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MaulingMonkey

Discussion: Untrusted, purely P2P, dynamic database

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MaulingMonkey    1728
ChrisJW suggested making a thread about this on #GameDev, and in the interests of brining up some interesting discussions besides "how do I do this" onto the forums, I thought, hey, why not :-). The "Goal" would be to create a completely peer to peer based database with the expectance of untrusted clients, a database which will hold data which in fact changes. I'll outline my thoughts on the subject in this opening post. First off, the target goal, or what this database is and is not suitable for.

So, what would such a database target?

I believe it is unsuitable for:

1) Sensitive data. Given the promiscuous nature of a peer to peer network, especially given that we do not trust the clients, sensitive data such as medical or bank records are straight out. 2) Integrity-cruicial data. Again, given the untrusted nature of our clients, we should not use this database for nuclear power plant control, or things of that nature.

I believe it would be suitable for:

1) A theoretical P2P MMO. No, I'm not planning on building one, just colasing ideas. It is more important that the server be extremely difficult and expensive in resources to "hack" (alter the data of) rather than impossible. If a dedicated hacker is more likely to get struck by lightning than successfully alter the database illegally, I would consider this "secure enough" for a MMO. 2) Any traditional filesharing P2P network. Further posts will cover some of the difficulties with such a P2P network, detection, and possible solutions. [Edited by - MaulingMonkey on August 11, 2005 7:42:14 PM]

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Ra    1062
It's a pretty cool idea. (As you know) I'm going to try and hack together something simple in Python. There are no guarantees on this; I've got a few other things to do right now and it's not high priority.

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PaulCesar    524
Theoreticaly yes, you could create a game system like that, the problem comes with administering it. Since there is no central server, administration is impossiable, hence if things do get unwieldy, passwords stolen, or hackers attack, there is no way to do anything about it.

A quick solution, but against your methodologies, is to use a napster like system. Have a central system that only checks in and out peers. You could then build the client to update patches from there, as well accept that ip address as the administrative contact. While this wouldent fix hacking quite as much, it would make recovery easier, as IP spoofing would be necessary to get into it.

EDIT: no, its not about hackers, but its common maintinance that would be impossiable. Even for a free MMORPG, it could get terriable quickly

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MaulingMonkey    1728

Mallicious Attack (Part 1, Independant)


Obviously, a P2P network is useless unless it can be partially trusted, if only for the reason that it'd have no reason to behave with malice. For a game, however, the desire of another player to have a level 99 omgwtfbbq instakillyou character can be considered mallicious, so this direct level of trust is not available.

Some of the forms of malicious attacks to attempt to alter the database that I can think of:

Independant, localized attack:


Where the attackee runs multiple clients on a single network which agree with each other with the intent of spreading misinformation on the "real" database. This is probably the simplest to both implement (attack with) and counter (defend against).

Detection Method(s):
1) Run redundant data-checks against peers accross varied (sub)networks.

Countersolution(s):
1) Ensure a client has peers from varied networks. When results don't match, preform a larger poll check. If only one specific network area seems to differ (in that particular manner), the client (temporarilly?) blacklists that area as untrustworthy. This causes a netsplit(!), the hacker believes themselves to be level 99 whereas the remainder disagree and ignore/block them as invalid data.

Independant, diffuse attack:


Where the attackee has access to multiple networks, either via proxies or similar.

Detection Method(s):
1) Run redundant data-checks against peers accross varied (sub)networks (should tumble through the results occasionally to have a chance of being alerted to a problem should all peers be from the single attacker, however unlikely).

Countersolution(s):
1) This one is a bit harder to deal with, since the user has infiltrated multiple networks. Larger polls, or checking with whitelists, may be the way to go here.

Sidenotes:


One of the nice things about independant attacks is they're self defeating in a large group. What I mean by this is in order to not be recognized as obviously bogus (i.e. disagreeing with every single thing in the database) most of the parts will be equal with the "correct" version - including the parts which other independants are trying to change. Given twenty peers, all of them seperate mallicious attackers, it would be possible to identify the bad data on each machine, as it would be different only on that single machine for that item (i.e. 5% of the clients).

[Edited by - MaulingMonkey on August 11, 2005 8:40:38 PM]

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MaulingMonkey    1728

Administration (Authorization & Moderation, Part 1)



Quote:
Original post by PaulCesar
Theoreticaly yes, you could create a game system like that, the problem comes with administering it. Since there is no central server, administration is impossiable, hence if things do get unwieldy, passwords stolen, or hackers attack, there is no way to do anything about it.


Let's expand on this. Here, we're assuming a traditionalistic user authorization method, a central authority that says who is who. What if we toss that out the window?

P2P user authorization?



Do we have any current examples of this? I'd say yes - PGP keys, commonly used in website authorization, are a prime example. If you can communicate with the same key given out before, you know it's the same user. There's no password in this situation, there's your private key. If a hacker gets it, they get your identity, however. Is it possible to re-key yourself? Prehaps, this would have client(s) with the private key for the old public one confirming through that old key that the new public key is in fact their own.

If that made any sense, I'm beginning to confuse myself :-).

P2P moderation



P2P moderation presents an interesting challange. For this to work effectively, users must collase to form groups which are partially trusted - enough to pass judgement on a user's expressiveness anyways.

Two basic methods I can see would be opt-in and opt-out.

With the opt-in model, a moderation group must selectively choose to add users for their names, chat, spraypaints, etc, to be visible/unfiltered. This allows for those who would prefer not to suffer the weight of all the morons in the world to select those whom they would listen to.

With the opt-out model, a moderation group must selectively choose to add users for their names, chat, spraypaints, etc, to be filtered/blured/hidden. This allows people to help each other select racists, swearers, and so forth, that they'd rather not hear.

This can also apply to user-groups, otherwise known as clans in many games. I might opt-in to a group of my friends (kicking those whom I dislike, individual muting, or leaving the group if it refuses to do this itself).

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hplus0603    11347
Take a look at FreeNet for a distributed database.

Also take a look at the "performance" of FreeNet.

In a game, there's state that changes. Any database needs to have clear rules for how to get that state to everybody who would want to know about that state, and clear rules for applying and distributing changes. All within tolerable latency limits.

I'm not very optimistic about P2P MMOs.

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choffstein    1090
Well, here is a cool idea for p2p -- nothing exists on the computer's harddrive ... the clients, upon recieving data, send it to another client, so data only exists on the wire. Easily corruptable, but infinite space ;)

I had this idea a year or two ago, and then a day later, a paper came out about something similar. Here it is.

<3 visage

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mike25025    494
To moderate the network all you need to do is put a public key in all clients and keep the privite key. When you need to change something on the network you broadcast a message that is signed with the private key.

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MaulingMonkey    1728
Quote:
Original post by hplus0603
Take a look at FreeNet for a distributed database.

Also take a look at the "performance" of FreeNet.


Will do.

Quote:
In a game, there's state that changes. Any database needs to have clear rules for how to get that state to everybody who would want to know about that state, and clear rules for applying and distributing changes. All within tolerable latency limits.

I'm not very optimistic about P2P MMOs.


I'm definately not optimistic about a traditional style MMO being hosted on a P2P framework, in large part due to the huge amount of redundancy checking required. I still think it'd work for low-bandwidth MMOs, starting with fast initial results which arn't exactly trustworthy, then later affirmation or disagreement with more umph by higher-latency peers.

However, I havn't studied this in terrible depth yet, it may not be as bad as all that. Greeting another player, you have to verify inventories, character stats, and the like. That said, some things can be entirely client-verified - e.g. if your client "knows" (verified against a larger group) that player B can only move 3 paces a second, and player B's client tells you it's decided to move 30 paces in a second, it by itself has contradicted the rest of the game world - no independant confirmation is required to see that something is wrong.

I suppose I'm not aiming for a truely unhackable server, rather, a hack resistant and automatic repelling/healing one.

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stake    158
Not sure what you mean by p2p database??
Maybe you mean distributed database?? Probably.

So is the querying distributed..or the data transfer..or both.
If you mean a distributed database, then concurrency control will
overshadow security as a major difficulty.
How do you plan on tackling concurrency control?

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MaulingMonkey    1728
Quote:
Original post by stake
Not sure what you mean by p2p database??
Maybe you mean distributed database?? Probably.


No, a distributed database (at least by the wikipedia definition) implies a central management system.

With the peer to peer system I'm envisioning, the clients would directly be part of the database hosting mechanism, and would modify the database contents directly through specific "actions" most likely. These actions and results in turn would be accepted or rejected by their peers. Again, the earlier example, a peer will reject a modification which transports a target over 30 meteres in one second if that peer knows the maximum speed of that target is 3 meters a second.

Quote:
So is the querying distributed..or the data transfer..or both.


Both. The exact mechanics of the querying - specifically, finding out what peers have relevant information - could pose an interesting challange.

Quote:
If you mean a distributed database, then concurrency control will
overshadow security as a major difficulty.
How do you plan on tackling concurrency control?


Redundancy and statistics is what comes to mind at the moment. Nodes can and will constantly be comming on and offline, so a lot of duplication is going to be required. Concurrency control will be partially based on the same measures as the anti-hacking ones, absolute concurency is fairly hard to imagine. I should create a post on this alone, most likely...

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stake    158
So who would be the moderator-or controller of the data?
Suppose a client changes the world state and every other peer
accepts this, except for one that is also trying to access the same data.
Who wins? And how are you going to do the roll backs..you could have a roll back storm if the state is constantly being rolled back.
You could have a heirarchy of who wins..but then how do you distribute this heirarchy to the clients?

As far as querying(or searching in p2p)..what will be you mechanism?
I like a lot of the literature on heirarchical p2p networks, but it has its strengths and weaknesses...too many of the p2ps designed are just inefficient and waste bandwidth with their searches. I am not aware of how search engines work, but i know most p2p searches are pretty crude.

Having 100% consistency would of course be impossible...but most distributed systems have mechanism to eventually change the inconsistenct state..without this
a game would be unstable an unpredictable. The solution to this problem would be the conner stone of the design.

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Quote:
Original post by stake
So who would be the moderator-or controller of the data?


Peer vote, in accordance to a predefined set of rules. Assuming every peer is playing nice, they will all vote the same (by the book/rules). If half of them are playing naughty together, for a certain goal, a netsplit occurs (both sides "win" by blacklisting the side they disagree with).

Quote:
Suppose a client changes the world state and every other peer
accepts this, except for one that is also trying to access the same data.
Who wins?


Depends. If A says I-move-here, And B says A-moves-over-here, A's description of itself takes presendence assuming it is in fact legal - the client is authoritative over it's own data within set bounds (the peers do checking and data redundancy).

Quote:
And how are you going to do the roll backs..you could have a roll back storm if the state is constantly being rolled back.
You could have a heirarchy of who wins..but then how do you distribute this heirarchy to the clients?


How the clients interpret the event depends on which side of the split they'll take - it depends on who lied, in effect.

Quote:
As far as querying(or searching in p2p)..what will be you mechanism?
I like a lot of the literature on heirarchical p2p networks, but it has its strengths and weaknesses...too many of the p2ps designed are just inefficient and waste bandwidth with their searches. I am not aware of how search engines work, but i know most p2p searches are pretty crude.


The data is going to need to be related to itself somehow. Your average MMO example would be spatial partitioning/grouping of some sorts. The client would engage in direct contact with it's local neighbors, plus distant peers for preventing peerdom from being merely a function of locality (which is open to absue). Some heiarchical partitioning along the lines of a quad/oct-tree, managed by peers, would also likely be in order - all redundant of course.

Quote:
Having 100% consistency would of course be impossible...but most distributed systems have mechanism to eventually change the inconsistenct state..without this
a game would be unstable an unpredictable. The solution to this problem would be the conner stone of the design.


Not 100% certain TBH. The main problem is to get nodes grouping around sections of related data to collase together when those two chunks become related in turn. Some form of matchmaking will be required, to locate peers in the first place, prehaps this will need to be an extension of that...

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hplus0603    11347
Please send e-mail to the gdnet folks to report the lack of this acknowledgement dialog as a bug, and copy/paste the URL of this thread in your report.

Regarding P2P MMOG, there are some academic papers on the topic, but I've never seen a convincing argument for why the system would "survive" after the "netsplit" when a gang of peers disagree with another gang; I've also not seen a convincing argument for how you can first proceed with an optimistic assumption about available resources, and then later undo based on higher-latency peer votes.

For example: there's a bag of gold on the ground over there; north of you, but south of me. You run up to it and pick it up on your computer; I run up to it and pick it up on my computer. I got the gold before your packets made it to me; you got the gold before my packets made it to you. Who gets the gold? This is the simplest of arbitration problems in a P2P system; once you go down that path, it starts becoming even harder (and if you include voting, well, please publish your results :-)

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stake    158
Not sure if i understood ya on a point.

Lets say A and B are both the same distance away from a gold coin. If both
pick the coin up at roughly the same time(assuming no clock syncronization)
you would have the peers(probably the closest in the virtual world) decide who wins. This works, but favors the peer with the lowest latency to its peer.
Depending on how much close interaction there is it could be a problem...only probably in very interactive 3d worlds or virtual reality.

The ideas for searching sound ok for games..but doesn't really work for p2p file sharing...unless you have mechanism to group like minded individuals close together...which is ok if you have certain groups you always search through...but any topic searched for out of the normal would require the peer to search away from its neighbors

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MaulingMonkey    1728
Quote:
Original post by hplus0603
Please send e-mail to the gdnet folks to report the lack of this acknowledgement dialog as a bug, and copy/paste the URL of this thread in your report.

Regarding P2P MMOG, there are some academic papers on the topic, but I've never seen a convincing argument for why the system would "survive" after the "netsplit" when a gang of peers disagree with another gang; I've also not seen a convincing argument for how you can first proceed with an optimistic assumption about available resources, and then later undo based on higher-latency peer votes.


Basically, it dosn't survive - as a single piece. If a server is simply misinformed, a poll of random network peers with the data can determine what the majority believes, and thus what it in turn should believe. If a server is mallicious, it will continue to spout the misinformation and become ignored/blacklisted by the majority. The "bad" chunk falls off, that's partly the point. But there's no reason you'd want to keep malicious servers around anyways, this casting off is actually a good thing - it's self policing.

Although, in theory some of the "sane" nodes could get cut off as well if they didn't get accurate peer sampling.... hmm... that would be bad :3.

Quote:
For example: there's a bag of gold on the ground over there; north of you, but south of me. You run up to it and pick it up on your computer; I run up to it and pick it up on my computer. I got the gold before your packets made it to me; you got the gold before my packets made it to you. Who gets the gold?

Whoever reaches it first ;-). The way to deal with that situation would be with timestamps from semi-syncronized clocks, but I do see the problem (and it involves attempting to "speed hack" or similar...)
Quote:
This is the simplest of arbitration problems in a P2P system; once you go down that path, it starts becoming even harder (and if you include voting, well, please publish your results :-)

If I get anything worthy of publishing, I most definately will :-).

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mike25025    494
Quote:
Quote:
For example: there's a bag of gold on the ground over there; north of you, but south of me. You run up to it and pick it up on your computer; I run up to it and pick it up on my computer. I got the gold before your packets made it to me; you got the gold before my packets made it to you. Who gets the gold?

Whoever reaches it first ;-). The way to deal with that situation would be with timestamps from semi-syncronized clocks, but I do see the problem (and it involves attempting to "speed hack" or similar...)


The problem is bigger than just hacks. Who gets the gold if both players really do reach it at the same time?

Heres my solution. Both players choose the same number (by asking peers for one or some other method). Then the players choose two private random numbers and preform Diffie-Hellman key exchange. Then after the exchange the players exchange there private numbers (in plain site with no encryption). Then both players put the numbers into a special hash function that tells who wins.

Diffie-Hellman is needed because if the players just sent there private numbers then one could wait for the others number to arrive, then change theres before sending. Diffie-Hellman prevents this assuming that no one can solve a discrete log in a short amount of time.

This should prevent hacks and solves the problem of who gets the gold.

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hplus0603    11347
Quote:
This should prevent hacks and solves the problem of who gets the gold.


Imagine doing Diffie-Hellman for each potion quaffed, spell cast, shot fired, ste taken (if you don't allow co-existence in world space), ...

Of course, they need to tell their peers about the decision, and sadly you can't easily un-hack that. Even if I do a diffie-hellman with the peer, I may decide that I always win, and tell the world that the peer sent me a 0 and I chose a 1; it's my word against the peer's word at that point.

Peer-to-peer consistency for anything resembling real-time is a Really Hard Problem; and it gets worse in an untrusted environment.

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mike25025    494
Quote:
Imagine doing Diffie-Hellman for each potion quaffed, spell cast, shot fired, ste taken (if you don't allow co-existence in world space), ...
I didn't think about that.

Quote:
Of course, they need to tell their peers about the decision, and sadly you can't easily un-hack that. Even if I do a diffie-hellman with the peer, I may decide that I always win, and tell the world that the peer sent me a 0 and I chose a 1; it's my word against the peer's word at that point.
No one knows what the private numbers are (except the creator) untill after diffie-hellman. Because of that the other peer cant chose a number that will make him win.

But like you pointed out its not practical to diffie-hellman so it doesn't really matter how secure it is.

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hplus0603    11347
Assuming two peers compete for a gold coin.

Peer A picks random number Ra; Peer B picks random number Rb; Diffie-Hellman exchanges these.

Now, how does the rest of the world know which peer got the gold coin? Both Peer A and Peer B need to tell the rest of the world about the update in world state (because the gold coin went away, and some peer got more gold in their inventory).

Now, suppose that after the Diffie-Hellman, Peer A tells the world "I won, because I picked the number 7 and B picked the number 2" and Peer B tells the world "I won, because I picked the number 5 and A picked the number 1" -- how do you tell who's cheating, and who's not?

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mike25025    494
Ill try to explain it better.

Alice and Bob are fighting for a gold coin. They contact some peers to watch and prevent cheating. Their peers chose a random number b and send it to Alice and Bob. Alice and Bob now have the same number b and can not change it because of the peers. Alice chooses a random number A and sends it to no one. Bob also chooses a random number B and sends it to no one. Then Alice computes i = b^A mod m (m is built into the program so there is no disagreement to its value) and sends i to Bob and all watching peers. Bob computes j = b^B mod m and sends it to Alice and the peers. At this point Alice cant change A without changing i because solving a discrete log is a hard problem. Same goes for Bob. Alice and Bob now compute s = b^(AB) mod m and send to all peers. They should not disagree. If they do then one is trying to cheat and no one gets the gold (or some other action takes place). Now and only now can Alice and Bob reveal the numbers A and B. They cannot be changed from there original value without detection. Every one computes w = hash(A,B) and then they will all know who the winner is.

I hope that made sense. If I'm wrong please point out my mistake.

Edit: s = b^(AB) mod m is computed without exchanging A and B.

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MaulingMonkey    1728
Quote:
Original post by mike25025
No one knows what the private numbers are (except the creator) untill after diffie-hellman. Because of that the other peer cant chose a number that will make him win.

But like you pointed out its not practical to diffie-hellman so it doesn't really matter how secure it is.


From what I gather, diffie-hellman arbitration would only be required when arbitration is in fact necessary. Not all items will have two users claiming to pick it up at the same time. If both clients are agreeing on a similar timing scale (e.g. keep semi-syncronous clocks, timestamping actions and the like with that), diffie-hellman would only occur when these timestamps are identical. In a scenario with a malicious party, their intent would be to either:

1) Break the rule(s) and move more than allowed by the logical timescale (combatted/countered by the verification of 3rd party peers as to modification legality)

or:

2) Follow the rule(s) but "bend" the actions taken in logical time (e.g. have no delay as the user moves his mouse, instead as soon as A is reached in logic-time proceed to B, even if the user delayed this movement). This would only be combatable by too-expert detection (e.g. "you can't get 100% hit ratio, all headshots" in the average FPS) and client-checksuming of some sort (e.g. punkbuster) - both nonpermanent solutions. This said, this only pushes the curve of what the player can do, rather than breaks it giving godlike powers. But, I think the big key in getting this conceptual P2P database working isn't in preventing hacks, it's in containment to the point where a hack would not ruin the experience for another player not alter the data by much, for it to remain relatively consistant (attempting to use more generic terminology).

One can also reduce the need for repeated lengthy arbitration is with a tit-for-tat mechanism - e.g. do a full contest to see who starts with the "favorable" position, then when fighting for items, alternate - e.g. if A wins the "favorable" position, the first item argued over is awarded to A, the second item argued over is awarded to B, third to A, fourth to B, and so forth. Expanding this to N > 2 would be tricky, but I believe doable.

[Edited by - MaulingMonkey on August 13, 2005 2:48:49 PM]

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hplus0603    11347
Quote:
Not all items will have two users claiming to pick it up at the same time.


Most of the time, you don't even know that you have a second user wanting to claim the same object, because of latencies. You don't hear about it until after you've already taken the local action, unless you somehow vote for arbitration for each action.

Anyway, what if the peer I contact to perform arbitration is, in fact, compromised, and running on the machine right next to me? With any voting system, as long as I can provide at least half of the voters, then I can rig the vote.

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MaulingMonkey    1728
Quote:
Original post by hplus0603
Quote:
Not all items will have two users claiming to pick it up at the same time.


Most of the time, you don't even know that you have a second user wanting to claim the same object, because of latencies. You don't hear about it until after you've already taken the local action, unless you somehow vote for arbitration for each action.


You won't know immediately, this is true, but this is also a seperatable issue - you want an undoable "pick-up-into-inventory" action, which in a simple scheme could be solved by first tracking objects as "tenatively picked up". Once the data for all "local" clients comes in as moving in a manner which would make it impossible for them to arrive at the item before you (in logic-time), then that item can safely be moved into your confirmed inventory.

Quote:
Anyway, what if the peer I contact to perform arbitration is, in fact, compromised, and running on the machine right next to me? With any voting system, as long as I can provide at least half of the voters, then I can rig the vote.


I was considering this earlier, there's a few solutions I can identify to deal with the problem of arbitration:

1) Trusted 3rd party (peer, server, etc) - if both parties can trust an external source to the generation of random numbers (such as explicitly set peers), that peer can be used (some websites do this for traditional pen and paper RPGs online, such as Irony Games' Dice Server. Here, the peers are the players and the website they both agree they can trust).

In a P2P setting, this would not be a guaranteed source of randomness in most cases - but two members belonging to a guild (following traditionalistic MMO guidelines) may decide they can trust other members of the guild, or maybe just their guild leader. This could help reduce bandwidth (they only need to check one peer), but admittedly dosn't work for the general case.

2) Psuedo-Random Number Generator - if both parties can agree on a RNG which produces identical results for both players given an initial seed, and agree upon when this RNG is used, and continue to use this single RNG for a set period (example: "the next 1000 uses"), although a malicious party may be able to affect the vote to some degree, they would not be able to dictate it. Precalculated random values agreed upon to be used would also work here.

3) Alternation - an initial semi-random value (the XOR of the public keys of the clients involved?) is used to start off the streak, then distribution occurs equally in order (A B A B A B ...)

4) Make it deterministic. If the clients reach it at the same "time", whoever's DEX attribute is higher gets it (they're a little faster). If those are the same, STR (whoever has a better grip on it). If those are the same, INT (whoever can figure out how to trick the other into letting go gets it). There will eventually be some loophole that you need third party arbitration for, but if it only avails itself in 1 out of a bazillion instances, the hackability of the third party arbitration won't be a major issue.

You can cascade most of these methods as well - if you share a mutual friend who's online, you can use him, failing that, a PRNG, failing being able to agree on an acceptable start for that, alternation, failing that, a nearly certain to resolve the dispute deterministic approach, failing that, peer vote.

Also, you can help stave off the situation where a malicious party controls +50% of the vote with geography limits - e.g. no more than 25% of the vote may come from a single class-B subnet.

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