How does a running-key cipher differ from Vigenère?

Standard Vigenère repeats a short keyword, which creates a period a cryptanalyst can find. The running-key cipher draws its key from a long text passage, so the key is as long as the message and has no repeating period.

How long must the key passage be?

It must contain at least as many letters as your message. If the passage runs out, the tool leaves the remaining message letters unchanged and warns you how many extra key letters are needed.

Do spaces and punctuation in the key matter?

No. Only the letters of the key passage are used; spaces, punctuation and digits are skipped. Likewise, non-letter characters in the message pass through and do not consume key letters.

Is the running-key cipher unbreakable like a one-time pad?

No. A one-time pad uses a truly random key once. A running key uses natural-language text, which has statistical structure, so techniques that exploit the redundancy of both message and key can break it.

Why must I keep the exact key passage to decrypt?

Decryption subtracts the same key letters that encryption added. Even a one-character difference in the passage — including punctuation that shifts the letter alignment — will corrupt the recovered text from that point on.

Running Key Cipher Encoder & Decoder

The running-key cipher is a polyalphabetic substitution cipher that works like the Vigenère cipher but takes its key from a long stretch of text — classically a passage from an agreed book — rather than repeating a short keyword. Because the key is as long as the message and never repeats, it removes the periodic structure that makes ordinary Vigenère vulnerable to Kasiski analysis.

How it works

Letters are numbered 0 (A) through 25 (Z). The letters of the key passage are lined up beneath the message, one key letter per message letter. Encryption adds them modulo 26:

C = (P + K) mod 26

Decryption subtracts the same key letters:

P = (C - K) mod 26

Only letters in the key passage are used; its spaces and punctuation are skipped. In the message, non-letter characters pass through unchanged and do not consume a key letter, so the alignment between message letters and key letters is preserved.

Example

With key passage KING and plaintext MEET:

P:  M  E  E  T
K:  K  I  N  G
C:  W  M  R  Z

M (12) plus K (10) is 22 → W, and so on. Decrypting WMRZ with the same key KING subtracts each key letter to recover MEET.

Notes

The key passage must supply at least as many letters as the message; if it is too short, the trailing letters of the message are left in the clear.
The running-key cipher is not a one-time pad: natural-language key text is highly redundant, so the cipher is breakable by methods that play the statistics of the message against those of the key.
Sender and receiver must agree on the exact same source text and starting point — including how punctuation is handled — or decryption drifts out of alignment.