Get public key of msg.sender in a smart contract

Question

How can I get the public key of msg.sender from inside the smart contract?

function getPublicKey() returns (string publicKey) {}

From Solidity docs, ecrecover returns an address:

ecrecover(bytes32 hash, uint8 v, bytes32 r, bytes32 s) returns
(address): recover the address associated with the public key from
elliptic curve signature or return zero on error

From ethereumjs-util library, ecrecover returns a public key:

/**
 * ECDSA public key recovery from signature
 * @param {Buffer} msgHash
 * @param {Number} v
 * @param {Buffer} r
 * @param {Buffer} s
 * @return {Buffer} publicKey
 */
exports.ecrecover = function (msgHash, v, r, s) {
  var signature = Buffer.concat([exports.setLength(r, 32), exports.setLength(s, 32)], 64)
  var recovery = v - 27
  if (recovery !== 0 && recovery !== 1) {
    throw new Error('Invalid signature v value')
  }
  var senderPubKey = secp256k1.recover(msgHash, signature, recovery)
  return secp256k1.publicKeyConvert(senderPubKey, false).slice(1)
}

I assume, that when the transaction is sent, there is all information necessary to derive a public key for the sender's address, but I'm struggling to find any clear documentation on how to do it.

---

My current approach is:

function getPublicKey(account) {
  return new Promise((resolve, reject) => {
    web3.eth.sendTransaction({
      from: account,
      to: account,
      value: 0,
    }, (err, txId) => {
      if (err) return reject(err)
      web3.eth.getTransaction(txId, (err, tx) => {
        if (err) return reject(err)
        return resolve(tx.publicKey)
      })
    })
  })
}

Answer 1

Contracts cannot access the public key of the sender, since the raw transaction data is not visible to contracts. However, given the public key, you can verify that it does in fact match the address of the sender by re-deriving the address from the public key.

In Solidity it would look something like this:

function checkPubKey(bytes pubkey) constant returns (bool){
    return (uint(keccak256(pubkey)) & 0x00FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF) == uint(msg.sender);
}