Cracking the Monoalphabetic Substitution Cipher

Abstract

Cryptography is the cornerstone of secure communication. It is the study of techniques by which a message, the plaintext, is transformed into an obfuscated form, the ciphertext. One of the earliest cryptographic techniques used by the Romans is the monoalphabetic substitution cipher. It replaces one letter in a message with another. This approach has one glaring weakness, in that the frequency of letters is preserved in the ciphertext. This renders messages encrypted via monoalphabetic substitution vulnerable to frequency analysis attacks. Frequency analysis is the knowledge of how frequently letters are used in a language. A persistent attacker that has intercepted the message can conduct frequency analysis to “crack” a monoalphabetic substitution cipher. An algorithm can also be created to automate the process and conduct frequency analysis attacks on ciphertext to crack the cipher in minutes, decrypting the message and exposing the plaintext and its contents to an unintended recipient. Key Terms—Cryptography, Decryption, Frequency Analysis, Monoalphabetic Substitution.