A Substitution Cipher is similar to a Caesar cipher, but instead of using a constant shift left or right, the plain alphabets and the cipher alphabets are mixed arbitrarily.
Plain Alphabet: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Cipher Alphabet: Z Y X W V U T S R Q P O N M L K J I H G F E D C B A
With the above, the Plain text "This is a sample" would encrypt to "Gsrh rh z hznkov." This particular substitution cipher, which relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards, is known as an atbash cipher.
With Substitution Ciphers, the secret is in the mapping between the plain and cipher alphabets. However, there are several analytical techniques to help break these ciphers with only the ciphertext. See Frequency analysis
Solving substitution ciphers
English-language ciphers be solved using principles such as these:
- Single-letter words are almost always A or I.
- As Edgar Allan Poe points out in The Gold Bug, "E predominates so remarkably that an individual sentence of any length is rarely seen, in which it is not the prevailing character."
- Apostrophes are generally followed by S, T, D, M, LL, or RE.
- Repeating letter patterns may be common letter groups such as TH, SH, RE, CH, TR, ING, ION, and ENT.
- Double letters are most likely to be LL, followed in frequency by EE, SS, OO, and TT (and on to less commonly seen doubles).
- 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 beginning and ending with the same letter is likely to be THAT. Others are AQUA, AREA, AURA, BARB, BLAB, BLOB, BOOB, BULB, CHIC, DEAD, deed, DIED, DYED, ease, edge, ELSE, FIEF, GANG, GONG, HASH, HATH, HUSH, KICK, LULL, MAIM, NEON, NOON, NOUN, ONTO, ORZO, PEEP, PIMP, PLOP, POMP, PREP, PROP, PULP, PUMP, REAR, ROAR, SAYS, SEAS, SEES, TACT, TART, TENT, TILT, TINT, TOOT, TORT, TUFT, URDU, and WHEW.
- McClung, O. William: Substitution Cipher Cracker — a useful tool that will perform a frequency analysis on ciphertext.
- CryptoClub: Crack a Substitution Cipher.
- American Cryptogram Association: Solve a Cipher.
- Olson, Edwin: Decrypto — a fast and automated cryptogram solver that can solve simple substitution ciphers often found in newspapers, including puzzles like cryptoquips and patristocrats.
- Ciphergram Solution Assistant — solves, or nearly solves, ciphergrams like those in the newspapers that are called cryptoquotes.