[Cryptography] Unicity distance of Playfair

[mok-kong shen]

Am 24.03.2016 um 22:41 schrieb Ray Dillinger:
>
>
> 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.

Sorry, I don't yet understand the "purpose" of your mentioning this
general fact, for another similar general fact for most ciphers
is that they produce one output bit per input bit and the bit is the
"most" basic unit of all encryption. The unit of processing is
another basic unit and for Playfair it happens to be 2 characters
in each step.

M. K. Shen