Re: VMs: Time to get hands dirty

[Jim Gillogly <jim@xxxxxxx>]

Jacques Guy wrote:
> There aren't many weapons in the arsenal. If the VMs is a cipher, then
> it is a cipher which _lowers_ the entropy of the text. The only cipher
> that can do that (Jim Gillogly, please correct me) is a cipher that 
> encodes single characters of the plain texts as sequences of characters,
> or whole words and sentences. E.g. cat -> cloakarmtower (c > cloak, etc.)
> Even so, the cipher has to be a bijection: c becomes cloak only and not other
> words. There is another possibility: lots of nulls.

Besides these cases, some ambiguous ciphers such as the Keyphrase
can lower the entropy.  As an English example, you can have:

plaintext:  abcdefghijklmnopqrstuvwxyz
ciphertext: THEPRESIDENTSPEAKSNONSENSE

In this case "my hovercraft is full of eels"
becomes      "SS IESRSESTEO DN ENTT EE RRTN"

Note the excess of S's.  The Keyphrase will frequently produce
runs of three or four of the same letter, and occasionally
adjacent ciphertext words which represent different plaintext
words.  However, the Keyphrase and other entropy-reducing
ciphers I know (except those that use lots of nulls, as Jacques
points out) also reduce the size of the alphabet.

Unless one is unlucky, the recipient will be able to get most
of the words right given the mapping, as will the cryptanalyst
given enough material.

While moderns wouldn't use this for mission-critical information,
Kahn reports that it may have been used by the Duchess of Berry
in the early 18th century.
--
	Jim Gillogly
	Highday, 18 Solmath S.R. 2003, 19:50
	12.19.9.17.14, 12 Ix 7 Pax, Third Lord of Night

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