Add Recipe: Util function to get v, r and s values from a signed message

[naddison36]

It would be nice to have a cryptographic utility function that returns the r, s and v values of a signature. These values are calculated as follows

const wallet = new Wallet(privateKey);
const signature = wallet.signMessage(message);
const r = signature.slice(0, 66);
const s = '0x' + signature.slice(66, 130);
const v = '0x' + signature.slice(130, 132);

For more details see https://ethereum.stackexchange.com/questions/1777/workflow-on-signing-a-string-with-private-key-followed-by-signature-verificatio

[ricmoo]

You can use the underlying SigningKey object to do this.

> (new ethers.SigningKey(privateKey)).signDigest(digest)
{ recoveryParam: 1,
  r: '0x8ce26421f2cfbc14a9917dbff12853bb4d1a1c9badfdc082647aba315341469c',
  s: '0x6d78375b703c4b3564c4b357b8510e3dd621719e4adcca90c6d8f3e8287391c0' }

Please keep in mind that v, depending on the context depends on the chain ID and in the future may further be hijacked (e.g. EIP 186) to have additional meanings. The recoveryParam is either 0 or 1. The value you need to pass into ecrecover in solidity is (27 + recoveryParam), which is the same as the unspecified network (i.e. chain ID 0). For other chain ID, v is ((35 + 2 * chainID) + recoveryParam).

Make sense?

[ricmoo]

Added: https://docs.ethers.io/ethers.js/html/cookbook.html#break-apart-r-s-and-recoveryparam-from-a-message-signature