How Do You Decode Ciphers?


Quick Answer

To decode a simple substitution cipher without the key, look for one-letter words, count the frequency of each letter or symbol and compare to the most common letters in English writing, look for repeating letter patterns, decipher two- and three-letter words, and look for letters following an apostrophe. A substitution cipher replaces each letter of the message with a different letter of the alphabet, or a symbol, number or character.

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Full Answer

The most common letters in English writing are E, T, A and O. If an encoded cipher is written in plain English and is more than a sentence or two long, the most frequently occurring symbols or letters likely correspond to one or more of these letters. One-letter words are almost always A or I. Common two-letter words such as OF, TO, IN, IS and IT, and the three-letter words SHE, AND, FOR, WAS and HIS are likely to be present in the message as well.

Specialized cases of substitution ciphers may be solved more quickly if a pattern is discovered. The Caesarian Shift cipher, for example, shifts each letter of the alphabet by the same amount when encoding. If the letters are shifted by one position, the letter A would be encoded as B, the letter B as C, the letter C as D and so forth. Once the pattern is recognized, the cipher may be quickly decoded.

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