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." _______________________________________________ Full-Disclosure - We believe in it. Charter: http://lists.netsys.com/full-disclosure-charter.html