What is a Running Key Cipher in Cryptography?

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Definition and Overview

The Running Key Cipher is a type of polyalphabetic substitution cipher. Essentially, it uses a long piece of non-repetitive text (the ‘running key’) to encrypt the plaintext.

The running key serves as the keystream for the encryption process, meaning it’s used in conjunction with the plaintext to produce the ciphertext.

The Encryption Process

Here’s the core of how it works:

  • Alignment: Align the plaintext and the running key so that each letter of the plaintext corresponds to a letter in the running key.
  • Substitution: Each letter in the plaintext is shifted according to the corresponding letter in the running key. The shift value is determined by the position of the running key letter in the alphabet.
  • Resultant Ciphertext: After the shifts are made for each letter, you’ll get the encrypted ciphertext.


To reverse the process:

  • Alignment: Align the ciphertext with the running key.
  • Reverse Substitution: Each letter in the ciphertext is shifted back based on the corresponding letter in the running key to retrieve the original plaintext.

Strengths and Weaknesses

Some considerations:

  • Strengths: When the running key is genuinely random, as long as the plaintext and never reused, the Running Key Cipher can be theoretically unbreakable. This resembles the One-Time Pad encryption in such cases.
  • Weaknesses: If the running key is derived from predictable sources (like books or known texts) or if it’s reused, the cipher can be vulnerable to attacks. Cryptanalysts might exploit patterns or known portions of the key to break the encryption.

Historical Usage


  • Vigenère Cipher: The Running Key Cipher is closely related to the Vigenère Cipher, but the key difference is the nature and length of the key used. The Vigenère Cipher uses a shorter, repetitive keyword, whereas the Running Key Cipher employs a longer, non-repetitive key.
  • Usage in War: In times of war and espionage, the Running Key Cipher was sometimes employed due to its perceived strength, especially when the key was a randomly generated sequence.


The Running Key Cipher offers an interesting perspective on polyalphabetic substitution methods in cryptography. Its strength largely depends on the randomness and non-repetitiveness of the key. While modern cryptographic methods have surpassed it in terms of security and efficiency, understanding the Running Key Cipher provides valuable insights into the evolution of encryption techniques.

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