Cryptology Cryptanalysis of Monoalphabetic Substitution (Approaches)

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The cryptanalysis of monoalphabetic substitution starts with its invariants—properties of a text that remain unchanged under a monoalphabetic substitution:

1. The distribution of frequencies of single characters is invariant.
   • That is a letter in the ciphertext occurs exactly as many times as the corresponding letter in the plaintext.
   • The same is true for bigrams (= pairs of letters), trigrams, etc.
2. Repeated patterns in the plaintext show up also in the ciphertext.

Both of these invariant properties suggest cryptanalytic approaches:

1. Statistical analysis,
2. Pattern recognition (for example matching with the words of a dictionary).

Often the cryptanalyst combines both of these approaches, and supplements them with systematic guesses:

Only in rare situations cryptanalysis is completely algorithmic. But no matter which method applies and how clean its theoretical basis is, the successful solution legitimates the cryptanalyst.