Folks,
Here is a revised version of the SSP Summary Description, based on comments
received:
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.
The new specification is for 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 misbehaviors, specifically related to
unauthorized use of the email address in a message's RFC2822.From field
<addr-spec>. 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.
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 is not legitimate. At best, the
mechanism is approximate, in that a legitimate message might begin with a
legitimate signature that becomes broken during transit. When SSP is used,
such messages will be treated by the recipient as exceptions.
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, and
lists one identity in the signature's i= parameter -- the identity on behalf
of which the message is signed -- but another in 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 may
modify and re-sign the message.
Messages that fail an SSP analysis are handled as exceptions. The publisher of
an SSP record may request that such mail be treated to:
1. Further consideration, where the exceptional 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
SSP-conformant receiving 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. SSP is likely to have a number of other interactions with email
handling practice.
Given that adoption of a new mechanism like DKIM's base signing takes many
years, adoption by any arbitrary 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
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