🚀 go-pugleaf

In article <1992Apr11.170301.4743@mnemosyne.cs.du.edu> bfairlan@isis.cs.du.edu (Brian Fairlane) writes:
>                     data : This is the text I want to hide from the NSA
>            key1 (nisse)  : nissenissenissenissenissenissenissenissenisse
>            key2 (bagare) : bagarebagarebagarebagarebagarebagarebagarebagare
>   key3 (towerofeiffel's) : towerofeiffel'stowerofeiffel'stowerofeiffel's

This was discussed recently, and I think I can recount the discussion
fairly easily.  Basically, the use of the three keys in this context,
of length 5, 6, and 15, respectively, is equivalent to the use of a
single key of length of the least common multiple of these, i.e. 30.
Thus, the unicity distance will be 30. Given a sample of say 60
characters of output, you would almost certainly be able to discover
this unicity distance by shifting, xoring, and then computing
frequencies. (The redundancies are quite significant; it came as a
surprise to me that so few characters would enable one to spot the
unicity distance.)

So then you are down to a polyalphabetic cipher, that is, effectively
30 separate ciphers. (Actually 29, as the combination 'sae' occurs
twice each thirty characters, another not-very-surprising
demonstration of redundancy at work.)

The permutation at this point contributes relatively little to the
security, as permutation of groups of say 5 characters merely means
checking 120 possible permutations. This is well within the bounds of
brute force, and I imagine that digram or trigram frequencies could be
used to unravel much larger permutations once the individual
characters are back in the right "alphabet". I don't think
permutation-only ciphers have ever been used successfully, although I
an happy to be proven wrong on this.
--
Greg Rose                 Australian Computing and Communications Institute
ggr@acci.com.au                                              +61 3 282 2517