Countermeasures
against cribbing
As an ENIGMA
type device (with a reflector) Typex was also vulnerable to the plaintext-ciphertext
attacks used by the Allied codebreakers against the German plugboard Enigma. In
order to hinder such attacks several measures were employed, such as burying
the address in the middle of the text, cyclic encipherment for short messages
and insertion of random letters in the text.
For example
report FO 850/171 ‘Preparation of
telegrams: use of code words: cypher machines and traffic: teleprinter
services: en clair messages. Code 651 file 1 (to paper 4968)’ (25) says:
‘When encyphering on the Typex
machine, the encyphered version of a letter can never be the letter itself.
This sometimes makes it possible to assign with absolute accuracy even a small
number of words known or estimated to be in a message to the actual letters of
the cypher version by which they are represented. To obviate this danger
operators must from time to time press a key not demanded by the text of the
message; the additional letters resulting will make the accurate fitting to the
cypher version of a piece of clear text quite impossible. Such an insertion
should be made on average once in every 10 words while the body of the message
is being encyphered; it should be made on average once in every three words
during the encypherment of the codress, the prefatory details and the
beginnings and endings, whichever of the methods of encypherment in paragraph
25 is being followed; it should also be made on average once in every three
words throughout very short messages when they have to be encyphered separately
in Typex (see paragraph 27). The insertion should be made within words and not
between them.’
