PEM question

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2 total messages Started by Robert Ames Fri, 02 Oct 1992 16:32

#1239

Author: Robert Ames
Date: Fri, 02 Oct 1992 16:32

17 lines
952 bytes

Question:

Let's say Alice and Bob are engaged in a criminal conspiracy.
They have an encryption system which uses RSA for key exchange, and DES
for encipherment.

Alice sends Bob an encrypted message, which is intercepted.  The plain
text of the same message also becomes available, and through some
cryptanalytic technique the secret DES key used for that encryption is
found.

The question is, to what extent does the discovery of the secret key
used in that one exchange compromise the system being used?  Can one
infer information about Alice's secret RSA key?

-- 
Robert Ames mirage1@gpu.utcs.utoronto.ca  P.O. Box 724, Station 'A', Toronto

Re: PEM question

#1314

Author: Christopher Browne
Date: Sat, 03 Oct 1992 22:25

34 lines
2041 bytes

In article <BvI5xx.6Mu@gpu.utcs.utoronto.ca> mirage1@gpu.utcs.utoronto.ca (Robert Ames) writes:
>Question:
>
>Let's say Alice and Bob are engaged in a criminal conspiracy.
>They have an encryption system which uses RSA for key exchange, and DES
>for encipherment.
>
>Alice sends Bob an encrypted message, which is intercepted.  The plain
>text of the same message also becomes available, and through some
>cryptanalytic technique the secret DES key used for that encryption is
>found.
>
>The question is, to what extent does the discovery of the secret key
>used in that one exchange compromise the system being used?  Can one
>infer information about Alice's secret RSA key?

A) It shows that the system is pretty seriously compromised at the DES
level.  It doesn't matter how good the key exchange system is, if
someone can break the code WITHOUT a key.

B) Alice's secret RSA key isn't particularly compromised; the "evil
decoders" haven't found out any information that they wouldn't have
been able to figure out given (1) The DES key and (2) Alice's public key.

If they COULD, then it means that Bob could, from the information HE was
intended to receive, infer information about Alice's secret RSA key.  If
THAT is the case, then RSA is very seriously flawed for ANY encryption
purpose.  It ain't a good public key system no more...

-- 
Christopher Browne
cbbrowne@csi.uottawa.ca
University of Ottawa
Master of System Science Program

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