[Cryptography] Unicity distance of Playfair

Ray Dillinger bear at sonic.net
Thu Mar 24 17:41:34 EDT 2016



On 03/24/2016 11:35 AM, mok-kong shen wrote:
> Am 24.03.2016 um 16:20 schrieb Jerry Leichter:

>> The replacement means "we can't distinguish I [from] J (or X [from]
>> Y)" after decoding - which has no significant effect on the
>> understandability of English-language text.

> My point is that since Playfair processes digrams (as single units),
> one should accordingly take that fact into consideration while
> determining its unicity-distance. Consider the analogy of a
> hypothetical (because very impractical owing to its size) Vigenere
> substitution table for digrams, i.e. one has entries of aa, ab, ac,
> .... zz instead of a, b, c, .... z. Then the "alphabet" concerned in
> that case is clearly of size 26**2 and not 26. Do you agree?

It doesn't matter.  The cipher produces one output letter per
input letter.  The letter in this case is the basic unit of
encryption.  This is true with almost all hand ciphers.

				Bear




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