Saturday, March 14, 2009

TAEB 0.03

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This is the first version that is actually going to be on the CPAN. I decided to put up with Module::Install for a little while longer. :)

TAEB, pubsub, and announcements

TAEB is a decently-sized, componentized program. Components need to be able to communicate with each other. For example, the Senses component (which tracks the state of TAEB's character) needs to know when we are indebted to a shopkeeper so that it can respond to the AI when it asks if TAEB is in debt, and for how much. The Cartographer component (which tracks the state of the dungeon map) needs to know when TAEB becomes indebted so it can mark the current room as a shop. In general, we want any component to be able to listen for any update. This will let us remain flexible and extensible; the completely-separate AI can listen for any update that the framework can.

We use the publish-subscribe pattern to control this complexity. Pubsub decouples those who generate updates (publishers) from those who listen for updates (subscribers). TAEB has been using pubsub for a long time now (since 2008-01-06, according to darcs trackdown). To publish a message, anything in the program can call TAEB->enqueue_message("foo" => @arguments). This will call the msg_foo method on all subscribers with arguments @arguments. We have a component, Publisher, that acts as message broker.

Pubsub has been a great tool for letting TAEB understand messages from NetHack. We have a component that is devoted entirely to figuring out what the current state of the screen is telling us: the ScreenScraper. The ScreenScraper deals mostly with transforming characters on the screen into published messages. For example, when the NetHack prints the message "You owe (somebody) (some amount of) zorkmids.", the ScreenScraper publishes a debt message with one argument: the number of gold pieces TAEB owes. The Senses has a msg_debt method that stores this amount of debt in an attribute. The Cartographer has a msg_debt method that floodfills the current room's tiles with the shop bit. The ScreenScraper does not need to know who cares about debt. The ScreenScraper publishes a lot of messages that no component subscribes to yet, and that's okay!

I'm happy with this, but there is room for improvement. Simple methods are not great message handlers. There are painful bugs lurking when subscriptions interact with inheritance. Suppose the AI defines a generic "go to a specific tile" behavior. This behavior would need to handle walked so it can track TAEB's progress. This GotoTile behavior is then subclassed to produce behaviors such as GoUpstairs and GotoCorpse. GotoCorpse might need to handle walked so it can age the corpse (so that TAEB doesn't eat rotten corpses). When the publisher sends the walked message to the GotoCorpse behavior, it does not invoke GotoTile's msg_walked method, because GotoCorpse didn't invoke it. While you may argue that GotoCorpse should have known to invoke GotoTile's msg_walked as that is part of its public interface, it certainly sucks for usability. It's especially painful when GotoTile begins subscribing to walked months after GotoCorpse is written!

The potential fixes for this are easy. The one I like best is providing some "subscribe to a message" sugar. Where we previously wrote sub msg_walked { ... }, we would now write message walked => sub { ... }. This sugar would handle publishing to parent classes if there are any. It would still use method calls behind the scene; message would install a msg_walked method. This gives us maximum flexibility. A subscriber could define an unusual message that would only conditionally be published to its parents.

The arguments we pass to each method could be improved as well. It's not immediately apparent what arguments the msg_debt method would receive. Currently we pass the amount of debt. However, we should also be providing the name of the shopkeeper TAEB is indebted to. This would help the Cartographer resolve ambiguities when there are multiple rooms and shops in sight. Should we rewrite every msg_debt method (including those potentially written by third-party, unknown AI hackers) to take named parameters? Should we pass in the shopkeeper's name as the second parameter? Should we pass in the name as the first parameter? After all, the name does come first in the message NetHack prints. We could provide one positional parameter (amount) and one named parameter (shopkeeper). Hey wait, it would be useful to also pass which items we're buying as well...

The best answer is to make each message an object. We would have a TAEB::Message::Status::Debt class with attributes amount and shopkeeper. This class could have a method to ask the inventory what items are currently in TAEB's shopping cart. If we're feeling cute, we could even overload this message to stringify to the amount of debt.

Pubsub with objects as messages is generally called Announcements. The concept of announcements is more general than pubsub. Announcements lets subscribers of a message communicate with the other subscribers, and even with the publisher.

Currently, when NetHack presents TAEB with a menu to select which items to pick up, TAEB will ask the AI whether it wants to pick up each item. The AI is asked about the item without the context of the other items that it can pick up. This really sucks! If TAEB is toeing the burden line, it needs to be pretty strict about what items it will pick up. This means it may refuse to pick up a useful item because there just might be an even more useful item further down the list.

Instead, TAEB should publish a "pick up items" announcement. This announcement, TAEB::Message::Query::PickupItems, would have the list of items. Subscribers would select which items they want by invoking methods on the announcement. When all subscribers have had a chance at it, the ScreenScraper would select the items in NetHack's menu accordingly. The selection doesn't have to be binary either; each subscriber could assign a numeric "desire" to each item. The ScreenScraper would then select items that have a sum desire greater than some cutoff (which would be another decision that some subscriber could set). Using announcements would better decouple the AI from the framework.

This is not the first time that turning plain strings into classes has been a major improvement for TAEB. Previously, the AI would return a string, a NetHack command, as the action to perform next. We then reified actions into classes. Letting the current action subscribe to messages, and respond to NetHack prompts, vastly improved TAEB's interactions with NetHack. Problems with unexpected prompts (such as "Eat this corpse on the ground?") quickly and completely vanished. Actions subscribing to messages lets us handle ambiguous messages better; if we just applied a unicorn horn, then we can figure out that "You feel sick." means the unicorn horn was cursed and TAEB can mark it so.

TAEB does not have announcements yet, but that's my next big project. I'm excited by the many possibilities here.

Monday, March 2, 2009

Predicting and controlling NetHack's randomness

NetHack, being a single-player game of imperfect knowledge, incorporates a large element of randomness. Random numbers control how much damage that minotaur will deal you, what the identity of each purple-red potion is, whether dipping into that fountain will net you a wish, etc. To generate random numbers, NetHack employs a pseudorandom number generator. Wikipedia says:

A pseudorandom number generator (PRNG) is an algorithm for generating a sequence of numbers that approximates the properties of random numbers. The sequence is not truly random in that it is completely determined by a relatively small set of initial values, called the PRNG's [seed].

Because NetHack uses a PRNG, providing identical input to two separate games, sharing the same seed and PRNG algorithm, will lead to the same results in each game. This should not be surprising as this is a property fundamental to PRNGs. The input must be identical for both games, otherwise the games will diverge; on the same turn, casting a spell in game A will do a different amount of damage than simple melee in game B. Since the two actions use a different amount of random numbers, it would be annoying (but not difficult) to reconcile the two games.

NetHack uses the current time (specifically the number of seconds since midnight, January 1st, 1970) as the seed for its PRNG. Thus, two games started in the same second will have the same seed. You can verify this by typing "nethack" into two separate terminals, then quickly hitting enter in each. Once you select the same characters, you should be greeted with the same map and stats. If not, your clock may have ticked between starting the two games, so just try again. If you play both games identically you'll see that you obtain the same results in both games. You can, of course, diverge once you experience very unfavorable results in one game, such as death.

That's a lot of work just to fool the game. If you're going to those lengths, you're better off playing in the immortal "explore mode", or changing the code so that it's easier to win. These are some of the reasons that "public server ascensions" are more highly valued than local ascensions. You can't change the code on the server, or play in god mode, so everyone knows that your server ascensions are legitimate.

However, public NetHack servers still use PRNGs, so there still exist some serious exploit vectors. On the public server, I abused the PRNG for demonstrative purposes (though never for actual ascensions). On the WowDeath account, I killed myself by kicking wands of wishing on the first level — three times in one day.

Though NetHack uses the current time for its seed, it's very easy to change that to be, say, an option to NetHack. You can tell NetHack to start a game in advance, fooling it into thinking it's 11:00:00. If that game doesn't work out so well (perhaps you got a lame starting inventory), you can try again, using a seed 11:00:01. Repeat until you find a great starting haul, then start a game on the server at that exact time. I went one step further, generating thousands of starting levels until a wand of wishing was created as part of the initial level. paxed, one of the admins of, patched nethack to use a truly random seed so that this specific exploit can no longer be used on that server.

The trickiest part, which was the only point of failure, was in starting a game on the server at some exact second. Due to clock skew, the server could think it is several seconds (or even minutes) earlier or later than your clock does. The server doesn't broadcast what it thinks the current time is, either. One way to actually figure out the current time is through the random seed! Start a game on the server, noting the current time on your clock. Then generate NetHack games for that time, that time plus a second, that time minus a second, plus two seconds, and so on until you get the same starting map and stats. The offset you used is the number seconds between your machine and the server, so you can use it to know exactly when to start a server game. It's still not as exact as I would like, since you only get resolution of a second.

That method actually describes a distinct avenue of exploitation. Since you now have a server game and a local game with the same seed, you can play the local game (remembering to mirror the input to the server game as well) until you get to some point at which you want to diverge. It's actually more powerful than even that, because you can modify your local nethack to display object identities instead of appearances, or not require exploration for the map. You will appear to have x-ray vision or prescience to the audience of your server game.

This was actually how I was introduced to practical methods of exploiting nethack's PRNG. A nethacker named switch told me to start up a game and wait. He then said "See that gold in the upper right corner of the room? It has 70 gold pieces in it." That was viciously consternating. What he had done is what I described in the previous paragraph. He found my game's seed then looked at the pile of gold before I did: prescience. has patched the initial seed calculation, but as nethacker Adeon has demonstrated, it's still feasible to figure out the game's seed by observing random effects. Being able to observe random effects without advancing the game's state is very helpful in sussing out the game's seed, since monsters moving, level messages, and many more per-turn effects use an unknowable amount of random numbers. By using something similar to the artifact naming exploit, you can observe the effects of a knowable amount of random numbers. Adeon has code that will do exactly this for server games. Once you have the seed, you're in control of the random number generator. You can trash as many random numbers as you want by using the same actions you used to figure out the seed. This will let you make every fountain quaff produce a wish by throwing out every random number that wouldn't produce a wish.

The fix for this would be to re-seed the PRNG every n (say, 100) random numbers. That way, well before you have a chance figure out the current seed, a new one takes its place. Re-seeding must use true random numbers. If not, the re-seeding is useless; pseudorandom seeds fall victim to the same exploits. [Update March 3rd: paxed has patched this fix into as well]

You might wonder why we use PRNGs at all; why not use a truly random source for every number? Unfortunately, a computer's pool of truly random numbers is very limited; it has to build up by measuring minute temperature changes or internal timing variations. NetHack, especially being played by fifty people on a public server, would keep that pool empty and harm the game's playability.

This type of exploit could affect many single-player games like NetHack, but I don't think there would be value in doing this for other games, since one of the preconditions is that server play is more highly valued than local play. Commercial games use closed-source servers anyway. The real applicability of these techniques is in cryptanalysis. Use truly random numbers for your keys!

Sunday, March 1, 2009

Where's 0.01?

Due to an issue with Module::Install, the 0.01 tarball I uploaded to CPAN last night was empty. I've long disliked Module::Install, so I'm going to get rid of it in favor of a plain ExtUtils::MakeMaker or something.

Update June 3rd, 2009: The issue with Module::Install was that it generates an empty tarball when there's no MANIFEST. More recent versions of MI are thankfully very reluctant to do that.