We've been working on some game ideas where different people play against each other.

I'd just like to check that a secret for a provably fair game can be made out of 2 transaction ids; where the secret is used to create a signature with those tx ids (i.e. satoshi dice)

Let's say that there were 2 transactions in a 2 player game:

player_1 = "d200c3d88a578e32c6d024aacdac11b8f790c34679e60a114b4af9bcc07b39ad"
player_2 = "6bfafd42a665e14b3aa2f107e9c09164697bfe6f5dfb48ba163198a749fca3cb"

Apply a 512 Digest to the combined hash of the 2 players

secret = Digest::SHA512.hexdigest("d200c3d88a578e32c6d024aacdac11b8f790c34679e60a114b4af9bcc07b39ad6bfafd42a665e14 b3aa2f107e9c09164697bfe6f5dfb48ba163198a749fca3cb"))

secret == "0775c9758a8d63f66098e461bb185f7adc1c080f7d3f0656e3d968cdc45de4512abfff756eea6b3 379a8eb7eae110d3f95363a1c570fcbea6a2a48eaca5e0282"

Things to bear in mind:
- Players play against each other - 2 players could gang up and try to match their tx ids together in order to screw other players, but they don't know the tx ids of those other players.
- Because people are playing against each other, can any one (or many) users exploit the game?

Thanks for your help 

Currently supporting e-mail and webhooks (HTTP callbacks):

http://www.blocknotify.com

If you want some other integrations, or have any feedback; just let us know 

We've just launched http://www.blockrun.com , a play on penny auctions built on top on the blockchain.

- Each auction has its own bid address
- Auctions last for a certain number of blocks
- When blocks reach 0, each new bid extends the auction by 3 blocks
- There needs to be 3 new blocks with no transactions associated with that auction for a winner to be declared
- The highest bid wins, all other bids lose (we send winning, and losing transactions when you have been outbid)
- Winners are confirmed after 6 confirmations
- Winner creates a signed message to confirm they own the address

How it works: http://www.blockrun.com/how_it_works
FAQ: http://www.blockrun.com/faq
Auction for 5 coins: http://www.blockrun.com/auctions/5-bitcoins

We're always looking to improve, so if anyone has any feedback (e.g. "this is too complicated, how does it work??), any thoughts are much appreciated

We've just launched http://www.blockrun.com , a play on penny auctions built on top on the blockchain.

- Each auction has its own bid address
- Auctions last for a certain number of blocks
- When blocks reach 0, each new bid extends the auction by 3 blocks
- There needs to be 3 new blocks with no transactions associated with that auction for a winner to be declared
- The highest bid wins, all other bids lose (we send winning, and losing transactions when you have been outbid)
- Winners are confirmed after 6 confirmations
- Winner creates a signed message to confirm they own the address

How it works: http://www.blockrun.com/how_it_works
FAQ: http://www.blockrun.com/faq
Auction for 5 coins: http://www.blockrun.com/auctions/5-bitcoins

Get bidding  

I'm trying to compile a list of wallets (hosted or local) that support message signing.

So far I have:
- Bitcoin QT v. 0.8.1
- Electrum v. 1.7.3
- Blockchain.info "my wallet"

- Armory v. 0.88.1 (Currently incompatible with Bitcoin QT, or anyone else)

Am I missing any? Also, if anybody knows the earliest versions of Bitcoin QT/Bitcoind, Electrum and Armory that supported message signing, that would be really useful.

Thanks for your help  


Edit: added Armory to the list

I've been reading through Contracts on the Bitcoin wiki and just wanted to check something about one of the use cases: https://en.bitcoin.it/wiki/Contracts

Consider the example of an old man who wishes to give an inheritance to his grandson, either on the grandson's 18th birthday or when the man dies, whichever comes first.

To solve this, the man first sends the amount of the inheritance to himself so there is a single output of the right amount. Then he creates a transaction with a lock time of the grandson's 18th birthday that pays the coins to another key owned by the grandson, signs it, and gives it to him - but does not broadcast it. This takes care of the 18th birthday condition. If the date passes, the grandson broadcasts the transaction and claims the coins. He could do it before then, but it doesn't let him get the coins any earlier, and some nodes may choose to drop transactions in the memory pool with lock times far in the future.

Forgetting the scenario where the grandfather dies for the time being, what happens if the grandfather spends his coins before the 18th birthday? Or are the coins "spent" by the grandfather and in a locked state so that they can only be accessed by the Grandson once the lock time has past?

Thanks!