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 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…!