Re: RC4 encrypt/decrypt keys

From: Roger Schlafly (rogersc_at_mindspring.com)
Date: 09/28/03 

Date: Sun, 28 Sep 2003 08:10:37 GMT

"TC" <a@b.c.d> wrote
> brute-force. But if there is a theoretical possibility that the data could
> be decrypted with a different, possibly *very short* key, doesn't that
> affect the whole "brute-force" idea?

Whether that is possible or not, there is always the possibility
that a brute-force attack will be unusually lucky, and find the
key much more quickly than expected.

Eg, suppose the key has 64 bits. An exhaustive brute force key
search would take 2^64 key attempts. On average, the key will
be found about half-way thru the search, or after 2^64/2 = 2^63
keys. But if the search is extraordinarily lucky, it might find the
key after only 2^40 key attempts, in which case the key will have
turned out to be no better than a 40 (or 41) bit key.