Re: time dependant

[Zulfikar A Ramzan]

One nice number-theoretic approach to the problem of preventing someone
(including the original sender) from decrypting a message before a
certain amount of time elapses can be found in the paper:  

Time-lock puzzles and timed-release Crypto
    by Ronald L. Rivest, Adi Shamir, and David A. Wagner.

which is avaialable from:

http://theory.lcs.mit.edu/~rivest/publications.html

The difficulty with this problem, in general, is that the notions of
cpu time and real time don't often coincide.    

Hope this help,

Zulfikar.