Lecture 16: Introduction to Encryption

Substitution Ciphers

This is the simplest (and oldest) technique, whereby each character in the message is replaced by another using some rule. The order of the encrypted characters is the same as in the plaintext.

There are many examples of this technique. Most fall into the general category of monoalphabetic substitution, where the output alphabet is the same as the input. For example, in the classic Caesar Cipher, letters of the alphabet were shifted by 3 positions, hence a becomes D, b becomes E, etc. In this case, the key could be said to be "3" -- the distance by which each character was translated. A more complex example uses a "random" reordering of the letters:

plaintext:  a b c d e f g h i j k l m n o p q r s t u v w x y z
ciphertext: Q W E R T Y U I O P A S D F G H J K L Z X C V B N M

hence bad is encrypted as WQR^[1].

This type of cipher turns out to be relatively easy to break, despite the huge (26!) keyspace, by using known statistical characteristics of English (or other languages), eg:

letter frequencies - e is most common, then t, etc.
trigrams (eg the, and) are common.
Some words may be more likely in the particular context.

^[1] using the common convention that plaintext is shown in italicised lowercase and ciphertext is ITALICISED UPPERCASE.

Lecture 16: Introduction to Encryption

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