Section 2.3: Substitution Ciphers• A substitution cipher is a cipher in which

correspondents agree on a rearrangement (permutation) of the alphabet in which messages are written.

• Shift and affine ciphers are special cases of the substitution cipher.– A mathematical formula is used to define the cipher.

• Other techniques for defining substitution ciphers:– Mixed Alphabet with simple keyword substitution

cipher.– Mixed Alphabets with keyword columnar substitution

cipher…

Mixed Alphabets with a Simple Keyword Substitution Cipher

• Keyword Substitution Cipher– Write the letters of a keyword without repetition of

letters in order of appearance. List the remaining letters in alphabetical order following the keyword. The substitution is written below the plaintext in the usual manner.

• Example: Suppose we want to use the keyword “NEILSIGMON” to create a simple keyword substitution cipher.

• Note: One of the flaws of the substitution cipher is the last few plaintext and ciphertext letters are the same.

– This can make this type of cipher more vulnerable to cryptoanalysis….

Mixed Alphabets with Keyword Columnar Substitution Cipher

• We write the letters of a keyword on one line without repetition.

• Below the key word we write the remaining alphabet, generating columns of letters, one column for each letter of the keyword without repetition.

• Now associate the letters of the alphabet with the letters in your table reading down each column.– Example 2: Keyword “RADFORDVA…

Cryptanalysis of Substitution Ciphers

• We use frequency analysis for decryption of a substitution cipher.– We look at single letter, digraphs (two letter

sequences), and trigraphs (three letter sequences).

– Example 3: Decrypt the cipher given.– Example 4: Decrypt the cipher given…!