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Homophonic Cipher Decryption

The receiver deciphers the text by performing the inverse substitution process to extract the original message. Substitution ciphers can be compared with transposition ciphers. In a transposition cipher, the units of the plaintext are rearranged in a different and usually quite complex order, but the units themselves are left unchanged. By contrast, in a substitution cipher, the units of the plaintext are retained in the same sequence in the ciphertext, but the units themselves are altered. There are a number of different types of substitution cipher. If the cipher operates on single letters, it is termed a simple substitution cipher; a cipher that operates on larger groups of letters is termed polygraphic.

Figures (8)

A typical distribution of letters in English language text. Weak ciphers do not sufficiently mask the distribution, and this might be exploited by a cryptanalyst to read the message.
Tabula recta
First page of Al-Kindi's 9th century Manuscript on Deciphering Cryptographic Messages
Arabic Letter Frequency distribution
Letter frequency in Spanish
ROT13 is a Caesar cipher, a type of substitution cipher. In ROT13, the alphabet is shifted 13 steps.
The forged nomenclator message used in the Babington Plot in 1586
A French nomenclator code table

Key Points

  • Frequency analysis is based on the fact that, in any given stretch of written language, certain letters and combinations of letters occur with varying frequencies.
  • Each letter of the keyword is used in turn, and then they are repeated again from the beginning.
  • A monoalphabetic cipher uses fixed substitution over the entire message, whereas a polyalphabetic cipher uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa.
  • The ciphertext alphabet is sometimes different from the plaintext alphabet; for example, in the pigpen cipher, the ciphertext consists of a set of symbols derived from a grid.
  • Polyalphabetic substitution ciphers were later described in 1467 by Leone Battista Alberti in the form of disks.
  • The first advantage is that the frequency distribution is much flatter than that of individual letters (though not actually flat in real languages; for example, 'OS' is much more common than 'RÑ' in Spanish).

Terms

Tap a term for a plain-language explanation.

Sources & licensing(4)