[Cryptography] Unicity distance of Playfair

[Jerry Leichter]

> One point I don't yet understand: You wrote: "Using Playfair we
> either can't distinguish IJ, or we can't distinguish YZ." Clearly
> you refer 'IJ' to input and 'YZ' to output. But 'IJ' has a (variable)
> space of 25*25 (like 'I' has a space of 25), isn't it? So 'IJ'
> would be an alphabet of 25*25 in my current view.
Playfair uses a 5x5 table - 25 cells.  Each input character has to correspond to a unique cell, but there are 26 letters.  So traditionally we replace every I in the input with a J (or every X with a Y).  Now the input has only 25 different characters.

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.
                                                        -- Jerry