Construction Principles

Let’s get back to classic cryptography after all of that fun we had in our last chapter with the null. We are going to focus our attention on the cipher construction process in this issue. It is the process of converting plaintext message text into disguised ciphertext. You will find the construction process to be one of the best ways of appreciating and learning the deciphering technique.

Learning construction principles allows one to become familiar with the details of a cipher type. In fact, it is the reason that there are so very few ciphers that have been left unsolved, for all of the principles that have been used to put a piece of ciphertext together can be used to unscramble and decipher the very same message.

Continuing to work with the Aristocrat cipher, let’s take a look at how these ciphers are constructed by following our ACA guidelines for The Cryptogram (Cm) cipher publication. Where an ACA fundamental differs with real world of cryptography, we will indicate the difference.

Construction Principles

Taken from The ACA and You Handbook.

  1. Aristocrat ciphers are simple substitution ciphers which maintain their word divisions.
  2. Each ciphertext letter may stand for only one other plaintext letter.
  3. Each plaintext letter must be replaced by a ciphertext letter not equal to itself.  (This differs from some cryptography systems which allow self-encryption (allowing a letter to stand for itself.)
  4. An asterisk (*) is indicated to show the use of proper nouns which may, or may not, be part of the English dictionary (limited to 3).
  5. Aristocrats intended for Cm publication should be from 75 to 100 letters in length.
  6. A double hyphen (=) is used to indicate a true word hyphenation to distinguish it from a word split at the end of a line.
  7. Because of space limitations in the Cm compared to the material that would be available in the real world, cipher titles and tips are used to accompany the cipher.
  8. Cipher tips, also known as cribs may be in plaintext form or in an encrypted Caesar  shift format.
  9. No more than four letters may be used only a single time.
  10. At least 18 different letters should be used in each cipher.
  11. Keyword alphabets are most useful in the encrypting of plaintext to ciphertext. (See  Chapter Five.)

Construction Quiz

True or False:

C-1. The construction and decipherment process are totally unrelated.

C-2. Ciphertext letters may represent more than one plaintext letter in the same cipher.

C-3. ACA Aristocrat construction standards allow self-encryption.

C-4. Tips and cribs are synonymous.

We will continue our discussion of the construction of a cipher as an aid to decipherment with some discussion of keywords and the keyword alphabet as a tool in the building and deciphering of a disguised message.

In Chapter Five, we introduced you to the K1 keyword type (ACA and You Handbook, P. 26) where the plaintext alphabet has the key and the ciphertext alphabet is normal. In this chapter we will make you aware of another keyword type.

Keywords are used in many different cipher types. Their original and still useful purpose is to promote the ease of communication in ciphertext. Easily remembered keywords allow the ease of changing ciphertext to plaintext. The exchange of information between sender and receiver is rapidly accomplished as both are able to work with a common keyword.

Keywords transform our common everyday alphabet to a ciphertext alphabet that permits the construction of ciphertext with much more ease. One needs to only follow the keyword alphabet to avoid the repeat use of the same letter. An example will make its usage clear.

K2 Keyword Type

In the K2 keyword alphabet type, the plaintext alphabet is normal and the ciphertext alphabet has the key. The purpose, once again, is to allow an ease of construction of ciphertext as well as an ease of interpretation of the disguised message.

Continue to think of plaintext always appearing in lower case letters and CIPHERTEXT in UPPERCASE LETTERS to avoid confusion when working with two sets of alphabets.

K2 Alphabet Construction

pt abcdefghijklmnopqrstuvwxyz
CT XYZCIPHERABDFGJKLMNOQSTUVW

K2 Alphabet Construction Principles

  1. Plaintext alphabet is normal
  2. CIPHERTEXT alphabet has the key
  3. Each alphabet must have 26 letters with no repeated letters
  4. No CIPHERTEXT letter may stand for more than one plaintext letter
  5. No CIPHERTEXT letter may stand for the same plaintext letter (self-encryption)

A keyword may be started at any point under the plaintext alphabet as long as we avoid any CIPHERTEXT letter falling under the same plaintext letter. For this reason the keyword CIPHER in the above alphabet cannot begin under plaintext letter plaintext letter “c.”

The above K2 alphabet construction generates the CIPHERTEXT appearing beneath the plaintext message shown below:

Pt: this cryptography is a lot of fun.
CT: OERN ZMVKOJHMXKEV RN X DJO JP PQG.

Keyword Alphabet Questions

KW-1. Can the CIPHERTEXT alphabet construction above begin its keyword under  plaintext letter” a”?

KW-2. Which alphabet sequence is normal in the K2 keyword alphabet type?

KW-3. How many keyword types are described on page 26 of ACA and You Handbook?