Subject: [ietf-dkim] prviate review of SSP summary description draft

From: Dave Crocker <[email-masked]>
Date: Thu, 13 Dec 2007 12:59:11 -0800

Folks,

The following is a revised version of the draft SSP Summary. It factors in the 
comments that were sent for the previous version.

I'd appreciate suggestions for specific changes.

d/

SSP Summary Description  -- DRAFT
=======================

The IETF's DKIM working group has followed its specification of a base method
for associating a responsible identity to an email, via cryptographic signing,
with a draft, titled DKIM Sender Signing Practices (SSP).  The SSP
specification describes itself as defining a mechanism "senders may use to
advertise how they sign their outgoing mail, and how verifiers should access
and interpret those results." That is, SSP permits potential DKIM signers to
publish statements about how they use DKIM, and also to publish directions for
DKIM validators (receivers) on how they are to handle a class of received
messages.

The SSP mechanism permits a potential signer -- that is, the owner of a domain
name -- to publish an SSP-specific DNS record -- a TXT record in an
SSP-specific branch under the domain name.  On the receive-side, the domain
name under which the DNS query is made is taken from the author's mailbox
address -- the rfc2822.From <addr-spec> portion of an address -- in a received
message.

By associating an organization's verifiable identity to a message, the
reputation of that organization can then be used by a message-receiving
engine, for determining message handling, such as whether to deliver the
message to the designated recipient.  This is what DKIM Base permits.

By contrast, SSP seeks to detect problematic uses of a domain name,
specifically related to use of the email address in a message's RFC2822.From
field <addr-spec>, where the use cannot be validated as acceptable.

SSP does not seek to deal with other identity fraud, such as in the 
human-readable RFC2822.From <display-name>, the Subject field, or in the
message body, or any use of "cousin" domains that can be confused with a
target domain. Unauthorized or intentionally confusing domain names and 
addresses -- or other deceptive text -- can appear in any of these places and
will not be prevented or detected by use of SSP.

SSP is motivated by a desire on the part of message senders, to inform message
recipients about constraints on the senders' practices.  The premise is that
receivers with this additional information will be able to detect, and
possibly reject, a class of mail that cannot be verifies as legitimate. At
best, the mechanism is approximate, in that a legitimate message might begin 
with a legitimate signature, but then have the signature get broken during 
transit. When SSP is used, such messages will be treated by the recipient as 
suspect.

The current SSP draft provides for two basic conditions which will trigger a
query:

    1. Unsigned message.  When a receiver gets a message that has no DKIM
signature, they can query the DNS for an SSP record that is associated with
the domain name in the (first) rfc2822.From field header mailbox address.

    2. Signed message.  When a receiver gets a message that is signed, but
which with the identity on behalf of which this message is signed -- the
signature's i= parameter -- as different from the (first) address in the From
field, then perform the SSP query, described in step 1.

The publisher of an SSP record can say that:

    1. All mail that they send is signed by them

    2. All mail that they send is signed by them and they do not send mail via
intermediaries -- called "third parties" -- such as mailing lists that might
modify and re-sign the message.

Messages that fail an SSP analysis are treated as exceptions. The publisher of
an SSP record may request that exceptional mail be treated to:

    1. Further consideration, where the suspicious status is only one factor in
determining handling.

    2. Rejected.

SSP also permits the publisher to declare that the record applies to all of
its sub-domains, although there is a DNS limitation on reconciling deeply
nested sub-domains with this record.

The SSP specification defines a 10-step "check procedure" that is a decision
tree for performing SSP analysis.

As an example of implications, the SSP rejection semantics would mean that a
confirming site would reject a message that has a broken author signature,
even if it still had a valid signature by an operator with a good reputation.

Given that adoption of a new mechanism, like DKIM's base signing, takes many
years, adoption by any random sender/receiver pair is unlikely for many years,
absent prior arrangement.  So most publishers of SSP records will be sending
to sites that are not checking them.  Equally it should be assumed that
receivers will almost always obtain a failed SSP DNS query, for every message
with a new (un-cached) domain name in the From field.

d/
-- 

   Dave Crocker
   Brandenburg InternetWorking
   bbiw.net

-- 

   Dave Crocker
   Brandenburg InternetWorking
   bbiw.net
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