full-disclosure - Time Expiry Alogorithm??

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From: peak at argo.troja.mff.cuni.cz (Pavel Kankovsky)
Subject: Time Expiry Alogorithm??

On Fri, 19 Nov 2004, Gautam R. Singh wrote:

> I was just wondering is there any encrytpion alogortim which expires
> with time.  For example an email message maybe decrypted withing 48
> hours of its delivery otherwise it become usless or cant be decrypted
> with the orignal key

No.

If a certain deterministic computation (e.g. decryption) can be made in
time T, then it can be made in any time T' > T. Even if the computation 
needs cooperation by your computer that refuses to cooperate when the 
time limit expires (e.g. the recipient of the message needs to ask you 
for an extra key), you can always do the computation once and save the 
result (e.g. the plaintext). Well, I admit, this holds unless your 
computer has been possesed by Palladium (and is not *your* computer 
anymore).

On the other hand, the power of hardware as well as the knowledge of
cryptanalysis oincreases as the time passes, ergo any cipher is going to
expire...in the sense someone will become able to break it and recover 
the plaintext without the (a priori) knowledge of the encryption key.

--Pavel Kankovsky aka Peak  [ Boycott Microsoft--http://www.vcnet.com/bms ]
"Resistance is futile. Open your source code and prepare for assimilation."