[Basic] Skill of the week: Cryptography

[johndallman]

Cryptography is the IQ/H TL-dependent skill of making and breaking encryption systems - usually codes and ciphers. It isn't required for simply using an encryption system, which comes under the skill used to operate a communications system, such as Electronics Operation (Communications), probably as a familiarity. The skill is boosted by Mathematical Ability, and the only default is Mathematics (Cryptology)-5, which seems a bit harsh: -3 would feel more reasonable, given that this is the major application of that Mathematics specialisation. The skill covers all the normal techniques of its TL, from the simplest to the most up-to-date, and an optional specialisation in either creating or breaking codes is common.

Creating a new encryption is done with a skill roll, and taking extra time is common. There's a special rule using the speed/range table for getting a skill bonus for taking extra time (use days in place of yards, read off the bonus) which could usefully be applied to other creative skills. Breaking encryption is done with a quick contest of Cryptography, with a base time of one day, but the code-maker always uses his original roll. The code-breaker ought to be allowed to take extra time, and using the speed/range table rule might well be reasonable.

Both sides benefit from computers, by up to +4. A code-breaker gets +5 if he has a translation of some or all of his sample of the code, and -5 if his sample is very small. Higher-TL ideas can make breaking some low-TL cryptography much easier; there's a case for a higher-TL code-breaker to get bonuses instead of penalties for the TL difference, if he has information about the lower TL's usual encryptions, and tools of his own TL.

There is a special case default for Cryptography to IQ-5, for making trivial codes, which won't stand up to anyone with the skill for long, and for breaking trivial codes, but not professionally designed ones. This rule seems OK from personal experience, having broken a copy-protection scheme produced by a former employer, which the creator thought was sound, in well under a day. I might be able to claim a Dabbler perk in Cryptography but I certainly don't have a point in the skill.

High-Tech has more detailed rules, which say that the Basic Set rules are for manually devised encryption, not the modern kinds based on rotors or hard mathematical problems, and give new timescales and equipment costs. Ultra-Tech has less specific rules for future TLs. Bio-Tech offers DNA computing for cryptography. Action 2: Exploits has action-movie rules for modern-day code-breaking. Spaceships 5 covers reading alien civilizations' digital communication protocols. Supers has the Coded Thoughts technique of Mind Block, which allows a sufficiently powerful mind to think in code, and thus resist mind-reading, and Thaumatology considers magical books written in codes.

Cryptography appears on templates or lenses in Action, Fantasy, Horror, Madness Dossier, Infinite Worlds, Locations: Metro of Madness, Worminghall, Martial Arts, FCCT, Monster Hunters, Mysteries, Psis, Space, Tales of the Solar Patrol, Thaumatology: Magical Styles, Ritual Path Magic, and Urban Magics. PU3: Talents and PU7: Wildcard skills both have examples that include Cryptography.

I don't think I've ever seen Cryptography used during a game session. Our hackers have had players who weren't into the technical details, and in Transhuman Space, the encryption is all very strong, making amateur attempts fairly pointless. The characters in the WWII secret agent campaign carefully pass cipher material they capture back to higher authority, but as field operatives they have need-not-to-know about code-breaking on their own side, and are independent enough that they rarely need to talk to HQ from the field.

It's interesting to note how much cryptography there is in the Thaumatology series. I suspect this is because in the modern world, cryptography can seem "magical", pulling information out of nowhere. The occasional crazes for things like steganography fit that pattern.

What have you done with cryptography in a game?

[RogerBW]

For Reign of Steel, I looked at the history of machine cryptography (for example, the way Enigma was breakable with difficulty at early TL7, more easily at mid TL7, and trivially at TL8) and ruled that a TL(n+1) Cryptography skill and computer can reliably break TL(n) cryptosystems in useful time. Which has a game application, of course: it means that resistance groups need to carry messages to each other, not just squirt encrypted data about the place. (I didn't collude with David Pulver on his introductory fiction piece for Will to Live, but it's a good example of another approach to this problem.)

(Bearing in mind that this is not just encryption but authentication.)

I think there's a place for a skill in operating pen-and-paper cryptosystems such as the Playfair cipher without necessarily being able to devise new ones, but I'm not sure what that skill would be. At higher technologies you can just fold it into Computer Operation.

I don't remember actual skill rolls coming up in game.

[McAllister]

Originally Posted by johndallman

What have you done with cryptography in a game?

Personally, nothing: RogerBW's assertion implies that TL(N) cryptanalysis cannot reliably break TL(N) cryptography in a useful time scale without something like massive samples or other assistance, and I generally assume that to be true. Thank you for your analysis, though, especially the references to cryptography rules throughout the different sources.

[johndallman]

Originally Posted by RogerBW

I think there's a place for a skill in operating pen-and-paper cryptosystems such as the Playfair cipher without necessarily being able to devise new ones, but I'm not sure what that skill would be.

I think that might be Administration:

Originally Posted by Wikipedia's page on Playfair

It [Playfair] was rejected by the British Foreign Office when it was developed because of its perceived complexity. When Wheatstone offered to demonstrate that three out of four boys in a nearby school could learn to use it in 15 minutes, the Under Secretary of the Foreign Office responded, "That is very possible, but you could never teach it to attachés."

[Anaraxes]

Originally Posted by McAllister

RogerBW's assertion implies that TL(N) cryptanalysis cannot reliably break TL(N) cryptography

Actually, it doesn't. His assertion is that higher tech cryptanalysis can defeat lower tech encryption. That statement in itself says nothing about equal tech levels. He didn't assert that _only_ higher tech can defeat encryption.

An important part of the tech advance in this case is in mathematics, not just speed of computation. Historical cryptosystems tend to fail when someone figures out a new method for attacking them, whether that's letter and n-graph frequencies for substitution ciphers or differential cryptanalysis.

Copy protection is a bit different than encryption in that the protected system has to be capable of decrypting itself. So, it contains any necessary keys, algorithms, and a way to trigger execution. An encrypted message keeps those separated. So, the strength of the copy protection doesn't rely on just the strength of the cipher used. They have the additional weakness of securing the whole system, which is usually the weaker link in the chain. (Analogies for encryption might be bribing the legitimate recipient to give you the contents of the message, or ignoring the encrypted email transmission and just copying the data from the receiving network switch after it's been decrypted.)

I'm not sure if this kind of attack is really Cryptography skill or one of the relevant Engineering skills (whether mechanical, computer, or social engineering).

[vicky_molokh]

I've been playing Transhuman Space over a year, and, as stated by others, THS is very unfriendly towards characters wanting to play cryptologists, since its encryption TL significantly exceeds its cryptoanalysis TL.
Which kinda encourages sending tactical-level information straight through radio without second thoughts: interception is never a concern, only moles and other 'extracryptic' things.

Now, for a campaign where both were TL9, I once planned a character specialised in real-time tactical decryption. It was barely doable with a sufficiently large portable quantum computer available in the setting and significant Time Spent penalties. The campaign never took off.

I'm also currently GMing a campaign where cryptoanalysis TL slightly exceeds encryption TL (there's no strategic encryption, though tactical encryption is semi-viable), and while there hasn't been any decryption work to speak of, it does seem to significantly change the level of carefulness about what stuff people dare to send over radio/wires instead of personal meetings.

[johndallman]

Originally Posted by Anaraxes

Copy protection is a bit different than encryption in that the protected system has to be capable of decrypting itself. ... I'm not sure if this kind of attack is really Cryptography skill or one of the relevant Engineering skills (whether mechanical, computer, or social engineering).

The full explanation would have been a bit long for the starting post. The system in question had a dongle, with various flags enabled according to which software modules you had bought. It also had a macro language which could be used to write fairly sophisticated programs. Those macros (which were just text files) could be encrypted, and there was a dongle flag without which the system wouldn't decrypt the macros when it was told to run them. This enabled the company to sell add-ons written in the macro language for significant money.

But if you could separately decrypt a macro, the system couldn't tell it from one you wrote yourself and would run it, thus enabling you to run the add-ons with a dongle that didn't enable them.

Debugging the software to find the decryption code looked hard-ish, because it was pretty complex. So I tried breaking the encryption as a stand-alone task, starting with one reasonably-sized encrypted macro at 9:00am and had it by lunchtime. It really wasn't very good encryption.

[RogerBW]

Originally Posted by Anaraxes

Actually, it doesn't. His assertion is that higher tech cryptanalysis can defeat lower tech encryption. That statement in itself says nothing about equal tech levels. He didn't assert that _only_ higher tech can defeat encryption.

In fact I would argue that TL(n) cryptanalysis normally can't defeat TL(n) cryptography in useful time (i.e. at a point when knowing the content of the message is still useful), because if it can then people won't use that cryptography in the first place. The exceptions happen when advanced cryptanalysis techniques are kept locked up by governments or other organisations.

But yes, I was specifically thinking of mathematical advances as well as increases in computing power. When someone develops a much faster algorithm for factoring large numbers, that's a whole class of TL8 crypto that's just gone from "decently secure" to "kids' stuff" even if the attacker doesn't have a building-sized computing array to throw at the problem.

[RogerBW]

Originally Posted by RogerBW

In fact I would argue that TL(n) cryptanalysis normally can't defeat TL(n) cryptography in useful time (i.e. at a point when knowing the content of the message is still useful), because if it can then people won't use that cryptography in the first place.

Considering John's comment, I should probably amend that to "smart people who are aware of the scope of the problem won't use" etc.

Because there are always suckers who end up with sub-par systems.

[Anaraxes]

Originally Posted by johndallman

The full explanation...

Yes, that does sound like straight-up Cryptography.

Originally Posted by RogerBW

In fact I would argue that TL(n) cryptanalysis normally can't defeat TL(n) cryptography in useful time... because if it can then people won't use that cryptography in the first place.

I'd agree, with the caveat that it's more like "believed that cryptanalysis can't defeat". (My earlier objection was to the point that was statement implied the other, not that this particular assumption isn't reasonable.) This is particularly true in the current era, where there's a lot of emphasis on widespread public review of algorithms in the hopes of getting people to have to insights that allow the system to be broken. But there are still a lot of secret-keepers.

And in RPG terms, the skill has to allow all those cinematic hackers that can just somehow break codes even if they're not singular mathematicians.