Cryptanalysis of The Enigma - World War II - Crib-based Decryption

Crib-based Decryption

The term crib was used at Bletchley Park to denote any known plaintext or suspected plaintext at some point in an enciphered message.

Britain's Government Code and Cipher School (GC&CS), before its move to Bletchley Park, had realised the value of recruiting mathematicians and logicians to work in codebreaking teams. Alan Turing, a Cambridge University mathematician with an interest in cryptology and in machines for implementing logical operations—and who was regarded by many as a genius—had started work for GC&CS on a part-time basis from about the time of the Munich Crisis in 1938. Gordon Welchman, another Cambridge mathematician, had also received initial training in 1938, and they both reported to Bletchley Park on 4 September 1939, the day after Britain declared war on Germany.

Most of the Polish success had relied on the repetition within the indicator. But as soon as Turing moved to Bletchley Park—where he initially joined Dilly Knox in the research section—he set about seeking methods that did not rely on this weakness, as they correctly anticipated that the German Army and Air Force might follow the German Navy in improving their indicator system.

The Poles had used an early form of crib-based decryption in the days when only six leads were used on the plugboard. The technique became known as the Forty Weepy Weepy method for the following reason. When a message was a continuation of a previous one, the plaintext would start with FORT (from Fortsetzung, meaning "continuation") followed by the time of the first message given twice bracketed by the letter Y. At this time numerals were represented by the letters on the top row of the Enigma keyboard. So, "continuation of message sent at 2330" was represented as FORTYWEEPYYWEEPY.

Cribs were fundamental to the British approach to breaking Enigma, but guessing the plaintext for a message was a highly skilled business. So in 1940 Stuart Milner-Barry set up a special Crib Room in Hut 8.

Foremost amongst the knowledge needed for identifying cribs was the text of previous decrypts. Bletchley Park maintained detailed indexes of message preambles, of every person, of every ship, of every unit, of every weapon, of every technical term and of repeated phrases such as forms of address and other German military jargon. For each message the traffic analysis recorded the radio frequency, the date and time of intercept, and the preamble—which contained the network-identifying discriminant, the time of origin of the message, the callsign of the originating and receiving stations, and the indicator setting. This allowed cross referencing of a new message with a previous one. Thus, as Derek Taunt, another Cambridge mathematician-cryptanalyst wrote, the truism that "nothing succeeds like success" is particularly apposite here.

Stereotypical messages included Keine besonderen Ereignisse (literally, "no special occurrences"—perhaps better translated as "nothing to report"), An die Gruppe ("to the group") and a number that came from weather stations such as weub null seqs null null ("weather survey 0600"). This was actually rendered as WEUBYYNULLSEQSNULLNULL. The word WEUB being short for wetteruebersicht, YY was used as a separator and SEQS was common abbreviation of sechs ("six"). Field Marshal Erwin Rommel's Quartermaster started all of his messages to his commander with the same formal introduction.

With a combination of probable plaintext fragment and the fact that no letter could be enciphered as itself, a corresponding ciphertext fragment could often be tested by trying every possible alignment of the crib against the ciphertext, a procedure known as crib-dragging. This, however, was only one aspect of the processes of breaking a key. Derek Taunt has written that the three cardinal personal qualities that were in demand for cryptanalysis were (1) a creative imagination, (2) a well-developed critical faculty, and (3) a habit of meticulousness. Skill at solving crossword puzzles was famously tested in recruiting some cryptanalysts. This was useful in working out plugboard settings when a possible solution was being examined. For example, if the crib was the word WETTER (German for "weather") and a possible decrypt before the plugboard settings had been discovered, was TEWWER, it is easy to see that T with W are stecker partners. These examples, although illustrative of the principles, greatly over-simplify the cryptanalysts' tasks.

A fruitful source of cribs was re-encipherments of messages that had previously been decrypted either from a lower-level manual cipher or from another Enigma network. This was called a kiss and happened particularly with German naval messages being sent in the dockyard cipher and repeated verbatim in an Enigma cipher. One German agent in Britain, code named Treasure, who had been turned to work for the Allies, was very verbose in her messages back to Germany, which were then re-transmitted on the Abwehr Enigma network. She was kept going by MI5 because this provided long cribs, not because of her usefulness as an agent to feed incorrect information to the Abwehr.

Occasionally, when there was a particularly urgent need to break German naval Enigma, such as when an Arctic convoy was about to depart, mines would be laid by the RAF in a defined position, whose grid reference in the German naval system did not contain any of the words (such as sechs or sieben) for which abbreviations or alternatives were sometimes used. The warning message about the mines and then the "all clear" message, would be transmitted both using the dockyard cipher and the U-boat Enigma network. This process of planting a crib was called gardening.

Although cillies were not actually cribs, the chit-chat in clear that Enigma operators indulged in amongst themselves, often gave a clue as to the cillies that they might generate.

When a captured, interrogated German Enigma operator revealed that they had been instructed to encipher numbers by spelling them out rather than using the top row of the keyboard, Alan Turing reviewed decrypted messages and determined that the word eins ("one") appeared in 90% of messages. He automated the crib process, creating the Eins Catalogue, which assumed that eins was encoded at all positions in the plaintext. The catalogue included every possible rotor position for EINS with that day's wheel order and plugboard connections.