Re: Mandatory email verification

[Adam Shostack <adam@bwh.harvard.edu>]

DC Williams writes:

| Is anyone aware of a way to modify sendmail to require a verified digital
| signature for all mail sent? This subject came up after a discussion
| of the possible repercussions of forged email through port 25.
| Even a good PGP user can't use manual encryption on a message-by-
| message basis as a defense for false mail attribution. For example,
| someone forges a defamatory message and signs your name to it. The
| recipient brings it to public attention and you try to claim "it doesn't
| have my PGP sig, therefore I didn't send it". The obvious counter is that
| you purposely failed to sign it to preserve your plausible deniability.
| The only way this would work is if the system you're on won't accept
| mail unless accompanied by a digital signature, which would allow the
| user to claim innocence if it wasn't his sig. The mailer would also have
| to check the sig to ensure that it belongs to an authorized user on the
| system to prevent people from creating one-time keys just to appease the
| mailer and prevent their real sig from being used. Running this version
| of "SIGmail" (<-- note flashy new marketing name) on your system would
| seem to be a reasonable defense against claims of false attribution.
| 
| Has anyone done any work along these lines? Is there an obvious fault
| with a system which would operate in this manner?

	Design areas to be worked out:
	Will the system drop such mail silently, or return it to the
sender?  Will the messages returned to sender be signed by the mail
system?  If so, will they contain any reference to the message sent?
How will you protect the keys used for signing?  If the 'bounce'
messages aren't signed, a great way to generate flamage would be to
send messages to the user claiming that his recent mail was not
properly signed, causing him to send another copy, annoying the hell
out of all the recipeints.

	I'd like to close this message by saying that mandatory
signing is not a good idea.  People will generate a low security key,
and leave it totally unsecured.  The way most folks with a clue deal
with forged mail is they see the writing style is different, the
person is advocating a new & different position, or the mail is just
random flammage.  Most folks regularly disregard this sort of thing as
children playing with a new toy.  Requiring the use of signatures for
all mail is silly.

Adam