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Ethereum: How are SIGHASH flags encoded into a signature?

Published by Terlumun Aondover on February 13th, 2025.

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Ethereum: How are SIGHASH flags encoded into a signature?

In Ethereum, signatures are used to prove ownership and authorship of transactions on the blockchain. One of the key components that makes this possible is
SIGHASH (Signature Hash Algorithm) flags

. In this article, we will look at how SIGHASH flags are encoded into a signature.

What are SIGHASH flags?

SIGHASH flags allow the signer to control which parts of a transaction they want to sign and verify. These flags allow for more efficient use of memory and computing resources on the Ethereum network.
A signature is created by hashing the input data with a public key, and then hashing the resulting hash with a private key.

Encoding SIGHASH flags into signature

When a signer wants to extract specific parts of a transaction to sign, they need to encode their SIGHASH flags into a unique identifier that can be used for authentication. This is done by extracting the required field from the input data and then hashing it with their private key.

Let’s assume we are working with a simple transaction example:


pragma strength ^0,8,0;
contract SimpleTransaction {
function sendMoney(address receiver, unit amount) public {
// Input: sender, receiver, and amount to be transferred
bytes32 _inputs = abi.encodePacked(sender, receiver, amount);
// Encode SIGHASH flags (OP_CHECKSIG)
SIGHASH[] memory _flags = new SIGHASH[_inputs.length];
for (uint i = 0; i < _inputs.length; i++) {
_flags[i] = SIGHASH(_inputs[i]);
}
// Hash input data with private key
bytes32 _dataHash = abi.encodePacked(_inputs, msg.sender);
// Hash result with public key (OP_CHECKSIG)
bytes32 _sigHash = keccak256(abi.encodePacked(_dataHash, _flags));
// Sign signature hash with private key
bytes32 _sig;
if (message sender == address(0)) {
assembly {
_sig := sha3_256(memo := abi.encodePacked(_sigHash))
}
} else {
_sig := keccak256(abi.encodePacked(_sigHash));
}
// Verify signature
request(msg.sender == owner, "Invalid signature");
}
}

In this example, we encode the SIGHASH flags into a unique identifier using the “abi.encodePacked” function. We also hash the input data with the private key to create a hash value.

SIGHASH Flag Encoding

The _flags array contains one element per flag, which is a
SIGHASH structure that contains, among other things, the following information:

0x1 (OP_CHECKSIG): Extracts the off-stack argument for each signature it evaluates.

Other flags (e.g. OP_CHECKSIGVERIFY, OP_CHECKSIGSIGN): Provide additional verification options.

By encoding these flags into unique identifiers using the _sigHash variable, we can ensure that the correct part of the transaction was signed and verified.

Conclusion

In this article, we have explored how SIGHASH flags are encoded as signatures in Ethereum. By understanding how these flags work, you will be better able to optimize your code to improve network performance. Always remember to follow best practices when working with cryptographic and hash functions.

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