Keyword substitution cipher

How it works

A simple substitution cipher replaces each letter by its partner in a scrambled alphabet. Since $26!$ (about $4 \cdot 10^{26}$) alphabets are possible, a convenient way to agree on one is a keyword: write it without repeated letters and append the rest of the alphabet. ZEBRAS gives ZEBRASCDFGHIJKLMNOPQTUVWXY, so A becomes Z, B becomes E, and so on.

Decryption reads the same table from the bottom row to the top. Some variants continue the alphabet after the last letter of the keyword instead of starting from A; this tool offers both.

The huge number of keys does not help, because every plain letter always becomes the same cipher letter and the letter frequencies of the language shine through. Al Kindi described frequency analysis in the ninth century, and newspaper cryptograms are solved with it by hand every day.