Substitution of single letters separately—simple substitution—can be demonstrated by writing out the alphabet in some order to represent the substitution. This is termed a substitution alphabet. The cipher alphabet may be shifted or reversed (creating the Caesar and Atbash ciphers, respectively) or scrambled in a more complex fashion, in which case it is called a mixed alphabet or deranged alphabet. Traditionally, mixed alphabets may be created by first writing out a keyword, removing repeated letters in it, then writing all the remaining letters in the alphabet in the usual order.
Using this system, the keyword "zebras" gives us the following alphabets:
| Plaintext alphabet: | ABCDEFGHIJKLMNOPQRSTUVWXYZ |
| Ciphertext alphabet: | ZEBRASCDFGHIJKLMNOPQTUVWXY |
A message of
flee at once. we are discovered!enciphers to
SIAA ZQ LKBA. VA ZOA RFPBLUAOAR!Traditionally, the ciphertext is written out in blocks of fixed length, omitting punctuation and spaces; this is done to help avoid transmission errors and to disguise word boundaries from the plaintext. These blocks are called "groups", and sometimes a "group count" (i.e., the number of groups) is given as an additional check. Five letter groups are traditional, dating from when messages used to be transmitted by telegraph:
SIAAZ QLKBA VAZOA RFPBL UAOARIf the length of the message happens not to be divisible by five, it may be padded at the end with "nulls". These can be any characters that decrypt to obvious nonsense, so the receiver can easily spot them and discard them.
The ciphertext alphabet is sometimes different from the plaintext alphabet; for example, in the pigpen cipher, the ciphertext consists of a set of symbols derived from a grid. For example:
Such features make little difference to the security of a scheme, however – at the very least, any set of strange symbols can be transcribed back into an A-Z alphabet and dealt with as normal.
In lists and catalogues for salespeople, a very simple encryption is sometimes used to replace numeric digits by letters.
| Plain digits: | 1234567890 |
| Ciphertext alphabet: | MAKEPROFIT |
Example: MAT would be used to represent 120.
Read more about this topic: Substitution Cipher
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—Henry David Thoreau (1817–1862)
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—Max J. Friedländer (1867–1958)