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Caesar Cipher Decoder

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Caesar Cipher Decoder

This tool can be used to encode and decode messages using a Caesar cipher.

Type Message Here
Message After Shift
Suggested Solution (Message Shift #)

Using This Solver

The Caesar cipher is one of the oldest substitution ciphers, used by the Roman leader Julius Caesar to encrypt military messages ( wikipedia article). The system shifts every alphabetic character in a message by X positions in the alphabet; to get back to the original position, you reverse the shift. The keyspace for this cipher is very small (26) making it vulnerable to brute force attacks ("brutus force attacks"?). At the time it was considered secure, primarily since most of Caesar's enemies were illiterate and innumerate.

This caesar cipher translator helps you decode caesar cipher messages. Set the cipher to encode (to encrypt using the caesar cipher) or decode (to decrypt a message). Set the caesar shift amount (how many alphabet positions you want to shift). The caesar cipher translator will encode (or decode) the message by the shift amount and display the result. It will also show the mapping between the plaintext and ciphertext alphabets.

But wait..there's more. The caesar cipher translator also does a "best fit" analysis to assess which shift produces a result closest to english. This can help you crack an encrypted message. The cipher solution is calculated by breaking the message into parts (trigrams - groups of three letters) and encoding them using different values for the caesar shift. We then compare the distribution of the trigrams to the nature trigram distribution of the English language. The value of the caesar shift which produces the closest result will be suggested as the solution.

You can also attack a Caesar cipher using frequency analysis. However, many cipher puzzles are designed to defeat this attack through short messages and unusual letter frequencies. That's a good cautionary tale about cipher decoding. The shorter the message, the harder it is to identify patterns. The longer the message, the easier it is to spot basic patterns. In the case of using the caesar cipher to encode messages, one key weakness is that it will still continue to maintain the basic letter distribution of the English language. Improved ciphers (eg. the history of cryptography since the conquest of gaul) use improved forms of substitution and transposition to break up this pattern.

We have a more advanced substitution cipher workbench which can help you work with keyed caesar ciphers and mixed alphabet ciphers. We also have a Rot13 Decoder which addresses a narrow case of the caesar cipher system (where the shift = 13).

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