just created Recipient Contexts in case of unsuccessful message verification (see Sections 6.2 and 6.4).¶
• Handling of replied/repeated responses on the client (see Section 6.4).¶
• New IANA Registry "Counter Signature Parameters" (see Section 9.1).¶

D.19. Version -02 to -03

• Revised structure and phrasing for improved readability and better alignment with draft-ietf-core-object-security.¶
• Added discussion on wrap-Around of Partial IVs (see Section 2.2).¶
• Separate sections for the COSE Object (Section 3) and the OSCORE Header Compression (Section 4).¶
• The countersignature is now appended to the encrypted payload of the OSCORE message, rather than included in the OSCORE Option (see Section 4).¶
• Extended scope of Section 5, now titled " Message Binding, Sequence Numbers, Freshness and Replay Protection".¶
• Clarifications about Non-confirmable messages in Section 5.1 "Synchronization of Sender Sequence Numbers".¶
• Clarifications about error handling in Section 6 "Message Processing".¶
• Compacted list of responsibilities of the Group Manager in Section 7.¶
• Revised and extended security considerations in Section 8.¶
• Added IANA considerations for the OSCORE Flag Bits Registry in Section 9.¶
• Revised Appendix D, now giving a short high-level description of a new endpoint set-up.¶

D.20. Version -01 to -02

• Terminology has been made more aligned with RFC7252 and draft-ietf-core-object-security: i) "client" and "server" replace the old "multicaster" and "listener", respectively; ii) "silent server" replaces the old "pure listener".¶
• Section 2 has been updated to have the Group Identifier stored in the 'ID Context' parameter defined in draft-ietf-core-object-security.¶
• Section 3 has been updated with the new format of the Additional Authenticated Data.¶
• Major rewriting of Section 4 to better highlight the differences with the message processing in draft-ietf-core-object-security.¶
• Added Sections 7.2 and 7.3 discussing security considerations about uniqueness of (key, nonce) and collision of group identifiers, respectively.¶
• Minor updates to Appendix A.1 about assumptions on multicast communication topology and group size.¶
• Updated Appendix C on format of group identifiers, with practical implications of possible collisions of group identifiers.¶
• Updated Appendix D.2, adding a pointer to draft-palombini-ace-key-groupcomm about retrieval of nodes' public keys through the Group Manager.¶
• Minor updates to Appendix E.3 about Challenge-Response synchronization of sequence numbers based on the Echo option from draft-ietf-core-echo-request-tag.¶

D.21. Version -00 to -01

• Section 1.1 has been updated with the definition of group as "security group".¶

• Section 2 has been updated with:¶

  • Clarifications on establishment/derivation of Security Contexts.¶
  • A table summarizing the the additional context elements compared to OSCORE.¶

• Section 3 has been updated with:¶

  • Examples of request and response messages.¶
  • Use of CounterSignature0 rather than CounterSignature.¶
  • Additional Authenticated Data including also the signature algorithm, while not including the Group Identifier any longer.¶
• Added Section 6, listing the responsibilities of the Group Manager.¶
• Added Appendix A (former section), including assumptions and security objectives.¶
• Appendix B has been updated with more details on the use cases.¶
• Added Appendix C, providing an example of Group Identifier format.¶
• Appendix D has been updated to be aligned with draft-palombini-ace-key-groupcomm.¶