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Re: Only accepting e-mail from known parties


	Erik:

On Mon, 25 Dec 1995, Eric Murray wrote:

> > On Mon, 25 Dec 1995, Dr. Dimitri Vulis wrote:

> Ok.  If I want to get my email ad for the Ronco turnip-twaddler past a filter
> like that, all I need to do is to create a PGP key with
> a user name that's the same as one that the victim already
> receives.
> 
> i.e. if I know that [email protected] exchanges email with [email protected], then
> I just create a PGP key with the name "[email protected]", and sign
> the turnip-twaddler ad with that.  It'd have a valid signature, and
> one coming from Joe's friend phred.   Mail accepted.

	But will the signature match that of [email protected]'s PGP
	key.  I doubt it.

> In addition to checking for a valid signature, the filtering software
> would have to also check the PGP key id of the key used.  It would

	To check a signature, you need the public key the signature 
	was created with.  You allready have [email protected]'s public
	key on your keyring.  If that key does not demonstrate an
	authentic signature for the messge, then the message is 
	a fake.  

	Now, if you assume that your keyring has been compromised,
	then you can also check the signatures of who signed the
	keys.  At a minimu, your signature should be on the authentic
	key.  If it is missing, then you can place the message in
	a "suspected to be forged bin", or just send it to dev/null,
	unread. 

> also need to make sure that there is ONLY PGP-signed content in the
> mail.  Otherwise Mallet could grab an innocuous mail message that

	I hadn't thought of that, but here is one solution.

	Run a perl script that automatically deletes everything 
	that is not signed by pgp, with the exception of the date,
	the sender, and the subject line.  

> I'm sure there's other caveats, these are just the ones I can think of now.

	Let's figure out some more threat models.  And how to counter
	them.  

	Man in the middle --- he has your public key, [email protected]'s
	public key, and access to both your pbulic ring, and 
	[email protected] public ring.   I don't know know how to counter
	this one using filters with perl --- yet.

        xan

        jonathon
        [email protected]


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