Many cryptographers throughout history have claimed that a particular code is the most-unbreakable ever written. But does a rarely-used code, invented in 1917 and briefly employed during World War II, have a potential claim to the throne?
Commenter mwhite66 shared some of this history of one such code, the Vernam Cipher:
Fun fact: there is at least one truly unbreakable code. The Vernam Cypher uses a random key stream equal in length to the message. The plaintext is XORed with the key stream, creating the cyphertext. If the key stream is truly random, and is only used once, the resulting cyphertext is unbreakable, even in principle. It contains no information at all; the information exists in the relationship between the cyphertext and the key stream. No matter what analysis is performed, an eavesdropper has an equal chance of extracting any message whatsoever from the cyphertext. Vernam cyphers are very cumbersome to use, as the keys are long and must be securely exchanged before messages can be passed.
A version of this method called the Letter One Time Pad was used extensively in World War II by agents in occupied Europe; the key was random letters, added modulo-26 to the plaintext. British cryptographer Leo Marks arranged for these and related materials to be printed on silk, as they wouldn't be noticed under clothing in a pat-down, were paper would. Read Marks' bookBetween Silk and Cyanide: A Codemaker's War 1941-1945 for much more.
Funner fact: Leo Marks was the son of famedbookseller Ben Marks who's Marks & Co. shop at 84 Charing Cross Road in London was immortalized in the book and movie of the that name. Young Leo got his start in cryptography by learning to decode the book prices hidden in their shelf stock numbers. The shop was sort of his unofficial crypto headquarters during the war.
The unwieldiness of the code wasn't the only roadblock that prevented it from becoming widely used, though. While it may be exceptionally secure for a single-time use, the difficulty of breaking the cipher decreases as it is reused.
Image: "Colossus", a computer used in breaking other WWII-era ciphers / National Archives (UK)