Using the javascript web3 API I can create a signature like this:
> web3.eth.sign(eth.coinbase, "0xdeadbeef")
"0xd3fe64b6f0920593cc4afb1321d592ae91e25fe1a0216e9002a4a6580fb2698c5ec62491c62557b8cc8f64533a5097b3ffb68208952b30cb27ed0a56ae21682201"
Now I'd like to verify that the signature is correct; doing so in a contract is already documented elsewhere, but I am looking for a way to do this using web3. (How) can this be done?
web3 does not support this feature yet, but it might be coming with web3 1.0.
In the meantime you can use ethereumjs-utils ecrecover feature.
Note that this function expects v to be in {27, 28}, and since your signature comes from geth, (since it doesn't return signatures in the canonical format yet) you will have to add 27 to your v.
Given a signature sgn of a the hashed message msg, you can use the ethereumjs-util library like this:
r = utils.toBuffer(sgn.slice(0,66))
s = utils.toBuffer('0x' + sgn.slice(66,130))
v = utils.toBuffer('0x' + sgn.slice(130,132))
m = utils.toBuffer(msg)
pub = utils.ecrecover(m, v, r, s)
adr = '0x' + utils.pubToAddress(pub).toString('hex')
To do the verification in a solidity contract, check out this answer.
0xdeadbeef and 0xd3fe64b6f0920593cc4afb1321d592ae91e25fe1a0216e9002a4a6580fb2698c5ec62491c62557b8cc8f64533a5097b3ffb68208952b30cb27ed0a56ae21682201 it is very unclear how to use ecrecover in that library to actually validate the address that signed it.
Commented
Aug 10, 2016 at 1:29
v = utils.toBuffer(27) //assuming that the given value was 0 should be 27 + the last hex value of the signature (0 or 1).
Commented
Aug 11, 2016 at 6:56
utils, the v parameter should be a number, so the v line should read: v = utils.bufferToInt(utils.toBuffer('0x' + sgn.slice(130,132)))
Starting v1.0 you can use web3.eth.accounts.recover
// message, signature
web3.eth.accounts.recover('Some data', '0xb91467e570a6466aa9e9876cbcd013baba02900b8979d43fe208a4a4f339f5fd6007e74cd82e037b800186422fc2da167c747ef045e5d18a5f5d4300f8e1a0291c');
> "0x2c7536E3605D9C16a7a3D7b1898e529396a65c23"
