This tool can be used to encrypt or decrypt text using a single letter substitution cipher. Each character of the original text is replaced by a single character of the substitution text. Possibilities to skip or replace unknown characters are given and whether the substitution is case sensitive.

In cryptography, a substitution cipher is a method of encoding by which units of plaintext are replaced with ciphertext, according to a regular system; the "units" may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing an inverse substitution.