Cryptology - Unofficial St. Mary's College of California Web Site

6.5. SUMMARY 101
our ciphertext much longer than it would otherwise have been. Nonetheless, all
modern cipher techniques involve the use of nulls.
Salting the message. Many messages begin and end with similar, repeated
information. It is standard practice to begin the message with whom it is for,
and who is sending it: “From: Capt. Thomas, USS Lexington. To: Admiral
Nelson, Enterprise Carrier Group, US South Pacific Force.” To prevent an
adversary from guessing such a stereotyped beginning of a message, one salts
the message by adding a meaningless strings to the beginning and/or end of the
plaintext before it is enciphered.
A different method with the same purpose is Russian Copulation. Cut
the message approximately in half and the swap the two halves. This hides the
beginning and ending somewhere in the middle of the message. Hopefully, this
is of no bother to the person who will decipher the message, but will prevent
the enemy from using information about the stereotyped beginning and ending
of the message. Unfortunately, it is a very small leap from guessing that
“Enterprise” or “Naval Task Force” appears at the beginning and/or end of a
message to using such words and phrases as cribs for the entire message. Cribs
are words or phrases thought to be part of the message. If, say, “carrier” is
thought to appear in a monoalphabetic cipher, a quick scan for its distinctive
letter pattern **XX**X, where X represents any particular letter, should be able
to determine this. And if it is found, the codebreaker has an immediate decryption
of 5 letters of the code! We will not study them further, but clearly cribs
are a very powerful tool to have when decrypting ciphers.
6.5 Summary
Decrypting shift ciphers involved only frequency analysis, really, just counting
the letters. For more general monoalphabetic ciphers we need additional information
about how the letters relate to one another. For example, while e and t
both are very common letters, and both are among the most frequent final letters
of words, e seldom starts words whereas t very frequently does. When the
ciphertext is presented with word breaks, this information alone usually allows
the identification of e and t, with h quickly following.
Even without word breaks, vowels are much happier mating with consonants
than with other vowels. We can thus use e and t as wedges to separate the vowels
of aonirsh from the consonants. Using the differences in letter behavior should
then give enough guesses at substitutions that we can then try out our guesses,
fixing and revising as we make progress.
Despite tricks like these, monoalphabetic ciphers simply are not very secure,
and have not been for a very long time.