Only accepting e-mail from known parties

[Jonathan Blake]

	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 joe at blort.com exchanges email with phred at none.net, then
> I just create a PGP key with the name "phred at none.net", 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 phred at none.net'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 phred at none.net'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, joe at none.net's
	public key, and access to both your pbulic ring, and 
	joe at none.net public ring.   I don't know know how to counter
	this one using filters with perl --- yet.

        xan

        jonathon
        grafolog at netcom.com


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