encryption program

[Paul Foley]

On Wed, 8 Jan 1997 19:32:20 -0600 (CST), Igor Chudov wrote:

   Cypherpunks, why don't we try to crack his encryption program? It seems
   like a fun exercise.

Well, a few minutes looking at it turned up the following:

* Capitalisation and word lengths are retained in the ciphertext, so
  there's no transposition involved (or he's being sneaky to make it
  look that way :-) )
* The two 'e's in the first word have different ciphertext equivalents,
  so it's not a single-alphabet substitution
* The words "wjq nwcc" occur together twice, 300 characters apart,
  the word "iyn" occurs three times, 375 and 300 characters apart,
  the word "fhw" occurs twice, 1400 characters apart, and
  the word "git" occurs twice, 500 characters apart.  This suggests
  a repeating key of length 5 or 25 characters.
* However, the letter 'r' in "Here" and "encryption" and the letters
  'am' in "example" and "program" occur 25 characters apart and have
  different ciphertext equivalents.  Possibly the key position is
  being incremented when "encrypting" digits and other non-alphabetic
  symbols, so my counts above (only letters) are wrong.

Also, I suspect that in the following ciphertext, EJMDLH.WKN and
ZFIYVZ.XGA are "CONFIG.SYS", KALMF is "FILES" and "ZPKQUST" is
"BUFFERS".

   ##>      Pbg qpypmjf EJMDLJ.WKN prbthucy hxzb tbcmsfs iyn bsfijsyey hdokz:
   ##> 
   ##>       KALMF=20
   ##>       ZPKQUST=20
   ##> 
   ##>      Z ixyh ZFIYVZ.XGA crxg ckqm wam wjq nwcc qjt QMUSAXH dsi ei:

-- 
Paul Foley <mycroft at actrix.gen.nz>       ---         PGPmail preferred

	   PGP key ID 0x1CA3386D available from keyservers
    fingerprint = 4A 76 83 D8 99 BC ED 33  C5 02 81 C9 BF 7A 91 E8
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