In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.-In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system　the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.