[rc5] key security question

Thomas A. Gunter tag at mail.utexas.edu
Thu Jun 5 18:00:18 EDT 1997

At 4:43 PM -0400 6/5/1997, Brian Carlton wrote:

>>Think keyspace: for each first key you have to search all the 2^56 second
>>keys. This makes 2^112 combinations, IOW a 112-bit key. This presumes that
>Not true, the work is 2^56 for each, for 2^57 total.

That's what I thought at first, too, but I think 2^56 * 2^56 because even
after you decrypt once, you can't tell which key was right since it's not
text (it's still encrypted).  So you have to try every key on each of the
results.  Right?

Here's a new question:  Is it possible to tell just by looking at encrypted
data what algorythm was used (RC5, DES, PGP, etc.) and/or how many bits the
key is?  If so, how?

If not, it would take a LOT longer to crack it, since you'd have to try


Thomas A. Gunter         | "This must be a Thursday....
Departmental Services    |  I never could get the hang of Thursdays."
Academic Computing       |   -- Arthur Dent
Univ. of Texas at Austin |      The Hitchhiker's Guide to the Galaxy

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