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Code (cryptography)
Code (cryptography)
Code (cryptography)
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A portion of the "Zimmermann Telegram" as decrypted by British Naval Intelligence codebreakers. The word Arizona was not in the German codebook and had therefore to be split into phonetic syllables.
Partially burnt pages from a World War II Soviet KGB two-part codebook

In cryptology, a code is a method used to encrypt a message that operates at the level of meaning; that is, words or phrases are converted into something else. A code might transform "change" into "CVGDK" or "cocktail lounge". The U.S. National Security Agency defined a code as "A substitution cryptosystem in which the plaintext elements are primarily words, phrases, or sentences, and the code equivalents (called "code groups") typically consist of letters or digits (or both) in otherwise meaningless combinations of identical length."[1]: Vol I, p. 12  A codebook is needed to encrypt, and decrypt the phrases or words.

By contrast, ciphers encrypt messages at the level of individual letters, or small groups of letters, or even, in modern ciphers, individual bits. Messages can be transformed first by a code, and then by a cipher.[2] Such multiple encryption, or "superencryption" aims to make cryptanalysis more difficult.

Another comparison between codes and ciphers is that a code typically represents a letter or groups of letters directly without the use of mathematics. As such the numbers are configured to represent these three values: 1001 = A, 1002 = B, 1003 = C, ... . The resulting message, then would be 1001 1002 1003 to communicate ABC. Ciphers, however, utilize a mathematical formula to represent letters or groups of letters. For example, A = 1, B = 2, C = 3, ... . Thus the message ABC results by multiplying each letter's value by 13. The message ABC, then would be 13 26 39.

Codes have a variety of drawbacks, including susceptibility to cryptanalysis and the difficulty of managing the cumbersome codebooks, so ciphers are now the dominant technique in modern cryptography.

In contrast, because codes are representational, they are not susceptible to mathematical analysis of the individual codebook elements. In the example, the message 13 26 39 can be cracked by dividing each number by 13 and then ranking them alphabetically. However, the focus of codebook cryptanalysis is the comparative frequency of the individual code elements matching the same frequency of letters within the plaintext messages using frequency analysis. In the above example, the code group, 1001, 1002, 1003, might occur more than once and that frequency might match the number of times that ABC occurs in plain text messages.

(In the past, or in non-technical contexts, code and cipher are often used to refer to any form of encryption).

One- and two-part codes

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Codes are defined by "codebooks" (physical or notional), which are dictionaries of codegroups listed with their corresponding plaintext. Codes originally had the codegroups assigned in 'plaintext order' for convenience of the code designed, or the encoder. For example, in a code using numeric code groups, a plaintext word starting with "a" would have a low-value group, while one starting with "z" would have a high-value group. The same codebook could be used to "encode" a plaintext message into a coded message or "codetext", and "decode" a codetext back into plaintext message.

In order to make life more difficult for codebreakers, codemakers designed codes with no predictable relationship between the codegroups and the ordering of the matching plaintext. In practice, this meant that two codebooks were now required, one to find codegroups for encoding, the other to look up codegroups to find plaintext for decoding. Such "two-part" codes required more effort to develop, and twice as much effort to distribute (and discard safely when replaced), but they were harder to break. The Zimmermann Telegram in January 1917 used the German diplomatic "0075" two-part code system which contained upwards of 10,000 phrases and individual words.[3]

One-time code

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See also: One-time pad

A one-time code is a prearranged word, phrase or symbol that is intended to be used only once to convey a simple message, often the signal to execute or abort some plan or confirm that it has succeeded or failed. One-time codes are often designed to be included in what would appear to be an innocent conversation. Done properly they are almost impossible to detect, though a trained analyst monitoring the communications of someone who has already aroused suspicion might be able to recognize a comment like "Aunt Bertha has gone into labor" as having an ominous meaning. Famous example of one time codes include:

  • In the Bible, Jonathan prearranges a code with David, who is going into hiding from Jonathan's father, King Saul. If, during archery practice, Jonathan tells the servant retrieving arrows "the arrows are on this side of you," it's safe for David to return to court, if the command is "the arrows are beyond you," David must flee.[4]
  • "One if by land; two if by sea" in "Paul Revere's Ride" made famous in the poem by Henry Wadsworth Longfellow
  • "Climb Mount Niitaka" - the signal to Japanese planes to begin the attack on Pearl Harbor
  • During World War II the British Broadcasting Corporation's overseas service frequently included "personal messages" as part of its regular broadcast schedule. The seemingly nonsensical stream of messages read out by announcers were actually one time codes intended for Special Operations Executive (SOE) agents operating behind enemy lines. An example might be "The princess wears red shoes" or "Mimi's cat is asleep under the table". Each code message was read out twice. By such means, the French Resistance were instructed to start sabotaging rail and other transport links the night before D-day.
  • "Over all of Spain, the sky is clear" was a signal (broadcast on radio) to start the nationalist military revolt in Spain on July 17, 1936.

Sometimes messages are not prearranged and rely on shared knowledge hopefully known only to the recipients. An example is the telegram sent to U.S. President Harry Truman, then at the Potsdam Conference to meet with Soviet premier Joseph Stalin, informing Truman of the first successful test of an atomic bomb.

"Operated on this morning. Diagnosis not yet complete but results seem satisfactory and already exceed expectations. Local press release necessary as interest extends great distance. Dr. Groves pleased. He returns tomorrow. I will keep you posted."

Idiot code

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An idiot code is a code that is created by the parties using it. This type of communication is akin to the hand signals used by armies in the field.

Example: Any sentence where 'day' and 'night' are used means 'attack'. The location mentioned in the following sentence specifies the location to be attacked.

  • Plaintext: Attack X.
  • Codetext: We walked day and night through the streets but couldn't find it! Tomorrow we'll head into X.

An early use of the term appears to be by George Perrault, a character in the science fiction book Friday[5] by Robert A. Heinlein:

The simplest sort [of code] and thereby impossible to break. The first ad told the person or persons concerned to carry out number seven or expect number seven or it said something about something designated as seven. This one says the same with respect to code item number ten. But the meaning of the numbers cannot be deduced through statistical analysis because the code can be changed long before a useful statistical universe can be reached. It's an idiot code... and an idiot code can never be broken if the user has the good sense not to go too often to the well.

Terrorism expert Magnus Ranstorp said that the men who carried out the September 11 attacks on the United States used basic e-mail and what he calls "idiot code" to discuss their plans.[6]

Cryptanalysis of codes

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While solving a monoalphabetic substitution cipher is easy, solving even a simple code is difficult. Decrypting a coded message is a little like trying to translate a document written in a foreign language, with the task basically amounting to building up a "dictionary" of the codegroups and the plaintext words they represent.

One fingerhold on a simple code is the fact that some words are more common than others, such as "the" or "a" in English. In telegraphic messages, the codegroup for "STOP" (i.e., end of sentence or paragraph) is usually very common. This helps define the structure of the message in terms of sentences, if not their meaning, and this is cryptanalytically useful.

Further progress can be made against a code by collecting many codetexts encrypted with the same code and then using information from other sources

  • spies
  • newspapers
  • diplomatic cocktail party chat
  • the location from where a message was sent
  • where it was being sent to (i.e., traffic analysis)
  • the time the message was sent,
  • events occurring before and after the message was sent
  • the normal habits of the people sending the coded messages
  • etc.

For example, a particular codegroup found almost exclusively in messages from a particular army and nowhere else might very well indicate the commander of that army. A codegroup that appears in messages preceding an attack on a particular location may very well stand for that location.

Cribs can be an immediate giveaway to the definitions of codegroups. As codegroups are determined, they can gradually build up a critical mass, with more and more codegroups revealed from context and educated guesswork. One-part codes are more vulnerable to such educated guesswork than two-part codes, since if the codenumber "26839" of a one-part code is determined to stand for "bulldozer", then the lower codenumber "17598" will likely stand for a plaintext word that starts with "a" or "b". At least, for simple one part codes.

Various tricks can be used to "plant" or "sow" information into a coded message, for example by executing a raid at a particular time and location against an enemy, and then examining code messages sent after the raid. Coding errors are a particularly useful fingerhold into a code; people reliably make errors, sometimes disastrous ones. Planting data and exploiting errors works against ciphers as well.

  • The most obvious and, in principle at least, simplest way of cracking a code is to steal the codebook through bribery, burglary, or raiding parties — procedures sometimes glorified by the phrase "practical cryptography" — and this is a weakness for both codes and ciphers, though codebooks are generally larger and used longer than cipher keys. While a good code may be harder to break than a cipher, the need to write and distribute codebooks is seriously troublesome.

Constructing a new code is like building a new language and writing a dictionary for it; it was an especially big job before computers. If a code is compromised, the entire task must be done all over again, and that means a lot of work for both cryptographers and the code users. In practice, when codes were in widespread use, they were usually changed on a periodic basis to frustrate codebreakers, and to limit the useful life of stolen or copied codebooks.

Once codes have been created, codebook distribution is logistically clumsy, and increases chances the code will be compromised. There is a saying that "Three people can keep a secret if two of them are dead," (Benjamin Franklin - Wikiquote) and though it may be something of an exaggeration, a secret becomes harder to keep if it is shared among several people. Codes can be thought reasonably secure if they are only used by a few careful people, but if whole armies use the same codebook, security becomes much more difficult.

In contrast, the security of ciphers is generally dependent on protecting the cipher keys. Cipher keys can be stolen and people can betray them, but they are much easier to change and distribute.

Superencipherment

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It was common to encipher a message after first encoding it, to increase the difficulty of cryptanalysis. With a numerical code, this was commonly done with an "additive" - simply a long key number which was digit-by-digit added to the code groups, modulo 10. Unlike the codebooks, additives would be changed frequently. The famous Japanese Navy code, JN-25, was of this design.

References

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Sources

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  • Kahn, David (1996). The Codebreakers : The Comprehensive History of Secret Communication from Ancient Times to the Internet. Scribner.
  • Pickover, Cliff (2000). Cryptorunes: Codes and Secret Writing. Pomegranate Communications. ISBN 978-0-7649-1251-1.
  • Boak, David G. (July 1973) [1966]. "Codes" (PDF). A History of U.S. Communications Security; the David G. Boak Lectures, Vol. I (2015 declassification review ed.). Ft. George G. Meade, MD: U.S. National Security Agency. pp. 21–32. Retrieved 2017-04-23.
  • American Army Field Codes In the American Expeditionary Forces During The First World War, William Friedman, U.S. War Department, June 1942. Exhibits many examples in its appendix, including a "Baseball code" (p. 254)

See also

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Wikimedia Commons has media related to Codebooks.
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Code (cryptography)

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In cryptography, a is a that substitutes words, phrases, sentences, or other meaningful units of text with arbitrary code groups—such as numbers, letters, or symbols—according to a predefined , thereby obscuring the message's semantic content. This method operates at the level of meaning, distinguishing it from ciphers, which systematically transform individual letters or symbols through substitution or transposition rules without regard to semantics. Codes are typically designed for brevity, security, or efficiency in transmission, often requiring both sender and receiver to possess identical codebooks for encoding and decoding. The use of codes dates back to antiquity, with early examples appearing in diplomatic and contexts to protect sensitive communications, such as ancient Egyptian hieroglyphic substitutions or Greek signals. By the , more structured systems emerged; the first known nomenclator—a hybrid code combining substitution alphabets with arbitrary symbols for common words and phrases—was compiled around 1379 by Gabriele de Lavinde at the request of . Nomenclators became the dominant cryptographic tool in from the 15th to the mid-19th century, evolving from simple lists to complex dictionaries used by monarchs, diplomats, and spies to encode political intrigues and state secrets. Notable instances include the of , a nomenclator that remained unbroken for over two centuries until its solution in 1893. In the 19th and early 20th centuries, codes gained practical importance in commercial and military applications, particularly with the advent of , where codebooks like the Commercial Code of 1870 abbreviated lengthy messages to minimize transmission costs. During and II, nations relied heavily on codebooks for secure radio and cable communications; for example, the U.S. and employed extensive code systems alongside ciphers, though many were vulnerable to due to repetitive use and interception. Types of codes include one-part systems, where code groups directly correspond to plaintext units in a single sequence, and two-part systems, which separate encoding and decoding indices for added security against . Often, codes were "superenciphered" by applying a overlay to the code groups, enhancing resistance to interception. Although traditional codes have largely been supplanted by digital ciphers in contemporary , their principles influenced early and remain relevant in specialized fields like secure and error-correcting systems. In modern contexts, the term "code" sometimes refers to code-based cryptography, a post-quantum approach leveraging error-correcting codes like those in the for public-key encryption resistant to quantum attacks; as of 2024, candidates like Classic McEliece remain under evaluation by NIST for standardization. However, this usage is distinct from classical codes and reflects the evolving intersection of and cryptographic security.

Introduction

Definition

In cryptography, a is a method of that operates at the semantic level by mapping words, phrases, or ideas to arbitrary code groups, such as sequences of numbers, letters, or symbols, thereby concealing the message's meaning through substitution of larger linguistic units rather than individual letters. This substitution preserves the overall structure and intent of the message while replacing meaningful elements with non-semantic equivalents, distinguishing codes from other techniques that focus on character-level transformations. The essential tool for implementing a is the , a document or systematic arrangement that provides a for both encoding (converting to code groups) and decoding (reversing the process). typically feature an encoding section organized by entries and a decoding section sorted by code groups, ensuring efficient use by authorized parties. Common formats for code groups include five-letter combinations, such as "EZNLJ" representing "," or sequences like "EZNYZ" denoting the number "500," or "EZNZA" for phrases such as "23 knots." Codes offer advantages in communication, particularly brevity in transmission by condensing lengthy phrases into short, fixed-length groups, which historically reduced costs in systems like where charges were per word or character. Additionally, they facilitate the handling of proper names, numbers, or specialized terms without alteration or phonetic spelling, allowing direct substitution that maintains accuracy and avoids ambiguities in encoding.

Distinction from Ciphers

In , ciphers and codes represent distinct approaches to concealing information, with ciphers operating on individual letters, characters, or bits through systematic algorithmic rules such as substitution or transposition, while codes replace entire meaningful units like words or phrases with predefined symbols or groups. This difference in granularity underscores a core structural contrast: ciphers treat text as syntactic symbols of fixed length, applying uniform transformations without regard to semantic content, whereas codes target variable-length semantic elements, often drawing from structures. Methodologically, ciphers rely on mathematical universality and shared keys to enable reversible transformations via algorithms, making them non-dependent on exhaustive lists and suitable for automation, in contrast to codes, which necessitate pre-shared codebooks for lookup-based substitutions and lack inherent algorithmic generality. Although overlaps occur in practice—such as when codes incorporate cipher-like substitutions for sub-elements—the fundamental distinction lies in this semantic versus syntactic focus, where codes emphasize meaning preservation through direct mappings and ciphers prioritize structural obfuscation. Historically, early cryptographic practices often blurred these lines, with systems like nomenclators combining codebook elements and simple substitutions, but modern definitions have solidified the separation to reflect their divergent operational principles. Practically, codes offer greater flexibility for encoding nuances and compressing messages, as seen in their utility for efficient transmission in constrained environments, yet they are more challenging to automate due to the scale of codebooks required for comprehensive coverage. In comparison, ciphers' rule-based nature facilitates computational implementation but demands careful to maintain security.

History

Early Development

The earliest precursors to cryptographic codes emerged in ancient times, with the Spartan serving as a notable example around the 5th century BCE. This device involved wrapping a strip of around a cylindrical baton to inscribe a message in a transposed form; upon unwrapping, the text appeared as a jumbled sequence that required the matching baton for proper alignment and reading. While primarily a transposition technique and thus more cipher-like than a true semantic code, the scytale demonstrated an early systematic approach to obscuring messages for during the Peloponnesian Wars. During the medieval period, codes evolved in diplomatic contexts to safeguard sensitive information, with the first known nomenclator compiled around 1379 by Gabriele de Lavinde at the request of . Nomenclators, hybrid systems combining substitution alphabets with arbitrary symbols for common words and phrases, became the dominant cryptographic tool in from the 15th to the mid-19th century. In the , French diplomat contributed significantly through his 1586 treatise Traicté des chiffres, which explored substitution methods. By the 17th century, such techniques advanced under state patronage; established France's first formal cipher bureau, known as the , around 1633 and utilized printed codebooks for protecting diplomatic and state secrets, with cryptographer Antoine Rossignol developing the earliest known two-part code in 1640 to enhance security through layered substitutions. The Rossignol family further advanced codes with the of , a complex nomenclator created around 1669 that remained unbroken for over two centuries until its solution in 1893. The 18th and 19th centuries marked a pivotal shift with the proliferation of printed codebooks, driven by military needs and the emergence of . Early naval signaling systems, such as Sir Home Popham's Telegraphic Signals, or Maritime Vocabulary, introduced numeric codes for brevity in fleet communications, laying groundwork for standardized formats. The of the electric telegraph in 1837 spurred commercial adaptations; by 1845, Francis O.J. Smith's Secret Corresponding Vocabulary exemplified these advancements, assigning numeric indices to common phrases to minimize transmission length and costs—such as reducing a 20-word that might cost $100 to a single code group. Telegraphy profoundly influenced code standardization, promoting numeric systems for efficiency across military and commercial spheres. These codes replaced verbose with concise symbols, enabling rapid global exchanges while maintaining secrecy, and set precedents for brevity in later cryptographic practices. For instance, merchant shipping codes like Frederick Marryat's 1817 Code of Signals used four-digit numbers to encode instructions, reflecting a broader trend toward modular, distributable codebooks that balanced security and practicality.

Major Historical Uses

One of the most notable applications of codes in was the Zimmermann Telegram, sent on January 16, 1917, by German Foreign Secretary to the German ambassador in via U.S. diplomatic channels. The message, proposing a German-Mexican against the in exchange for territorial concessions, was enciphered using Code 13040, a two-part containing approximately 10,000 numbered entries for words and phrases. British cryptanalysts in intercepted the telegram, recovered portions of the from previous captures, and decrypted it within days, revealing its contents and influencing U.S. toward joining the Allies. In , codes remained essential for secure military signaling, particularly in naval operations. The relied on JN-25, its primary operational codebook system introduced in 1939, which featured over 45,000 five-digit groups superenciphered with daily-changing additives to protect fleet movements and strategies. Meanwhile, the Allies utilized one-time codes embedded in broadcasts to communicate with European Resistance networks, employing unique, pre-arranged phrases broadcast as innocuous "personal messages" to trigger actions like sabotage without repetition for security. A parallel example of such one-time signaling occurred on the Axis side, when Japanese Admiral transmitted the phrase "Climb Mount Niitaka" on December 2, 1941, as the irrevocable order to proceed with the attack. Following and into the , traditional codebooks saw diminished prominence as electronic ciphers and automated systems became standard for high-volume, secure communications among state actors. However, simple idiot codes—ad hoc phrase substitutions or basic substitutions devised without formal cryptanalytic rigor—persisted in asymmetric conflicts, where low-tech operatives lacked access to sophisticated tools, such as in guerrilla operations during the or insurgent activities in later proxy wars. This shift underscored codes' retention in scenarios demanding minimal infrastructure, like field or non-state networks. Even in the digital age, rudimentary codes appeared in 21st-century terrorism, as planners for the , 2001, attacks used innocuous phrases in emails and communications to mask intentions, referring to the operation as "" to denote the coordinated hijackings. Building on 19th-century telegraph codes as for concise signaling, these historical uses illustrate codes' adaptation from diplomatic tools to wartime imperatives, though their role waned post-1940s in favor of machine-based digital ciphers for scalability and resistance to .

Core Types

One-Part Codes

One-part codes represent the simplest systematic form of cryptographic codes, employing a single in which code groups—typically numeric sequences or alphabetic strings—are assigned to plaintext words, phrases, or letters in a predictable, ordered manner that parallels the natural sequence of the plaintext itself. For instance, code groups might be allocated sequentially from a , such as 1001 for "A," 1002 for "abandon," and so on, facilitating straightforward substitution without requiring separate encoding and decoding sections. This structure often uses four- or five-digit numbers or five-letter groups arranged alphabetically or numerically to cover common terms, with options for homophones (multiple codes for frequent words) to obscure patterns. The encoding process in one-part codes involves direct lookup in the ordered sections of the , where the sender identifies the unit and replaces it with its corresponding group, preserving the message's logical flow for easy reconstruction by the recipient using the same book. This direct mapping, sometimes enhanced with basic superencipherment like adding a fixed numeric key to the code groups, prioritizes speed and simplicity over complexity. Historically, one-part codes found widespread use in early telegraphic and commercial communications to enhance and reduce transmission costs, as phrases could be compressed into brief code groups—such as the five-letter word "GULLIBLE" representing " SEIZED BY " in shipping contexts—thereby minimizing the character count in expensive wire transfers. In applications, they served for rapid signaling, as seen in early 20th-century field systems where dictionary-sequenced groups encoded operational terms. Despite their efficiency, one-part codes exhibit significant vulnerabilities due to their ordered predictability, making them highly susceptible to , where cryptanalysts exploit the parallel structure between and code sequences to identify common words or patterns. Recovery is often feasible with partial —known segments—allowing attackers to deduce mappings and reconstruct the from intercepted messages, particularly if volume is high or keys change infrequently. A hypothetical example from military signaling illustrates this: in a codebook ordered by dictionary sequence, the plaintext "Advance to position" might encode as 0456 for "advance," 7231 for "to," and 8904 for "position," enabling quick transmission but risking exposure if an analyst guesses frequent terms like "advance" based on message context and positional clues.

Two-Part Codes

Two-part codes enhance security by employing separate indices or codebooks for encoding and decoding, with plaintext entries listed in a logical order (such as alphabetical) paired with randomly assigned code groups, while the decoding index arranges those code groups in a different, non-sequential order to obscure direct correlations. For instance, a plaintext phrase like "abaft" might be encoded as the arbitrary letter group "TOGTY" in one index, with no predictable relationship to nearby entries. This randomization contrasts with the sequential predictability of one-part codes, where plaintext and code groups align in a single list, facilitating easier cryptanalytic recovery. The encoding process involves consulting the plaintext-to-code index to substitute words or phrases with their corresponding random code groups, typically numerical (e.g., five-digit numbers like 72541) or alphanumeric sequences, before transmission. Decoding requires a separate , where the received code groups are looked up independently to retrieve the original , ensuring that even if an intercept includes partial data, full recovery remains challenging without both components. This dual-table lookup eliminates the one-to-one mapping vulnerabilities inherent in simpler systems, as the non-parallel organization between indices prevents straightforward reversal. Historically, two-part codebooks often contained over 10,000 entries to cover extensive diplomatic and military vocabulary, as seen in the German Code 7500 used in 1917, which featured 10,000 alphabetically ordered phrases for encoding and numerically disarranged equivalents for decoding. This code was employed for high-stakes transmissions, including the Zimmermann Telegram sent from to Washington on January 16, 1917, proposing a secret alliance between and . Such large-scale implementations became standard in early 20th-century diplomatic , reflecting the need for comprehensive phrase coverage in international communications. A primary advantage of two-part codes lies in their resistance to basic and pattern-based attacks, as the random assignment of code groups disrupts any statistical predictability tied to plaintext order or frequency. By separating encoding and decoding processes, they reduce the risk of compromise from partial captures, making significantly more labor-intensive compared to one-part systems. Their deployment in sensitive diplomatic contexts, such as negotiations, underscores their role in safeguarding strategic secrets against interception. Despite these benefits, two-part codebooks are notably larger and more cumbersome than one-part alternatives, often doubling the physical size and printing costs due to the need for dual indices. This bulk increases logistical challenges in distribution and handling, heightening the to loss, , or during in field or diplomatic operations. Additionally, the complexity of managing separate components raises the potential for operational errors, such as mismatched indices or transmission garbles in numerical formats.

Specialized Variants

One-Time Codes

One-time codes are disposable cryptographic systems consisting of pre-arranged phrases, word groups, or entries that are used only once to convey specific, short messages, ensuring perfect secrecy through their non-reusable design. These codes function on a principle of shared secrecy, where the assigned meaning or action triggered by the phrase is known exclusively to the sender and intended recipient, eliminating detectable patterns and providing analogous to one-time pads but applied to linguistic or semantic elements rather than individual characters or digits. In historical applications, one-time codes were frequently embedded within public broadcasts to signal actions covertly during espionage and resistance operations. The British Broadcasting Corporation's French Service, during World War II, transmitted "personal messages" as part of its daily programming from 1940 onward; these innocuous-sounding phrases served as one-time signals for the French Resistance, including 1940s Maquis guerrilla groups coordinating sabotage and intelligence efforts against German occupation forces. A prominent example occurred with the broadcast of the first stanza of Paul Verlaine's poem, "Les sanglots longs des violons de l'automne," on June 1, 1944, followed by the second stanza, "Blessent mon cœur d'une langueur monotone," on June 5, 1944, alerting resistance networks to commence widespread disruptions in support of the impending D-Day landings, mobilizing thousands without alerting Axis monitors. Prearranged one-time code phrases also featured in diplomatic and military prelude signaling. In late 1941, Japanese Imperial and Foreign Ministry communications incorporated the phrase "higashi no kaze ame" (east wind rain) within routine weather reports as a one-time indicator of severed relations and imminent hostilities with the , directly preceding the attack on December 7 and alerting overseas posts to destroy sensitive documents. Theoretically, one-time codes offer unbreakable security when the shared secret remains intact and the phrase is never reused, as cryptanalysts lack sufficient material for or , rendering decryption impossible without prior knowledge of the code's meaning. However, practical vulnerabilities include the capture of recipients leading to premature disclosure, operator errors in phrasing or reception that could expose intent, or forced under , which compromised several resistance signals during wartime operations. Limitations of one-time codes arise from their design for brevity and specificity, making them unsuitable for lengthy or improvised communications that require ongoing or detailed content. Precise synchronization is essential, as recipients must monitor designated channels—such as at fixed evening slots—without missing the single transmission, a challenge exacerbated by wartime jamming or unreliable reception in occupied territories.

Idiot Codes

Idiot codes, also known as simple substitution codes in terminology, are informal cryptographic methods that employ ad-hoc phrases, symbols, or words whose meanings rely entirely on pre-arranged shared between a small group of users. Unlike structured systems, these codes lack a formal or book, allowing communicators to substitute everyday language with innocuous terms that hold specific operational significance only to insiders—for instance, referring to "apples" as grenades or "" as an EU entry visa in operations. This structure makes them particularly suited to low-tech, low-resource environments where rapid, covert signaling is essential without the need for complex tools or training. These codes are typically created on-the-fly by small, trusted groups, drawing from personal knowledge, cultural references, or immediate circumstances to ensure mutual understanding without documentation that could be compromised. In practice, they have been employed in for coordinating ambushes or movements in resource-scarce settings, such as during insurgent operations where fighters use local idioms to signal threats without alerting patrols. Similarly, in , operatives have utilized such codes in and phone communications; for example, the phrase "the wedding cake is ready" served as a signal for imminent attacks, including in plots like the 2009 New York City subway bombing attempt. They also appear in informal signaling among prisoners or operatives to evade , as noted in jihadist recruitment handbooks that warn against detection. The primary advantages of idiot codes lie in their simplicity and adaptability: they can be implemented quickly in the field with minimal preparation, rendering them ideal for dynamic, high-stakes scenarios, and they are exceedingly difficult for outsiders to decipher without the underlying contextual , often appearing as benign . However, their reliance on trust introduces significant vulnerabilities; they can be readily compromised through by a group member or by intercepting multiple communications that reveal recurring patterns, allowing analysts to infer meanings through or contextual clues. In or monitored environments, such codes have been broken when authorities identify consistent phrasing across intercepted messages, underscoring their fragility in sustained operations.

Codebook Mechanics

Design Principles

The design of cryptographic codebooks prioritizes and obscurity to thwart cryptanalytic attacks, ensuring that no discernible patterns emerge from linguistic or structural cues. Code groups are selected as random, non-phonetic symbols, often consisting of five-letter nonsense words or arbitrary numeral sequences, to eliminate predictable associations with frequencies such as common digraphs like "EN" or "TH". This approach, exemplified in historical systems where groups like "parmesiel" or "oshurmi" represent entire phrases, minimizes the risk of partial recovery through or garble correction. Groups are further engineered to differ by at least two characters, reducing transmission errors while maintaining . Indexing methods emphasize a balanced distribution of code groups across the to prevent frequency biases that could reveal probabilities. In one-part codes, entries are ordered alphabetically with corresponding code groups listed sequentially, while two-part codes randomize the code group order, necessitating a separate decoding index for added . Nulls—meaningless filler groups—and dummies—deceptive inserts—are incorporated at rates of 25% or more per message to obscure true content length and disrupt statistical patterns, often prefixed with indicators like dashes for identification. These elements, drawn from low-frequency letters or arbitrary sequences, are distributed unevenly to simulate natural variability without compromising . Coverage in codebooks must be comprehensive, tailored to the operational domain such as , encompassing specialized vocabulary like tactical terms, place names, and personnel designations, alongside idioms, phrases, and numerical values for quantities or coordinates. For instance, systems include homophones—multiple groups for high-frequency words like "attack"—to flatten usage statistics, as well as provisions for unlisted terms using or letter codes. This ensures fluid expression of complex ideas, such as "advance to position at 1430 hours" rendered as a single group, while extending to non-verbal elements like or dates. Size considerations involve inherent trade-offs between enhanced from expansive books—containing thousands of groups for exhaustive coverage—and practical , including portability and rapid in field conditions. Larger codebooks, such as those with 10,000 entries across 50 pages, provide greater depth and variant options to counter interception but increase weight and lookup time, often limiting them to headquarters use; smaller variants, like pocket-sized editions with 800-3,000 groups, prioritize mobility for frontline operators at the cost of reduced . Standard five-letter or five-figure groups strike a balance, yielding millions of possible combinations while keeping volumes manageable. Erasure techniques focus on irreversible destruction or alteration of codebooks post-use to prevent compromise, particularly for one-time or short-lived systems. Physical methods include burning or shredding paper copies, as practiced in operations where worksheets and pads were incinerated immediately after encipherment to eliminate traces. For reusable books, alteration via overlays, detachable pages, or chemical erasure ensures periodic renewal without full replacement, while one-time pads mandate complete disposal after a single cycle to uphold perfect secrecy. These protocols, enforced through operational discipline, mitigate risks from capture or defection.

Implementation and Distribution

In classical cryptography, codebooks were typically implemented as physical printed documents, often in the form of bound volumes containing lookup tables that mapped words or phrases to arbitrary code groups such as sequences of letters or numbers. These formats allowed for manual encoding and decoding but posed logistical challenges in settings, where bulky volumes could weigh several pounds and required durable materials like cross-section paper for grids or aluminum components in mechanical variants to withstand field conditions. To address space and portability issues, alternatives such as microfilm or compact matrices were sometimes employed, though printed books remained the standard for their ease of reference during operations. Distribution of codebooks relied on secure, controlled methods to ensure only authorized personnel received identical copies, often managed centrally by offices that produced editions for peace and wartime use. Couriers or trusted channels were the primary means of delivery, with issuance based on predefined allowances to units, preventing through prearranged routes and strict accounting procedures. Synchronization for updates involved timely replacement of editions, typically coordinated via indicators in messages to align encoding keys without direct transmission of the itself. Operational protocols emphasized precise handling to maintain security and accuracy, including rules for encoding where was substituted using prearranged keys or matrices, followed by grouping messages into fives for transmission compatibility. Error-checking mechanisms, such as reciprocal tables or 2-letter differentials in groups, helped detect transmission mistakes, while recovery from loss required immediate reporting to higher authorities and fallback to reserve editions or pre-shared designs. Clerks were trained to avoid mixing with and to use indicators for key selection, ensuring both sender and receiver could reconstruct messages without ambiguity. In modern contexts, codebooks have largely been supplanted by electronic ciphers due to their , though digital adaptations persist in niche secure applications as encrypted lists within specialized software or apps, where access is controlled via key-derived permissions. These implementations prioritize computational efficiency over physical handling but remain uncommon, as algorithmic methods better suit high-volume data protection. A primary risk in codebook deployment was theft or capture, which could expose the entire substitution system and compromise all related communications, necessitating protocols like destruction of materials upon threat and frequent key changes to limit damage. Physical safeguarding, including limited distribution and on-site guarding, was essential to mitigate interception during transit or storage.

Advanced Techniques

Superencipherment

Superencipherment is a cryptographic technique that involves applying an additional layer of encipherment—typically a substitution or additive —to the output of a after the plaintext has been encoded into code groups using a . This process disguises the code groups, which are usually numeric or alphanumeric sequences, by transforming them further; for instance, in additive superencipherment, a random additive value selected from a prepared table is arithmetically combined with each code group, often a base like 10,000, to produce the final . The recipient must first reverse the superencipherment to recover the original code groups before applying the codebook to decode the message. The primary purpose of superencipherment is to obscure identifiable patterns in the -derived groups, such as recurring sequences or frequency biases, thereby complicating cryptanalytic attacks like or code recovery. It enforces a two-stage decryption process, enhancing security by requiring possession of both the and the superencipherment key, which could be a table of additives or a substitution mapping. A prominent historical example is the Japanese Navy's JN-25 code system used during , where 5-digit code groups from the were superenciphered by adding 5-digit values drawn from a 300-page additive book, with the starting point indicated by an indicator in the message preamble. The system underwent multiple revisions, including changes to the superencipherment additives in 1942, which temporarily delayed Allied codebreaking efforts but ultimately allowed U.S. Navy cryptanalysts at Station Hypo to recover portions of the system, aiding operations such as the . Variants of superencipherment include homophonic approaches, where each code group is mapped to multiple possible cipher substitutes proportional to its expected frequency of use, thereby flattening the overall symbol distribution in the ciphertext to resist statistical attacks. Despite its benefits, superencipherment introduces limitations by increasing the operational complexity, as manual addition or substitution steps are prone to human error, potentially elevating transmission error rates and requiring meticulous key synchronization between sender and receiver.

Hybrid Code-Cipher Systems

Hybrid code-cipher systems integrate codebooks, which substitute high-level phrases or concepts with concise symbols for semantic compression and obfuscation, with ciphers that apply mathematical transformations to the resulting code output for additional and . This combination extends beyond basic superencipherment by embedding code elements directly into cipher processes or using codes to preprocess messages before algorithmic encipherment, enhancing overall security through layered semantic and syntactic protection. Early examples include nomenclators from the , which merged small codebooks for key terms with homophonic substitution ciphers to balance brevity and resistance to . In post-World War II , hybrid systems often employed code preambles—short sequences indicating message type, priority, or routing—followed by ed body text to streamline processing while maintaining deniability. For instance, U.S. diplomatic communications in the late used indicators to select keys dynamically before applying rotor-based encipherment, allowing rapid adaptation to threats without full exposure. These approaches leveraged for in high-volume traffic, while provided mathematical robustness against partial intercepts. The primary advantages of such hybrids lie in combining the semantic depth of , which reduce message length and obscure meaning through contextual substitution, with the probabilistic strength of , yielding systems more resilient to partial breaks than either alone. This duality supports robustness in noisy or intercepted channels, as code errors may not propagate like cipher bit flips. In modern applications, hybrid systems persist in low-bandwidth military operations, where codebooks compress tactical data before AES encipherment to minimize transmission overhead on satellite or RF links. For example, AI-assisted codebook generation creates dynamic mappings tailored to mission-specific jargon, which are then enciphered using AES-256 for secure dissemination in denied environments, quadrupling effective bandwidth compared to uncompressed plaintext. Steganography further integrates these by hiding code phrases within innocuous media, such as embedding codeword sequences in image metadata before symmetric cipher application, evading detection in digital channels. Despite these benefits, challenges include dual key management—synchronizing codebook updates with cipher keys across distributed users—and vulnerability to side-channel attacks if layers are not perfectly isolated. Usage has declined with the dominance of pure digital ciphers like AES in high-throughput networks, though hybrids retain niche value in bandwidth-constrained or hybrid analog-digital scenarios.

Cryptanalysis

Breaking Methods

Frequency analysis serves as a foundational technique in codebook , where the relative frequencies of code groups in intercepted messages are compared to expected frequencies of elements, such as common words, phrases, or semantic clusters, to identify probable mappings. Unlike simple substitution ciphers, codes often encode multi-letter or multi-word units, requiring adjustments for semantic clustering—frequent code groups may correspond to high-utility phrases like orders or salutations, revealing patterns when aggregated across multiple messages. This method exploits the non-random nature of , where certain concepts recur predictably, allowing cryptanalysts to hypothesize and test entries based on statistical deviations. Crib-based attacks involve hypothesizing likely phrases, known as , and aligning them with sequences of code groups in the to deduce mappings within the . Common include stereotypical expressions such as "attack at dawn" or standard message preambles, which, when matched against message structures, enable the recovery of associated symbols through and positional verification. This approach leverages contextual predictability in communications, iteratively refining the reconstruction as more alignments succeed, particularly effective when overlap across multiple interceptions. Known-plaintext attacks capitalize on access to partial or complete alongside corresponding , often from captured , recovered fragments, or collateral intelligence, to directly map code groups to their meanings. Once a segment of is confirmed, the associated code groups can be cataloged, allowing to similar patterns in other messages and accelerating the compromise of the entire system. This method is particularly potent when partial recoveries expose systematic encodings, such as numerical sequences for dates or locations, enabling broader decryption without full seizure. Error exploitation targets procedural lapses by operators, such as the reuse of groups for the same across messages, inadvertent inclusion of predictable nulls (e.g., low-frequency symbols like certain letters or numbers inserted for ), or transmission anomalies that disrupt intended . Repeated equivalents—where the same group appears in identical contexts—can betray mappings, while operator habits like omitting nulls or duplicating phrases introduce exploitable redundancies. These human-induced vulnerabilities often amplify statistical weaknesses, providing entry points for deeper analysis without relying solely on volume of traffic. Computational aids enhance cryptanalysis by automating and statistical processing across large volumes of data, using tools like matrices for digraphic substitutions, tables for variant reconstructions, and software for correlating code group frequencies with linguistic models. Modern implementations employ algorithms to scan for recurring sequences and simulate hypotheses, significantly reducing manual effort in identifying semantic clusters or crib alignments. These methods, building on traditional aids like sliding strips and additive tables, enable efficient handling of extensive through probabilistic matching and error-tolerant searches.

Case Studies

The Zimmermann Telegram of January 1917 exemplified the vulnerability of diplomatic codebooks to partial recoveries and contextual cribs. British cryptanalysts in intercepted the message, encoded in the German Foreign Office's Code No. 13040, and decrypted it using fragments of the codebook salvaged from the sunken German cruiser SMS Magdeburg in 1914, combined with cribs derived from predictable diplomatic phrasing about U.S. neutrality. The revealed proposal for a German-Mexican alliance against the , including offers of , , and , was publicly disclosed on March 1, 1917, galvanizing American public opinion and prompting U.S. entry into on April 6. This break demonstrated how even robust codebooks could fail against accumulated intelligence from prior captures. The U.S. Navy's cryptanalysis of Japan's JN-25 naval code during World War II illustrated iterative breaking amid system changes. JN-25 employed a codebook of approximately 45,000 five-digit groups superenciphered with daily additives from an additive book, but U.S. analysts at Station HYPO in Pearl Harbor exploited "depths"—multiple messages enciphered with the same additive—to recover values through subtraction and frequency analysis of repeated code groups. After Japan introduced a new additive table in May 1942, partial recoveries from accumulated traffic allowed decryption of key messages identifying "AF" as Midway Atoll and predicting an imminent attack, enabling Admiral Chester Nimitz to ambush the Japanese fleet. The resulting U.S. victory at the Battle of Midway on June 4-7, 1942, shifted the Pacific War's momentum, sinking four Japanese carriers with minimal losses. Allied resistance codes in occupied , particularly those using BBC "personal messages" as one-time phrases, were compromised primarily through agent captures rather than technical flaws alone. In the Dutch operation known as (1941-1944), the German captured SOE agent Hubertus Lauwers in March 1942 along with his radio and code materials, enabling them to impersonate him and decrypt subsequent transmissions using revealed procedures and phrase mappings tied to . This led to the arrest of 54 agents and infiltration of resistance networks, as the Germans fed false intelligence to London while avoiding detection of the compromise. The operation's success highlighted how physical security breaches could nullify the theoretical security of one-time systems, resulting in disrupted efforts across the until its exposure in late 1943. Post-9/11 investigations into 's communications uncovered rudimentary phrase substitutions and contextual signals, such as "" for major attacks, decrypted through contextual analysis of seized documents and intercepts rather than sophisticated . U.S. intelligence reviewed captured materials from , including notebooks and electronic files, to map codewords against known operational patterns from detained operatives. This approach revealed planning details for the hijackings but exposed the limitations of simplistic coding, which relied on shared cultural knowledge vulnerable to post-capture contextual decryption. In contrast to these classical vulnerabilities, modern code-based cryptography, such as the , leverages error-correcting codes to provide public-key encryption resistant to quantum , where breaking the system would require solving hard problems in rather than exploiting semantic patterns or operator errors. These historical breaks, often aided briefly by general techniques like for expected phrases, emphasize the necessity of regular code changes to counter evolving threats from partial compromises and human factors. Failure to update systems periodically, as seen in prolonged use of vulnerable codebooks, amplified strategic impacts, from hastening U.S. involvement in to enabling pivotal victories and network collapses.

References

  1. https://www.nsa.gov/portals/75/documents/news-features/declassified-documents/friedman-documents/publications/ACC21609/41768429080752.pdf
  2. https://www.khanacademy.org/computing/computer-science/cryptography/ciphers/a/ciphers-vs-codes
  3. https://pi.math.cornell.edu/~morris/135/timeline.html
  4. https://www.thesslstore.com/blog/the-great-cipher/
  5. https://utimaco.com/service/knowledge-base/post-quantum-cryptography/what-code-based-cryptography
  6. https://csrc.nist.gov/projects/post-quantum-cryptography
  7. https://www.johndcook.com/blog/2019/03/23/code-based-cryptography/
  8. https://cryptiana.web.fc2.com/code/nomenclator.htm
  9. https://www.exploratorium.edu/explore/secret-language
  10. https://csrc.nist.gov/glossary/term/codebook
  11. http://www.quadibloc.com/crypto/pp0104.htm
  12. http://www.cs.columbia.edu/~cs4hs/talks/codebooks.pdf
  13. https://www.cerias.purdue.edu/education/k-12/teaching_resources/lessons_presentations/cryptology.html
  14. https://www.nsa.gov/portals/75/documents/news-features/declassified-documents/friedman-documents/reports-research/FOLDER_535/41772109081119.pdf
  15. https://www.osti.gov/biblio/1671059
  16. https://www.nsa.gov/portals/75/documents/news-features/declassified-documents/friedman-documents/publications/ACC15281/41785109082412.pdf
  17. https://www.cs.columbia.edu/~smb/papers/codebooks.pdf
  18. https://www.archives.gov/milestone-documents/zimmermann-telegram
  19. https://cosec.bit.uni-bonn.de/fileadmin/user_upload/publications/pubs/gat07a.pdf
  20. https://www.nsa.gov/portals/75/documents/about/cryptologic-heritage/historical-figures-publications/publications/wwii/west_wind_clear.pdf
  21. https://www.bbc.com/historyofthebbc/100-voices/ww2/secret-war
  22. https://www.nsa.gov/portals/75/documents/news-features/declassified-documents/friedman-documents/publications/FOLDER_238/41748889078809.pdf
  23. https://histocrypt.org/past/2018/assets/HistoCrypt-2018-Codes-Nomenclator.pdf
  24. https://www.nsa.gov/portals/75/documents/news-features/declassified-documents/friedman-documents/lectures-speeches/folder_198/41766889080599.pdf
  25. https://www.bbc.com/news/entertainment-arts-49562570
  26. https://www.warhistoryonline.com/world-war-ii/the-bbcs-messages.html
  27. https://wwii-netherlands-escape-lines.com/links-to-other-escape-and-evasion-websites/world-war-ii-bbc-secret-messages-to-resistance/
  28. https://www.dday-overlord.com/en/bbc-messages-announcing-d-day
  29. https://warfarehistorynetwork.com/the-winds-message-controversy-the-intelligence-that-predicted-pearl-harbor/
  30. https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/s/statement-regarding-winds-message.html
  31. https://www.researchgate.net/publication/292976966_International_Relations_and_Security_in_the_Digital_Age
  32. https://abcnews.go.com/Blotter/zazi-arrest-nyc-terror-plot-suspect-bought-chemicals/story?id=8662211
  33. https://jamestown.org/brief/briefs-203/
  34. https://ia601800.us.archive.org/4/items/7-the-code-breaker/7-%20The%20Code%20Breaker.pdf
  35. https://www.nsa.gov/portals/75/documents/news-features/declassified-documents/friedman-documents/publications/FOLDER_239/41748809078800.pdf
  36. https://www.cryptologichistory.org/archive/a-general-introduction-to-the-japanese-jn-25
  37. https://cryptologicfoundation.org/file_download/abe98456-86ec-4135-9a49-fe87bb2abf4b
  38. https://www.cnrs-scrn.org/northern_mariner/vol12/tnm_12_1_17-37.pdf
  39. https://www.probabilisticworld.com/birth-evolution-cryptographic-codes/
  40. http://practicalcryptography.com/ciphers/codes-and-nomenclators-cipher/
  41. https://www.tandfonline.com/doi/full/10.1080/02684527.2023.2269512
  42. https://www.cs.columbia.edu/~smb/talks/fc-codebooks.pdf
  43. https://www.atombeamtech.com/
  44. http://strangehorizons.com/wordpress/non-fiction/articles/steganography-how-to-send-a-secret-message/
  45. https://history.state.gov/historicaldocuments/frus1917Supp01v01/d158
  46. https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/b/battle-of-midway-4-7-june-1942.html
  47. https://www.nsa.gov/History/Cryptologic-History/Historical-Events/Article-View/Article/2740680/jn-25/
  48. https://www.historynet.com/was-this-the-uks-worst-spy-failure-of-world-war-ii/
  49. https://www.tracesofwar.com/articles/4706/England-game.htm
  50. https://govinfo.library.unt.edu/911/report/911Report_Ch5.htm
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