The Running Key Cipher is very simple in concept, but very difficult to decipher. It is not a genuine cipher since it cannot be deciphered uniquely. Rather, it should be thought of as a puzzle. However, it has a real cryptographic use, because the ability to decipher a Running Key cipher is what allowed American counter-intelligence people to decrypt Soviet messages during the Cold War (see the Venona Project).
To create a Running Key Cipher, take your message (plaintext) and break it in half. Add an extra letter if necessary if there’s an odd number of letters. Use the first half as a key to encipher the second half in a Vigenere cipher. The resulting ciphertext will always be half the length of the original plaintext. Here’s an example:
THISORGANIZATIONANDITSSYSTEMSPLI TTHEAMERICANGRAPHOLOGYWORLDINTWO MAPWODKRVKZNZZOCHBOWZQOMJEHUFIHW
The top row is the key (and first half of the plaintext) and the second row the rest of the plaintext. The third row is the ciphertext. Now try to decipher that ciphertext without knowing any of the text. I’ve written a program that uses brute force to decipher all possible combinations of key and plaintext for the first eight letters (MAPWODKR) to see what makes sense. Here are some of the pairs of valid decryptions that resulted.
THESAMER TTLEORGA, HANDAMON FACTORWE, THESOLID TTLEASCO, THISISCO TTHEGLID.
All of these pairs, and many others, are valid looking possible sentence beginnings and midsections. It takes skill and judgment to pick out the correct decryption. Note how any given letter or word, even if valid, can be either part of the key or the other half. This is why Running Key cipher problems in the American Cryptogram Association are usually presented only with generous cribs. If you want to try your hand at a few to hone your skills or just pass the time, I’ve provided a few for you below. They can be fun diversions.
ULLWLPXCZQWLQHXVHIGSZBEQUPD (crib: succeeding)
FWBTAFOIKDMCHBS (crib: swill)
FWYJIFWDECKGIXMVBXNOSAMS (crib: theproof)