The passphrase length in RC4/CyberSaber doesn't have anything to do with the final key length. In fact, RC4 doesn't really have a "key" in the obvious sence the way block ciphers do.

Rather, the internal state is an array of 256 bytes, one holding each value, that is shuffled into some permutation. The input letters of the passphrase control the shuffling details.

So, the actual "strength" of the cipher is that it has 256! possible states. So, the "key" is approximatly 1683 bits, since there are 2**1683 possible states for the cipher to be set-up into.

If you know something about how the passphrase is chosen, then an attacker can search far less than that many possibilities. E.g. with 96 ASCII chars and 8 char length, only 96**8 or 2**52 of the possible keys will ever be used. In order to say the system has an effective key size of 52 bits, the attacker would have to know that, and know which 2**52 keys are available. That's the case with "40 bit" SSL.

—John

In reply to Re: Re: Encrypting Largish Files by John M. Dlugosz
in thread Encrypting Largish Files by filmo

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