GURPS Cryptology

Another article resurrected from my previous website. This was intended to be an article for Pyramid magazine, but was too dry or too long for acceptance there. I’ve never updated it for GURPS 4th Edition. (Perhaps playes of that system could tell me what would need to be changed.)

for GURPS 3rd Edition rules

Witeout was pretty far gone into Rio Largo’s main databank. He’d been in for over 10 seconds already, and time was running out. If only his Spanish was better! This was his first foreign run, and for the proggies, natch. The Chilentinan’s were doin’ some research in Antarctica, and Uncle Adam wanted to know whether it was bolshy or faex.

Bingo! Well, maybe bingo. Witeout found a file tagged ‘Antarctica’ but inside was shash data. Encrypted, drig! Better than nothin’, though, and it was too hot to hang around. Witeout slipped back through the Net to the good ol’ U. S. of A.

“All right, let’s hope it’s a simple cipher,” says Brian, Witeout’s player, looking up from his copy of GURPS Cyberpunk. “My Computer Programming is 17, so with the −6, I should get the real data on an 11 or less.”

“What does Computer Programming have to do with breaking codes, Brainiac? I found what you need here in High-Tech. See, roll against Mathematics−2 or IQ−8.”

“C’mon, Cryptography is in Terradyne. Hold on, I’ll look it up. (flip, flip, flip.) Here it is: Cryptography, mental hard, defaults to Mathematics−5 or IQ−6. Hmmm, but you get a +2 for simple ciphers.”

The GM cuts to the chase. “We’re going with what’s in the Basic Set. It’s got Cryptanalysis at Mathematics−3 or IQ−5. Your IQ is 13, so roll an 8 or less.”

Mark, playing the troop’s chiller, tries to decide which PC to zero first.

GURPS 3rd Ed Rules

Many GURPS books make reference to codes and ciphers. If they mention a skill to be used, it is variously called Cryptanalysis, Cryptography, or Cryptology. Its defaults vary widely, from Mathematics −2 to −5, IQ −5 to −8, or Computer Programming −6. Sometimes it’s a /TL skill, sometimes it isn’t. Not particularly universal, is it?

General Cryptology

Definition of Terms

First, the terms need to be defined. Cryptology is the combination of cryptography and cryptanalysis, cryptography is the creation of cipher and code methodologies, and cryptanalysis is the set of methodologies for obtaining plaintext from ciphertext without the proper keys. What that boils down to is cryptographers figure out how to encode stuff, cryptanalysists try to break the code, and cryptologists study both activities. A cryptosystem is what the cryptographer produces and what the cryptanalysist tries to defeat.

A cryptosystem can be secure (unlikely that anyone without the key can obtain any part of the original message) in two ways. If it is secure in the information-theoretic sense, then no amount of computational power and clever approaches can ever yield the original message. This is usually an impossible level of security to obtain. More often, a cryptosystem attempts to be secure in the computationally feasible sense, so that original messages cannot be obtained with any reasonable amount of computation.

There are other related activities. Cryptanalysis and cryptography are two opposing components of the opposing methods of signal intelligence and signal security (or SIGINT for those of you with GURPS Special Ops). These signals come in two varieties, communication and electronic.

Communication security can involve steganography, traffic security, and cryptography. Steganography is the set of methodologies for concealing the fact that a message is being sent. It includes things like invisible inks, microdots, and hiding messages in background noise. Traffic security involves keeping information from being derived from the patterns of communication traffic. For example, dummy messages can be sent so that when the time comes to send a large number of real messages, it doesn’t seem as unusual.

Communication intelligence includes interception and direction-finding, traffic analysis, and cryptanalysis. Interception and direction-finding try to defeat steganography by obtaining the message or the source of the message, respectively. Traffic analysis tries to deduce facts from patterns of communication traffic.

Electronic security can involve emission security and counter-countermeasures. Emission security is simply the electronic version of steganography—trying to keep the adversary from becoming aware that electronic signals are being used, for example, by shifting radar frequencies. Counter-countermeasures are used to successfully send electronics signals when the adversary is using countermeasures to prevent them from being successfully transmitted.

Electronic intelligence includes electronic reconnaissance and countermeasures. Electronic reconnaissance involves interception and direction-finding, just like with communication intelligence. Countermeasures include techniques like radar jamming and sending false radar echoes.

Historical Cryptology

Cryptography has existed since the fifth century B.C. when the Spartans developed and used a transposition cipher (in which the letters of the message are rearranged). Cryptanalysis is much younger. The Arabians were the first to document their cryptanalytic methods, in an encyclopedia completed in 1412.

A US Navy WAVE sets the Bombe rotors prior to a run. Photo by J Brew (CC-BY-SA-2.0)

Cryptology made intermittent gains through its lifetime. Advances in other fields sometimes lead to these gains. The use of radio communications in World War I led to the growth of cryptology into a mature science. The development of powerful computers in the 1960s made some of the complex modern protocols feasible. The development of public-key cryptography in the late 1970s stimulated academic interest, since it could be widely used in non-military situations. (Public-key cryptography involves two keys, the public key for encoding and the private key for decoding.) One of the most recent advances uses quantum mechanics and Heisenberg’s uncertainty principle to send messages encoded in beams of polarized light.

GURPS Rules

Character Types

Chemist/Computer Programmer/Engineer/Gadgeteer/Physicist: This group of character types can have some impact on the development of new methods of signal security and signal intelligence, such as invisible inks, password programs, cipher wheels, and quantum encryption devices.; Most of these scientists may find it useful to know Signal Security or Signal Intelligence. These are not required as long as someone else (with the appropriate skill) is handy to specify what needs to be developed.
Cinematic Spy: The master-of-all-trades superspy certainly needs to include Cryptography in his repertoire, and probably Signal Security as well. If the Bad Guys are taking steps to keep the Important Information from the Superspy, he would do well to have Signal Intelligence and Cryptanalysis as well.
Cipher Clerk: Unlikely choices for PCs in a heroic campaign, these individuals held important positions nonetheless. Simple mistakes in their daily tasks could provide the enemy with invaluable information. They all must have Professional Skill: Cipher Clerk.
Conspirator/Diplomat/Paranoid/Scribe/Telegraph Operator: These are the character types that may routinely use cryptographic protocols in their lines of work. Even if they don’t know the science of Cryptology, many should have the Professional Skill of Cipher Clerk, so that they can send and receive secure messages with some degree of success.
Femme Fatale: As pointed out in GURPS Espionage, the femme fatale need not be a female. In espionage, these manipulators will often need to send reports or receive instructions while in the, ahem, company of important figures. Even apart from the spy world, cryptology can be handy for secret lovers—Vatsyayana’s Kama-sutra lists secret writing as one of the 64 yogas that women should know.
Historian/Theologian: Occasionally a researcher finds an encrypted historical or theological text. Most often they will only study cryptology on the job—working with more experienced cryptanalysists as the need arises.
Intelligence Analyst/Black Chamber Member/Government Investigator/Intelligence Officer: Intelligence Analyst is the generic category that all of these character types fall into. Signals are intercepted and read without the sender or recipient realizing what was going on.; Black Chambers were the NSAs of their time, intelligence-gathering entities operating in peacetime. Mail from one European country to another would be opened by members of the Black Chamber, read, resealed, and delivered. If the message inside was encrypted, it would be held long enough to decrypt it. As modern diplomatic methods evolved, so did modern cryptographic methods evolve in the Black Chambers.; Black Chambers were common in Europe in the 1700s. Russia established one as well, the Spets-Otdel, during the reign of Peter the Great. England’s Black Chamber, Austria’s Geheime Kabinets-Kanzlei, and France’s Cabinet Noir all stopped operations in the 1850s, mostly because of public opinion. The United States also operated a Black Chamber after a success with cryptolologic service in World War I. The American Black Chamber was in operation from 1919 to 1929.; These other character types do the same kind of work, just in different arenas. Government Investigators would work within a government, possibly as Internal Affairs. Intelligence Officers are military officers who do their work in wartime, intercepting enemy communications. These characters would be skilled in Cryptography, Forgery, Languages, and Signal Intelligence.
Mage: In a world where magical spells are closely guarded secrets, mages would need cryptology the same way nations do. They would want to keep their knowledge hidden while learning the knowledge that their neighbors possess. See below for an example of a mage using cryptography as part of a security plan for his grimoire.
Netrunner: Some netrunners may need to act as intelligence analysts. They can either use an expert system or take the do-it-yourself approach. Most netrunners will not have security clearance to come by either option legitimately, but there is always the black market (or a good character story and Unusual Background).

New Skills

Cryptology/TL (Mental/Very Hard) No default: This is the science of textual information security. It allows the use of both the Cryptography and Cryptanalysist skills below. If the GM wishes to retain the simplified one-skill mechanic for this avenue of study, he should ignore Cryptography and Cryptanalysis and permit players to purchase only Cryptology. Mathematical Ability will add to this skill.; Cryptological training requires a Top Secret/Special Compartmentalized Intelligence Clearance in the modern U.S. and similar clearances in other settings. No one with disadvantages that would make him a potential security risk is likely to be selected for such training. An Unusual Background or other Advantage that the GM allows may be substituted for this clearance. Cryptology may be learned at lower TLs without the clearance or other explanation—it is assumed that earlier methods are publicly documented.; Historical note: the bonus from Mathematical Ability would be unavailable in the ‘real’ world until 1920, when William Frederick Friedman’s The Index of Coincidence and Its Applications in Cryptography was published. Friedman was the first to apply statistical concepts to the letter frequency distributions used in cryptologic techniques.
Cryptography/TL (Mental/Hard) Defaults to Cryptology: Cryptography is the science of developing code and cipher protocols and techniques. It allows the creation of new methods of encipherment, and the selection of appropriate techniques and combinations of techniques to define a protocol for a given security need. Mathematical Ability will add to this skill.; Cryptographical training requires a Top Secret/Special Compartmentalized Intelligence Clearance in the modern U.S. and similar clearances in other settings. No one with disadvantages that would make him a potential security risk is likely to be selected for such training. An Unusual Background or other Advantage that the GM allows may be substituted for this clearance. Cryptology may be learned at TLs lower than the current TL without the clearance or other explanation—it is assumed that earlier methods are publicly documented.
Cryptanalysis/TL (Mental/Hard) Defaults to Cryptology: Cryptanalysis is the science of obtaining plaintext from ciphertext without the proper keys. Computer equipment is extremely helpful, if available, in conjunction with this skill. The availability of other plaintext/ciphertext pairs in the same cryptosystem is an asset; however, message brevity is a serious impediment by even the most skilled cryptanalysist. Mathematical Ability will add to this skill.; Cryptanalytical training requires a Top Secret/Special Compartmentalized Intelligence Clearance in the modern U.S. and similar clearances in other settings. No one with disadvantages that would make him a potential security risk is likely to be selected for such training. An Unusual Background or other Advantage that the GM allows may be substituted for this clearance. Cryptology may be learned at TLs lower than the current TL without the clearance or other explanation—it is assumed that earlier methods are publicly documented.; Computer modifiers: +Complexity of the decryption program, with a successful Computer Operations roll.; Other modifiers: +5 for a known plaintext/ciphertext pair in the same cryptosystem. −10 for a message of a single word, −9 for fewer than 5 words, −7 for fewer than 10 words, −5 for fewer than 25 words, −3 for fewer than 50 words, −1 for fewer than 100 words, +1 for more than 150 words, +3 for more than 300 words, +5 for more than 500 words. Additional modifiers may be made based on the cryptosystem. [This skill originally appeared, in a different form, in GURPS Special Ops.]
Hobby Skill: Cryptogram Solving (Mental/Easy) Defaults to Language−4*, Cryptology+2, or Cryptanalysis+2: This is the ability to solve simple substitution ciphers, such as those published in puzzle magazines. Mathematical ability will add to this skill.; Note to GMs: Simple substitution ciphers are never used in modern times to secure important information. They are used to make information mildly inconvenient to obtain, or to provide a small delay if the information needs to remain unknown for only a short time. For example, some postings on the Internet’s newsgroups are encrypted using a method known as rot-13, where each letter is replaced by the letter 13 places away in the alphabet. It can be decrypted through the same method, and many newsreaders provide that method of decryption. No one expects the information to remain undeciphered, but it does allow the distribution of information that some members of the audience may not want to read immediately, such as movie spoilers, joke punchlines, or off-color remarks.; Note: This skill is not historically available until TL4. During medieval times, cryptology was considered a black art, like witchcraft. The ability to produce real information from a jumble of meaningless letters was viewed as divination.; *The default to a language skill should only be allowed in game worlds where cryptogram puzzles are common.
Professional Skill: Cipher Clerk/TL (Mental/Easy) Defaults to IQ−4, Cryptology−2, Cryptanalysis−2, or Special: This is the ability to use cipher equipment and to follow the appropriate methodologies for encoding and decoding transmissions given the proper keys. In a modern setting, using a computer-program-based encryption protocol with standard documentation, this could also default to Computer Operation−2.
Signal Security/TL (Mental/Average) No default: The Signal Security skill includes the areas of steganography, transmission and emission security, and traffic security (note that ‘signal’ can mean any type of information transfer). Steganography is used to hide the fact that a message is being sent at all. Transmission and emission security attempts to avoid direction-finding techniques by sending a long message in one short burst or shifting radar frequencies.; Traffic security similarly tries to render traffic analysis fruitless by changing call-signs, sending dummy messages, and applying radio silence. This skill allows the application of existing methodologies. To develop new methodologies would require rolls against the appropriate skill: Chemistry to develop a new invisible ink, Engineering for hidden compartments, etc.
Signal Intelligence Collection/Jamming/TL (Mental/Average) No default: This is the ability to operate sophisticated intelligence-gathering and direction-finding systems targeted at secure signals. In modern times this can include voice or Morse radios, telephones, radar, telemetry, etc. In earlier times it would be limited to detecting invisible inks and the like. Again, ‘signal’ can mean any type of information transfer.; Modifiers: −2 for isolated communications shorter than one minute, −1 to −10 for rough handling or poor maintenance of equipment. [This skill originally appeared in GURPS Special Ops.]
Traffic Analysis/TL (Mental/Hard) No default: This is the ability to identify target unit types and organizational structures from data such as callsigns, prosigns, and transmission patterns of intercepted communications traffic. [This skill originally appeared in GURPS Special Ops.]

Investigation of Problems and Solutions (Cryptographic Techniques)

The standard problem for cryptographers is to send messages from one party to another such that no other party can gain information from the message. Here are some other things that aspiring cryptographers can develop:

Anonymous Election: electronic election where every voter’s vote remains private and every voter can check that the election was not falsely manipulated. −6 penalty.
Anonymous Transactions: electronic transaction where one party remains anonymous, but the other party receives some assurance (such as ability to pay or authorization). −4 penalty.
Certified Mail: simultaneous exchange of a secret message and a signed receipt. −4 penalty.
Digital Signature: the recipient can be sure of the sender’s identity, but the message is not secret. No penalty.
Contract Signing: digital signatures are simultaneously exchanged. −2 penalty, or −4 if the signed document is secret.
Oblivious Transfer: the sender cannot be certain if the intended recipient receives the secret message. −2 penalty.
Secret Sharing: a group of people all hold pieces of a secret, which are unintelligible taken individually. If a large enough percentage of the group put their pieces together, the secret can be produced. −4 penalty.

Each of these additional properties would represent a −2 penalty: anonymity, authentication, multi-party usage, obliviousness, secrecy, and simultaneity.

Rules for ‘Attacking’ Cryptograms

If the game includes both cryptographers and cryptanalysts making and breaking codes, the GM should treat attacks on cryptographic systems as contests of skill rather than simple skill rolls. Whenever a new protocol is developed, record the amount by which the roll was made. When it is later attacked, the cryptanalyst must succeed by that amount or more. Usually, however, the GM will be creating all of the protocols (or using historical ones) and the players will be trying to break them. In this case, the GM assigns modifiers to the different protocols as he sees fit, and the cryptanalysts must make rolls at the specified penalties (or bonuses).

An attack on a secrecy protocol has varying degrees of success. If the cryptanalyst succeeds by 1 or less, the message is only partially decoded. The GM gives the player some interesting pieces of information, but not all of it. If he succeeds by 2 or more, the entire message is decrypted. If he succeeds by 6 or more, he can also decrypt any future messages sent in the same protocol with the same key. On a critical success, he can obtain the secret key as well. Note that if the cryptanalyst knows the protocol being used, he can often determine the secret key once an entire message has been decrypted without a critical success.

An attack on an authentication protocol also has varying degrees of success. If the cryptanalyst succeeds by 1 or less, he has achieved an existential forgery—he can forge a signature on one message chosen at random. If he succeeds by 2 or more, he can forge the signature on one message of his choosing. If he succeeds by 6 or more, he can forge the signature on any number of messages. On a critical success, he can obtain the signer’s private key as well.

In a cinematic campaign, all keys and passwords are words, numbers, or phrases that have special meaning to the user and can be guessed. Examples include ‘TRUSTNO1′ for Fox Mulder on The X-Files television series and ‘RAMESES II’ for Ozymandias in the Watchmen graphic novel. ‘PASSWORD’, ‘SECRET’, and ‘SEX’ are good guesses for unsophisticated users, along with birth dates and maiden names. In a cinematic campaign, or a realistic game with naive users, the GM can allow a character to guess such a password or key on a roll of 3 on 3 dice. Bonuses should be given for intimate knowledge of the subject’s biographical information, or for the importance to the game’s plot. Or if the character has time and resources for a brute force attack on a system that the GM (or player) has already determined is protected by one of the naive passwords, the GM can rule that success is automatic.

Note that the magical spell Encrypt (GURPS Grimoire, p. 78) will produce ‘encrypted’ text that cannot be deciphered through mundane cryptanalytical attacks. It must be resisted or magically dispelled, or the encrypted item must be taken to a no-mana area. Truly fiendish mages will translate their most secret works into an ancient or alien language, encrypt it using mundane techniques, and then enchant the resulting work with the magical spell. See GURPS Magic Items 2, pp. 40-41, for even more convoluted methods.

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J. Hunter Johnson—I'm just this geek you (should) know.