Running Key Cipher

Running Key Cipher

The Running Key Cipher is a cryptographic algorithm that encrypts plaintext by combining it with a random keystream. Unlike traditional ciphers that use a fixed key, the Running Key Cipher uses a key that is as long as the plaintext itself, making it more secure. The key is typically generated from a book, passage, or other text, which is known to both the sender and the receiver.

Operation

To encrypt a message using the Running Key Cipher, the sender and receiver must first agree on a key, which is a string of characters as long as the message itself. The key is then combined with the plaintext using a modulo operation to produce the ciphertext. The same key is used to decrypt the ciphertext back to the original plaintext.

Example

Suppose we have the plaintext "HELLO" and the key "WORLD". To encrypt the plaintext using the Running Key Cipher, we would combine each letter of the plaintext with the corresponding letter of the key using a modulo operation:

Plaintext: H E L L O
Key: W O R L D
Ciphertext: V C A V F

To decrypt the ciphertext back to the original plaintext, we would use the same key and perform the modulo operation in reverse.

Security

The security of the Running Key Cipher depends on the randomness and secrecy of the key. If the key is generated from a truly random source and kept secret, the Running Key Cipher can be very secure. However, if the key is predictable or known to an attacker, the security of the cipher is compromised.

Usage

The Running Key Cipher has been used historically for secure communication, especially in situations where both the sender and receiver have access to the same key material, such as a book or passage. However, due to the advent of more secure encryption algorithms, the Running Key Cipher is now primarily used for educational purposes and recreational cryptography.

Conclusion

The Running Key Cipher is a cryptographic algorithm that encrypts plaintext by combining it with a random keystream. While it can be secure if used with a truly random and secret key, it is no longer widely used in practice due to the availability of more secure encryption algorithms.