Note 4/20/17: Due to circumstances I have had to move my web pages here.
This page allows you to encrypt/decrypt using a monoalphabetic cipher. Start by creating a key that maps each letter of the alphabet to a (possibly the same) letter of the alphabet. A sample key might be:
To encrypt the message "meet me at nine", start by taking the first letter of the message, 'm', and look up the corresponding ciphertext letter 'p'. Repeat by looking up the next plaintext letter 'e', and noting it becomes 'x'. Continue this process for the rest of the message. Typically spaces, numbers, and punctuation are left alone. In this case "meet me at nine." would become "pxxe px ye jmjx."
Decryption is similar. Start with the first ciphertext letter 'p', and look at the table to find the corresponding plaintext letter 'm'. Continue with the next letter 'x', and find it maps to 'e.
Monoalphabetic encryption is very easy to break, for two main reasons. First, commonly used letters like 'e' show up very quickly as the 'x' in the example. Second, words with repeated letters like "meet" in the example show that repetition in the ciphertext. And indeed this is so weak that the daily cryptogram run by some newspapers is typically an monoalphabetic substitution.
10/27/06: I have decided to not allow the key to changed to reduce the chance of this page being used to solve homework problems
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