Cryptography 101: Basic Solving Techniques for Substitution Ciphers

By Denise Sutherland, Mark Koltko-Rivera

Part of Cracking Codes & Cryptograms For Dummies Cheat Sheet

It doesn’t matter whether a cryptogram presents you with letters, numbers, arcane symbols, lines and dots, or weird alien squiggles — if you’re asked to replace each letter in the alphabet with another symbol, you’re dealing with a simple substitution cipher.

All substitution ciphers can be cracked by using the following tips:

  • Scan through the cipher, looking for single-letter words. They’re almost definitely A or I.

  • Count how many times each symbol appears in the puzzle. The most frequent symbol is probably E. It could also be T, A, or O, especially if the cryptogram is fairly short.

  • Pencil in your guesses over the ciphertext. Do typical word fragments start to reveal themselves? Be prepared to erase and change your guesses!

  • Look for apostrophes. They’re generally followed by S, T, D, M, LL, or RE.

  • Look for repeating letter patterns. They may be common letter groups, such as TH, SH, RE, CH, TR, ING, ION, and ENT.

  • Try to decipher two-, three-, and four-letter words.

    • Two-letter words almost always have one vowel and one consonant. The five most common two-letter words, in order of frequency, are OF, TO, IN, IS, and IT.

    • The most common three-letter words, in order of frequency, are THE, AND, FOR, WAS, and HIS.

    • The most common four-letter word is THAT. An encrypted word with the pattern 1 – – 1 is likely to be THAT. However, the pattern 1 – – 1 also represents 30 other words, so keep this in mind!

  • Scan for double letters. They’re most likely to be LL, followed in frequency by EE, SS, OO, and TT (and on to less commonly seen doubles).