🚀 go-pugleaf

>>>>I posted a question on comp.unix.questions about how to get a shell by email,
>>>>that is a shell that would execute what's in the body of an email message.
>>>>All the details have been worked out except for one thing: security.
>>
>>>How about this:
>>
>>>Include in the header a string in both plaintext and encrypted form.  The
>>>receiving system would decrypt the encrypted form and compare it to the
>>>plaintext.
>>
>>>One problem with this is that it's quite amenable to known-plaintext
>>>attacks.  And it also requires a decryption key to be stored on the
>>>destination system, unless you use a public-key encryption system.
>>
>>Such a system would be easily spoofed by a "replay" attack.  If I ever get
>>hold of one of the messages, I just take the header (which has a valid key
>>on it) attach a different shell script, and send it!  Bang!
>
>How about encode a sequence number, and disallow repeated sequence numbers?
>
>The replay attack goes away.  However, a sysop on the route your message
>travels could copy your message, replace the commands, and send his message
>*before* your message.  Significantly harder, but it's not A1 security.

The right thing to do would be to use PGP to build a system that
required that the commands be send public key encrypted and signed. As
you noted, a sequence number is needed to stop replay attacks -- that
should be included in the plaintext. The only thing that an interloper
can do under those circumstances is stop your message from getting
through -- he can not see you message, replay it, or forge it.

--
Perry Metzger		pmetzger@shearson.com
--
		  Just say "NO!" to death and taxes.
			 Extropian and Proud.