Subject: Re: Encoding of enhanced content types in CMS

From: "Housley, Russ" <[email-masked]>
Date: Fri, 21 Dec 2001 14:17:39 -0500

In CMS, we anticipate the encapsulation of one content type with 
another.  In SMIME, there is in interposed MIME heading, so this capability 
is not used.  In other environments, CMS content types are directly 
encapsulated (see draft-ietf-smime-x400wrap-03.txt).

Russ

At 07:36 PM 12/21/2001 +0100, [email-masked] wrote:

>A major difference I find between CMS and PKCS#7 (from which CMS is
>derived) is the fact that in PKCS#7 it is well defined how to encode
>enhanced types to be used as content for another enhanced type.
>
>" Content types in the enhanced class contain content of some type
>(possibly encrypted), and other cryptographic enhancements. For example,
>enveloped-data content can contain (encrypted) signed-data content, which
>can contain data content. "
>
>Specifically, in Section "7.General Sintax", Note 2:
>
>" When a ContentInfo value is the inner content of signed-data,
>signed-and-enveloped-data, or digested-data content, a message-digest
>algorithm is applied to the contents octets of the DER encoding of the
>content field. When a ContentInfo value is the inner content of
>enveloped-data or signed-and-enveloped-data content, a content-encryption
>algorithm is applied to the contents octets of a definite-length BER
>encoding of the content field. "
>
>On the other hand. CMS does not define any encoding rules at all. The new
>draft of the CMS points out the question of compatibility with PKCS#7
>(section 5.2.1) with the inclusion or not of the tag and lenght octets in
>the encoding of a SEQUENCE in the encapContentInfo eContent field, but it
>eludes again the matter of which encoding rules should be used in CMS.
>Does not it implies that incompatibilities may arise between different
>implementations of CMS when the content processed (digested or encrypted)
>was other than Data? Suppose I receive a CMS EnvelopedData type, and the
>encryptedContentInfo contentType is SignedData. After the decryption
>process, how am I supposed to decode the resultant OCTET STRING? With
>PKCS#7 I knew I had to use definite-length BER, but now?
>I would like to receive some information or comments on this matter.
>
>Luciano Medina

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