Re:The two-time pad: midwife of information theory?
by tptacek on Wednesday July 18, 2018 @01:36PM
(#17560389)
I wrote a reply but then read 'SilasX's sibling and now I'm less sure of that reply. :)
Key size is less interesting to me than the number of times the key is reused.
Re:The two-time pad: midwife of information theory?
by a1369209993 on Wednesday July 18, 2018 @09:48PM
(#17563423)
> Is that really a fair characterization?
Kind of, but not quite; the problem with a 2-time pad is key cancellation: c1 ^ c2 = (p1 ^ k) ^ (p2 ^ k) = p1 ^ p2. The xor[0] of two plaintexts is only challenging to cryptanalyze if high-entropy sensitive information happens to line up in just the right way.
0: Technically sum mod 26 or so - xor is sum mod 2 - but the same tricks apply.
The two-time pad is like Vigenere with a very long key. Offhand, I'd expect that makes the cryptanalysis much more challenging than for Vigenere with a typical short key.