Running Key Cipher

How Running Key Cipher works

Running Key Cipher handles message letters with a key passage that must supply at least one alphabetic key letter for every alphabetic message character. Encoding adds corresponding key and message positions modulo 26; decoding subtracts them. Unlike a repeating-key system, the passage is never cycled when it runs short. The Running Key Cipher exposes its message, running key passage, operation controls so this particular convention stays inspectable instead of hiding behind an unexplained preset.

ATTACKATDAWN with QUEENLYISLAND produces QNXEPVYBVLWA, and the same full key passage recovers ATTACKATDAWN. Puzzle designers can use a memorable book line as a long keystream while students compare key-length effects among classical polyalphabetic ciphers. The Running Key Cipher presents a deterministic plain-text result that can be copied and compared with the worked value before it enters another workflow.

Limits and private processing

The tool rejects an undersized key, ignores nonletters for key indexing, and offers historical encoding only. Predictable prose keys are not secure. The Running Key Cipher performs its complete operation locally in your browser; neither the source text nor its settings are uploaded, stored remotely, or sent through an external request.