Affine cipher

An affine cipher is a type of simple substitution where each letter is encrypted according to the rule c = (a-p + b) mod 26 (see the Appendix for a discussion of mod). Here, p, c, a, and b are each numbers in the range 0 to 25, where p represents the plaintext letter, c the ciphertext letter, and a and b are constants. For the plaintext and ciphertext, 0 corresponds to “a,” 1 corresponds to “b,” and so on. Consider the ciphertext QJKES REOGH GXXRE OXEO, which was generated using an affine cipher.

Determine the constants a and b and decipher the message. Hint: Plaintext “t” encrypts to ciphertext “H” and plaintext “o” encrypts to ciphertext “E.”

Decrypt the ciphertext

IAUTMDCSMNIMREBOTNELSTRHEREOAEVMWIH

TSEEATMAEOHWHSYCEELTTEOHMUOUFEHTRFT

This message was encrypted with a double transposition (of the type discussed in the text) using a matrix of 7 rows and 10 columns. Hint: The first word is “there.”

Solution: