Encryption methods have historically been of two categories, namely, substitution ciphers and transposition ciphers. We discuss them below.
Category # 1. Substitution Ciphers:
While the above would be a simple method of coding—based on the belief that the enemy could not decipher the Navajo tongue—variations of this method was used in substitution ciphers.
In this method (sometimes known as a generalisation of the Caesar Cipher), every letter or group of letters is replaced by another letter or group of letters as given in the following example:
In the above table, the first line lists the letters of the alphabets in sequence as they are used in writing plain text, while the letters of the alphabet that will replace them exactly in the cyber text are listed precisely underneath them. Thus, if the word ‘bomb’ written in plain text it will be converted to ‘ylny’.
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Similarly, each character of the plain text can be replaced by its equivalent character in the cipher text. This will make the whole message appear to be gibberish until it is decoded. The above example is one of a mono-alphabetic substitution. This substitution can be broken fairly easily if a small amount of text is available.
Breaking such a code will use the statistical fact that in English e is the most frequently used letter followed by t, o, a, n, i in that order. Using this fact and a little knowledge of words, this code can be cracked.
This substitution may be done using two-letter combinations or three-letter combinations. The common two-letter combinations called diagrams are th, in, er, re, and an. The common three-letter combinations called trigrams are and, the, ing, and ion.
Category # 2. Transposition Ciphers:
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This method of coding does not preserve the order in which the plain text letters are written, unlike in substitution ciphers. Instead of disguising the letters, it reorders them. The following example should make this form of coding clear. The key must be a word or phrase that does not have any repeated letters. Suppose the key is XCABLES.
The purpose of the key is to number the columns, the number 1 being allotted to the letter, in the column, being closest of the start of the alphabet, the other letters being numbered according to their position in the alphabet in their increasing order.
Thus, we write the key horizontally and the number associated with each letter horizontally under each letter of the key as follows:
The plain text message is written horizontally under the key above and the message is read out column-wise, starting from the column numbered 1, followed by column numbered 2, and so on sequentially. Thus, suppose that in the above example, the message in plain text was: please transfer ten lakh rupees to my Swiss bank account number five one four.
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Then the whole exercise will look like the following:
The plain text message reads:
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Pleasetransfertenlakhrupeestomyswissbankaccountnumberfiveonefour
The equivalent cipher message reads:
EAEUMNNREANNPYKTFFLRTROAUENEFAEWCUVUSSLESANIOTEKSICMER
The reader may try to see how to crack this code without knowing the key. As a hint, it will depend on the frequency of each letter in a text.