On Oct 31, 2005, at 2:24 PM, Dave Crocker wrote:
I didn't see anything in the
spec about verifying that the arbitrary text matches the purported
From
address. Is this correct? Perhaps this could be addressed as a
possible threat in the analysis?
SSP deals with matching the From to the DKIM identity. Did you
have any
other matching in mind?
Although many wish to attribute an ability to directly relate the
From header with the DKIM signing-domain as a means to abate abuse,
this is a foolish quest. There is a prevalent use of "pretty-names"
by MUAs, use of different character-sets, look-alike domains,
convincing sub-domains, mixed together with many newly registered
domains. The abuse problem can not be confronted without the use of
reputation assessments on some identity.
A goal of DKIM should be to ensure the identity assessed for
reputation reflects the administrator of the system, the signing-
domain. In the past, there have been "authorization" mechanisms
shifting the burden onto the email-address instead. SSP is another
such "authorization" mechanism, especially when plans for a "third-
party" signer list is considered. The unfortunate affect of using
the email-address to assess reputation is this precludes the use of
independent signing-domains.
This loss of independent signatures will mean that email-addresses
become tied to the provider, and third-party services are forfeit.
This is done in the guise that "authorization" is a means to control
the use of a domain. In reality, the signature indicates how the
domain is used. This is an attempt to shift the burden onto the
hapless email-domain owners.
Using an indirect method to associate the signing-domain with an
email-address will offer better protections against all types of
spoofing, including phishing. Opaque-identifiers for example, will
prohibit the use of reputation against the email-address as a mean to
ensure that allowing and using independent signing-domains do not
become problematic for the email-address domain owner. As it
happens, this approach also deals directly with compromised systems,
and replay abuse.
With the indirect method where the signing-domain and the email-
address are considered independent identities, this ensures that DKIM
can be deployed without creating a great deal of havoc. With the
DKIM signature in place, the sending domains are better protected
from routing exploits. The MUA/MTA can also utilize opportunistic
security techniques that will prove superior to any attempts at
direct header relationships.
-Doug
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