The Short Weather Cipher (in German: Wetterkurzschlüssel, often abbreviated as WKS), sometimes referred to by its more descriptive moniker, the weather short signal book, was less a cipher in the traditional sense and more a meticulously crafted codebook. This particular manual found its purpose in the grim theatre of World War II, specifically aboard the U-boats of the German Navy, where it was employed by their dedicated radio telegraphists. Its primary function was to reduce complex meteorological observations into a remarkably compact 7-letter message. This compressed report was then subjected to further obfuscation, being enciphered using the notorious naval Enigma machine. Once transformed into an inscrutable string of characters, it was broadcast by radiomen across the vast, unforgiving expanses of the ocean to intercept stations positioned on the shore. There, the process was reversed: the message was first deciphered by an Enigma machine and subsequently decoded using a copy of the WKS, allowing the original, detailed 7-letter weather report to be painstakingly reconstructed. 1 2 It was a system designed for efficiency and secrecy, a testament to the era's relentless pursuit of tactical advantage through encrypted communication, though, as history would prove, its perceived impregnability was ultimately a fatal flaw.
History
During the tumultuous years of World War II, the Short Weather Cipher wasn't a static entity; it underwent several iterations, each designed, presumably, to enhance security or adapt to evolving operational needs. Different versions of the cipher were introduced and superseded at various points throughout the conflict. The initial release bore the rather poetic codename Weimar, a name perhaps chosen to evoke a sense of cultural heritage amidst the burgeoning chaos. This inaugural edition, however, was not destined for longevity. It was eventually replaced by a new, updated version, christened Eisenach, which was officially put into operation on 20 January 1942. The cycle of updates continued, and on 10 March 1943, the third significant edition of the weather key, designated with the codename Naumburg, officially entered into force, further complicating the already intricate dance of wartime signals intelligence.
A pivotal moment, one that would irrevocably alter the course of the Battle of the Atlantic, occurred on May 9, 1941. This date saw the culmination of Operation Primrose, a British naval action during the Norwegian Campaign aimed at occupying Åndalsnes and creating a strategic diversion to the south of Trondheim in Norway. 3 However, the true prize of this operation wasn't found on land but adrift in the frigid waters of the North Atlantic, specifically east of Cape Farewell, Greenland. Here, the German submarine U-110 was captured, and from its depths, an astonishing trove of intelligence was recovered. Among the salvaged items were an intact Naval Enigma (M3) cipher machine, a pristine copy of the "Weimar" version of the Short Weather Cipher, and, equally critical, a copy of the short signal book (known in German as Kurzsignalbuch or simply Kurzsignale). 4 This unprecedented recovery was nothing short of a godsend for the Allied war effort. It provided the brilliant cryptanalysts working tirelessly at Bletchley Park with the indispensable tools they needed to finally crack the seemingly impenetrable encryption of the M3 Naval Enigma. With these documents in hand, they could begin the arduous, yet ultimately successful, process of deciphering the previously unreadable radio messages sent by German U-boats, gaining invaluable insight into the Kriegsmarine's operations and intentions.
The significance of the Short Weather Cipher in the grand scheme of cryptanalysis cannot be overstated, particularly its role in the monumental task of breaking the Naval Enigma M4, the more complex four-rotor version. The cipher proved to be an exceptionally rich source of "cribs" – known plaintext segments that are crucial for cracking encrypted messages. Another daring and costly operation on 30 October 1942 yielded another critical intelligence coup. This involved a raid on the U-boat U-559, which was in the process of sinking in the Mediterranean. Three exceptionally brave Royal Navy sailors—Lieutenant Anthony Fasson, Able Seaman Colin Grazier, and NAAFI canteen assistant Tommy Brown—boarded the abandoned, sinking submarine. In a heroic 90-minute search, they managed to recover vital documents, including a copy of the Wetterkurzschlüssel (the Short Weather Cipher) and another copy of the Kurzsignale (the short signal book). 5 These documents, secured at such a high personal cost, were then rushed through a three-week journey, finally reaching the Government Code and Cypher School at Bletchley Park on 24 November 1942. The intelligence gleaned from these recovered materials, tragically purchased with the lives of Fasson and Grazier, proved to be absolutely instrumental in the eventual breaking of the Naval Enigma M4, a cipher that had, until then, largely defied Allied efforts.
The version of the Short Weather Cipher recovered from U-559 was the Eisenach edition. Unlike its predecessor, Weimar, the Eisenach version notably omitted the explicit listing of the 26 rotor positions, each indicated by a single letter, that were intended to be used when enciphering weather reports. This seemingly minor change led the Hut 8 cryptanalysts at Bletchley Park to initially believe that all four rotors of the Enigma M4 were always in play for weather message encryption. This assumption, while initially a challenge, led to a new avenue of attack. Testing on the formidable Bombes—the electromechanical devices designed to uncover Enigma settings—began in earnest, specifically targeting "weather kisses." These were identical messages that cryptanalysts suspected or knew existed in two different cryptosystems, providing invaluable cross-references. 6
A significant breakthrough came on 13 December 1942. A crib obtained directly from the newly acquired Short Weather Cipher provided a critical key for the Naval Enigma M4. This key revealed that the rotatable Umkehrwalze (the reversing roller or reflector) was in its neutral position, effectively rendering the M4 equivalent to a standard three-rotor Enigma for that particular message. This discovery was monumental, as it meant that messages from the B-Dienst (German naval intelligence) could potentially be broken using the existing Bombes already configured for three-rotor Enigmas. On that very same day, Hut 8 personnel made another crucial observation: the 4-letter indicators used for regular U-boat messages were directly related to the 3-letter indicators for weather messages. Specifically, they were identical except for one additional letter. This revelation meant that once the key for a weather message was determined for any given day, the cryptanalysts only needed to test the fourth rotor in its 26 possible positions to uncover the full 4-letter key for regular messages. By the close of Sunday, 13 December, Rodger Winn of the Submarine Tracking Room at Bletchley Park had the definitive, exhilarating confirmation: the Shark Enigma Cipher, which governed U-boat communications, had been decisively broken. The tide of the Battle of the Atlantic was about to turn. 7
The period of Allied dominance over Shark Enigma was, however, not without its challenges. When the third edition of the short signal book, the Naumburg version, was introduced on 10 March 1943, Hut 8 was immediately deprived of its invaluable cribs. The Germans, in their endless pursuit of secure communication, had changed the rules of the game. Yet, the ingenuity at Bletchley Park was relentless. By 19 March, a mere nine days later, cribs were again being successfully employed by Hut 8 personnel. Their new method leveraged "short signal sighting reports." These were brief, coded reports transmitted by U-boats whenever they made contact with other vessels, and they utilized the Kurzsignalheft code book. This adaptation proved highly effective. Hut 8 managed to solve Shark for an impressive 90 out of 112 days leading up to the end of June. Crucially, these Kurzsignalheft short sighting reports also employed the Enigma M4 in a three-rotor, or "M3 mode," configuration, making them more susceptible to analysis. By the end of June, the development of specialized four-rotor Bombes had advanced to the point where they entered service at Bletchley Park, further enhancing their codebreaking capabilities. These advanced machines were subsequently introduced by the US Navy by August. From September 1943 onwards, the Shark Enigma Cipher was generally solved within 24 hours of interception, providing the Allies with a near real-time understanding of U-boat movements and intentions, a strategic advantage that proved decisive in the latter stages of the Battle of the Atlantic.
Operation
The operational sequence for generating and transmitting a weather report using the Short Weather Cipher was a multi-stage process, meticulously designed for both brevity and security. First, the U-boat crew would consult the Wetterkurzschlüssel to encode their current meteorological observations. This step transformed raw data—such as temperature, atmospheric pressure, and wind speed—into a concise, single-word coded message. Following this initial encoding, the resulting short word was then fed into the Naval Enigma machine and enciphered, adding a layer of cryptographic complexity. Once the message was transformed into its encrypted form, it was transmitted by the U-boat's radio telegraphist.
On the receiving end, the shore patrol of the Kriegsmarine would intercept these transmissions. Their task was to first decipher the Enigma message using their own Enigma machine and then decode the resulting short word using a duplicate copy of the Wetterkurzschlüssel. Once decoded and the original weather report reconstructed, this crucial data was not immediately disseminated. Instead, it was forwarded to a central meteorological station. This station then aggregated the incoming weather data from numerous sources and subsequently rebroadcast it as "ship synoptics"—comprehensive weather summaries for broader naval use. However, even these rebroadcast synoptics were not sent in the clear; they were further enciphered using additive tables with a distinct cipher, which Hut 8 personnel at Bletchley Park later designated as "Germet 3." 8 This multi-layered approach underscored the German command's unwavering commitment to maintaining secrecy around even seemingly innocuous weather data, understanding its potential value to enemy intelligence.
The Short Weather Cipher itself was an intricate system, relying on a polyphonic single-letter code, with the letter 'X' notably missing from its alphabet. 9 This meant that a single letter could represent a range of values, depending on the context or the specific table being referenced. For instance, consider the coding for air temperature in whole Celsius degrees:
- A = +28° ◦ B = +27° ◦ C = +26° ◦ D = +25° ◦ . . . ◦ W = +6° ◦ Y= +5° ◦ Z = +4° ◦
However, the same letters could also represent a different range of temperatures within a different context or table:
- A = +3° ◦ B = +2° ◦ C = +1° ◦ D = 0° ◦ E =−1° ◦ F =−2° ◦ . . . ◦ Z = −21° ◦
This inherent ambiguity was a deliberate design choice, adding a layer of complexity for any intercepting party without the correct codebook. In a similar systematic fashion, other vital meteorological and navigational parameters were coded. These included water temperature, atmospheric pressure, humidity, wind direction, wind velocity (often referenced against the Beaufort scale), visibility, the degree of cloudiness, and even the geographic latitude and longitude of the U-boat's position. All these disparate pieces of information were condensed into a prescribed order, culminating in a single, short coded word that constituted the entire weather report.
The polyphonic nature of the code necessitated a degree of contextual understanding for accurate decoding. For instance, if a receiving Kriegsmarine telegraphist received the letter "S" in a temperature report, they wouldn't immediately know its precise value. Based on the approximate, pre-known position of the transmitting U-boat, however, they could infer which temperature range was applicable. The letter "S," according to the tables, could mean either 10°C or −15°C. The knowledge of the submarine's general location—for example, whether it was operating in the frigid North Atlantic or the warmer Mediterranean—would allow the telegraphist to correctly translate "S" to the appropriate temperature, thus resolving the ambiguity.
Beyond standard atmospheric conditions, even more nuanced environmental data, such as the direction and type of ocean swell, was meticulously coded using only a single letter. This table provided a concise mapping:
| Direction from which the swell comes | Type of swell |
|---|---|
| low | middle high |
| N | a |
| NE | b |
| E | c |
| SE | d |
| S | e |
| SW | f |
| W | g |
| NW | h |
| No swelling | |
| Intermittent |
| Type of swell |
|---|
| high |
| q |
| r |
| s |
| t |
| u |
| v |
| w |
| x |
| y |
| z |
To illustrate the remarkable compression achieved by this cipher, consider a hypothetical, yet entirely plausible, weather report. A U-boat positioned at 68° North latitude and 20° West longitude (an area typically found north of Iceland) observes an atmospheric pressure of 972 millibars, an air temperature of minus 5°C, a wind from the northwest at Force 6 on the Beaufort scale, a cloud cover of 3/10 cirrus clouds, and a visibility of 5 nautical miles. All of this detailed information, which would typically require a lengthy descriptive message, was condensed into the single, concise coded word: MZNFPED. 10 This efficiency was paramount for U-boats, where radio silence was often critical for survival, and every second of transmission increased the risk of detection by Allied direction-finding stations.
Publications
For those who find themselves drawn to the intricate dance of wartime cryptography and the subtle art of meteorological deception, the following publications offer further illumination into the Short Weather Cipher and its broader historical context:
- Bauer, Arthur O. (1997), Funkpeilung als alliierte Waffe gegen deutsche U-Boote 1939–1945 [Direction finding as Allied weapon against German submarines from 1939 to 1945] (in German), Diemen, NL: Selbstverlag, ISBN 978-3-00-002142-8
- Bauer, Friedrich L. (2007), Decrypted Secrets. Methods and Maxims of Cryptology (4., rev. and extended ed.), Berlin Heidelberg New York: Springer, ISBN 978-3-540-24502-5
- Pfeiffer, Paul N. (October 1998), "Breaking the German Weather Ciphers in the Mediterranean Detachment, 849th Signal Intelligence Service", Cryptologia, 22 (4): 354–369, doi:10.1080/0161-119891886975, ISSN 0161-1194
- Ulbricht, Heinz (2005), Die Chiffriermaschine Enigma – Trügerische Sicherheit. Ein Beitrag zur Geschichte der Nachrichtendienste [The Enigma cipher machine – Deceptive security. A contribution to the history of the intelligence services], Dissertation, Fachbereich Mathematik und Informatik, Technische Universität Braunschweig (in German)