[ietf-dkim] Attempted text for x=
hsantos at santronics.com
Wed Apr 19 11:15:58 PDT 2006
3.5 The DKIM-Signature header field
x= Signature Expiration (plain-text; RECOMMENDED, default is no
expiration). Signature expiration in seconds-since-1970 format
as an absolute date, not as a time delta from the signing
timestamp. The value is expressed as an unsigned integer
in decimal ASCII, with the same constraints on the value in
the "t=" tag.
The expiration is a DKIM signature descriptor exposing the
responsible domain intent to expire or revoke a DKIM
signature validity. Although, in practice, this control might
be viewed as a domain key management and security concept,
the domain's true intent and purpose is out of the scope.
If present, the expiration time MUST be compared against
the Transaction Reception Time (TRT).
Extracting the TRT depends on where the DKIM verifier
is implemented. For transport (SMTP) level verifier
operations, the TRT is the current system time. For non-SMTP
verifier operations, the TRT is the time extracted from
the 2822.Received: header as required to be added by the
transport SMTP receiver.
To satisfy step 2 in section 6.2, verifiers MAY choose to
perform a DKIM verification before the expiration
time is checked. This will help on deciding between
a permanent or temporary response code.
sig-x-tag = %x78 [FWS] "=" [FWS] 1*12DIGIT
6.2 Get the Public Key
[Editor note: Renumbered the list]
1. If signature has an expiration (x=) tag, check if the signature
has expired. Signatures MUST NOT be considered valid if the
current time at the verifier is past the expiration date.
Verifiers MAY choose to check key selector for revocation or
DNS removal. See Step 3.
2. Retrieve the public key as described in (Section 3.6) using the
domain from the "d=" tag and the selector from the "s=" tag.
3. If the query for the public key fails to respond and the
signature has not expired (in step 1), the verifier SHOULD
defer acceptance of this email (normally this will be
achieved with a 451/4.7.5 SMTP reply code).
If the query for the public key fails to respond and the
signature has expired (in step 1) the verifier SHOULD
reject the transaction of this email (normally this will be
achieved with a 551/5.7.5 SMTP reply code).
Editor Change Checklist:
[X] Consolidate Formatting/Type text
[X] Domain signer's reasons for expiration tag is out of scope.
[X] Change Current Time to "Transaction Reception Time" to satisfy
the two types of SMTP operations - SMTP plug-ins verifiers
versus post SMTP acceptance Verifiers. To use the "Current
Time" for post verifiers would be technically incorrect which
could promote premature expirations.
[X] Satisfy Step 2 in Section 6.2 (now step 3).
[X] Allow for optimization consideration (No Verification Required).
[X] Remove Information Note about replay attacks. Considered or not,
DKIM can't control what the VERIFIER will experience and the
knowledge gain to address such issues.
Hector Santos, Santronics Software, Inc.
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