Thread View: sci.crypt
5 messages
5 total messages
Started by KAHLER PAUL H.
Fri, 02 Oct 1992 20:33
Message interception
Author: KAHLER PAUL H.
Date: Fri, 02 Oct 1992 20:33
Date: Fri, 02 Oct 1992 20:33
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I have a new idea on how someone (NSA for example) could intercept and read PGP (maybe other) encrypted messages. This is simple based on how PGP works. To refresh your memory, PGP doesn't actually use RSA, instead, the RSA key is used to encrypt a second random key. It is this second key which is used to encrypt the message (actual RSA would be very slow). Now suppose they (whoever THEY are) do the following: 1) Modify PGP so that it only generates a small number of secondary keys. Since the user is unaware of what key is used, he/she would never know. 2) Make sure this version of the program is widely distributed, or even get it to the ftp sites. 3) Messages can now be intercepted (copied) at any point along the way and decoded by simply trying all of the secondary keys (not many). i.e. skip the RSA decryption. Please correct me if this is not possible. It is certainly a hell of a lot simpler than the other methods the paranoids were talking about. I suppose PGP could be obtained as source code and each person would have to compile it themself. But how big a change would be needed to restrict the number of secondary keys? could it be overlooked anyway? (as you can tell, I don't have source code, so I don't know.) Notice, only the sender would need a fake copy of the program, for this to work. And you can (sometimes) tell a lot from only one end of a conversation. Just pretending to be paranoid... or am I? Paul
Re: Message interception
Author: Don Alvarez
Date: Sat, 03 Oct 1992 18:39
Date: Sat, 03 Oct 1992 18:39
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In article <1992Oct2.203303.9914@vela.acs.oakland.edu> phkahler@vela.acs.oakland.edu (KAHLER PAUL H.) writes: >I have a new idea on how someone (NSA for example) could intercept and read >PGP (maybe other) encrypted messages. > >1) Modify PGP so that it only generates a small number of secondary keys. > Since the user is unaware of what key is used, he/she would never know. If you are going to allow the bad guy to modify the user's copy of PGP (or any other encryption routine), then of course the bad guy can win. Forget PGP for the moment, and assume that you just have a generic routine with access to the plaintext (your text editor, for example). If you can't trust the routine, then you can never be confident that the plaintext wasn't written to a file, inserted into a virus for floppy-based distribution, squirted out through a modem while the keyboard and phone were idle, or whatever. I know your proposal doesn't require access to the plaintext, but other parts of PGP do, and if the bad guy can modify routines which have no access to plaintext, he can certainly also modify other routines at the same time. Now for the rub: Unless you have digitally signed cryptographically secure checksums of all the software you run on your machine, you can't trust the machine not to divulge your plaintext. BUT even this won't help because you have no way to trust that the digital signature computing software and checksum computing software weren't the first things the bad guy modified. >Please correct me if this is not possible. It is certainly a hell of a lot >simpler than the other methods the paranoids were talking about. Regarding your accusations of "paranoia", modern cryptography is nothing more than a mathematical framework for debating the *implications* of various paranoid delusions. This is a completely disjoint process from debating the *merits* of said paranoid delusions. You can't discuss cryptography without discussing the types of attacks that the information may be subject to. If you don't like having people explain to you how a certain type of protocol is vulnerable to a certain type of attack, then you might as well give up on the subject of cryptography, because that is what it's all about. > >Paul > -don dla@athena.princeton.edu
Re: Message interception
Author: lane
Date: Sun, 04 Oct 1992 10:46
Date: Sun, 04 Oct 1992 10:46
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Obviously one way that messages could be intercepted is by tapping off
of a line (phone or RS232) between a terminal and the mainframe it is
connected to....no doubt this is part of the reason why PGP is discouraged
on mainframe systems.
Wasn't there some mention a while back of "secure" terminal-to-mainframe
session software (I vaguely recall KA9Q tcp/ip ....) ? It seems to me
that transferring session keys for securing a terminal line would be
easiest using public-key cryptography.
--
Drexel University \V --Chuck Lane
----------------->--------*------------<--------lane@duphy4.hepnet
(215) 895-1545 / \ Particle Physics lane@duphy1.bitnet
FAX: (215) 895-5934 /~~~~~~~~~~~ lane@duphy4.physics.drexel.edu
Re: Message interception
Author: Vesselin Bontchev
Date: Mon, 05 Oct 1992 14:29
Date: Mon, 05 Oct 1992 14:29
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dla@athena (Don Alvarez) writes: > >1) Modify PGP so that it only generates a small number of secondary keys. > > Since the user is unaware of what key is used, he/she would never know. > If you are going to allow the bad guy to modify the user's copy of PGP > (or any other encryption routine), then of course the bad guy can win. Now comes the paranoid question. How do we know that our copy of PGP has not been modified in the way suggested by the original poster? I have never seen the program and its sources authenticated by its authors... > Forget PGP for the moment, and assume that you just have a generic > routine with access to the plaintext (your text editor, for example). > If you can't trust the routine, then you can never be confident that > the plaintext wasn't written to a file, inserted into a virus for > floppy-based distribution, squirted out through a modem while the > keyboard and phone were idle, or whatever. I know your proposal True, but I have not got my text editor from an ftp site and it is not advertised to be able to do public key encryption, which makes it a less tempting target for the subversion you are suggesting... Regards, Vesselin -- Vesselin Vladimirov Bontchev Virus Test Center, University of Hamburg Tel.:+49-40-54715-224, Fax: +49-40-54715-226 Fachbereich Informatik - AGN < PGP 2.0 public key available on request. > Vogt-Koelln-Strasse 30, rm. 107 C e-mail: bontchev@fbihh.informatik.uni-hamburg.de D-2000 Hamburg 54, Germany
Re: Message interception
Author: David Sternlight
Date: Tue, 06 Oct 1992 16:08
Date: Tue, 06 Oct 1992 16:08
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In article <1992Oct4.064606.582@duphy4.physics.drexel.edu> lane@duphy4.physics.drexel.edu writes: >Obviously one way that messages could be intercepted is by tapping off >of a line (phone or RS232) between a terminal and the mainframe it is >connected to....no doubt this is part of the reason why PGP is discouraged >on mainframe systems. The wise user with security needs having financial value will use a desktop or portable computer, not a dumb terminal. He will use PGP on his portable, and send the encrypted text to the mainframe for retransmission. So does anyone know when the rumored PGP for the Mac will be ready?
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