Classical cipher

Running Key Cipher

Encrypt or decrypt with a long, non-repeating text key. Message formatting is preserved, while only A-Z letters advance the key stream.

Operation

0 / 12,000 characters | 0 A-Z letters

ASCII letters are transformed in place. Spaces, punctuation, digits, line breaks, and non-ASCII characters pass through unchanged and do not consume key letters.

0 / 30,000 characters | 0 usable letters

Only A-Z letters form the key stream. All other key characters are skipped. The key must supply at least one letter for every message letter and is never repeated.

Ready. Enter a message and a running key.

Key stream used None yet

How the arithmetic works

Letters use values A=0 through Z=25. Each message letter is paired with the next usable key letter.

Encrypt: C = (P + K) mod 26
Decrypt: P = (C - K + 26) mod 26

Running key, not repeating Vigenère

A repeating Vigenère cipher cycles a short keyword. A running key uses consecutive letters from a longer external passage and stops if that passage is too short.

For education and puzzles: natural-language running keys are not secure modern encryption.