Issue while trying to verify signature

Question

I am using the following to sign a message

 var str="Trial with signature";
 text=web3.sha3(str);
 var sig=web3.eth.sign(eth.accounts[0],text);

Now after obtaining the hash value from the text and the signature, to verify the same, I use the following contract

contract signature
{
    event ret_addr(address addr);
    function sig_verify(bytes32 hash, bytes sig) returns(address)                       //verifying the signature
    {
        bytes32 r;
        bytes32 s;
        uint8 v;
        assembly 
        {
                r := mload(add(sig, 32))
                s := mload(add(sig, 64))
                v := byte(0, mload(add(sig, 96)))
            }
        if(v<27)
            v+=27;
        ret_addr(ecrecover(hash, v, r, s));
    }
}

The issue is I am not getting the correct address back. Why is this so? It used to work fine earlier but now it is giving some random addresses.

Possible duplicate of workflow on signing a string with private key, followed by signature verification with public key — atfornes, Jun 09 '17 at 15:46
Although, that answer focuses on breaking up the signature in javascript, while this one is trying to break up the pieces within a solidity contract, so I don't feel like they're the same thing. — Jeff Coleman, Jun 10 '17 at 16:30
Is this still an issue? Your code here is working fine as-is for me... — benjaminion, Aug 01 '17 at 15:22
Should be mload(add(sig, 65)) instead of mload(add(sig, 96)) and see answer below about Ethereum Signed Messageprefix. — Mikhail Vladimirov, Dec 11 '17 at 20:23

score 2 · Answer 1 · answered Nov 11 '17 at 05:12

You might need to add a prefix to the data as discussed in issue #3731:

bytes memory prefix = "\x19Ethereum Signed Message:\n32";
hash = sha3(prefix, hash);

score 1 · Answer 2 · answered Dec 04 '17 at 12:02

Something is wrong with assembly section. You can debug by extending your event like this

event ret_addr(address addr, uint8 v, bytes32 r, bytes32 s);

and call it with corrwsponding variables

ret_addr(ecrecover(hash, v, r, s), v, r, s);

My result:

[
    {
        "topic": "1171b064e2eda38d82ce5fadc8a911d493f2d22ff88013d965c9885638dee60d",
        "event": "ret_addr",
        "args": [
            "0x0",
            "97",
            "0x3078613963356435666332336333663762313130313134313262663536366432",
            "0x3638333437353534666561666665333164363662343539383739373435373562"
        ]
    }
]

As you can see, v is neither 27 nor 28. r and s are not hex encoded but with 0x prefix.

Issue while trying to verify signature

2 Answers2