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Sign and Verify Messages

The Simple Way
var signer = new EthereumMessageSigner();
var signature = signer.EncodeUTF8AndSign("Hello from Nethereum", new EthECKey(privateKey));
var recovered = signer.EncodeUTF8AndEcRecover("Hello from Nethereum", signature);

Sign a message and recover the signer address — no gas, no transaction needed.

Message signing lets you prove you own an address without sending a transaction. Common uses: wallet authentication (Sign-In with Ethereum), off-chain approvals, and proving ownership of an address to a dApp.

Installation​

dotnet add package Nethereum.Signer

Sign a UTF-8 Message​

var signer = new EthereumMessageSigner();
var message = "Hello from Nethereum";
var signature = signer.EncodeUTF8AndSign(message, new EthECKey(privateKey));

Console.WriteLine("Signature: " + signature);

The message is UTF-8 encoded, then prefixed with "\x19Ethereum Signed Message:\n" + length before hashing and signing. The prefix prevents a signed message from being replayed as a transaction -- it makes the signed data unambiguously a message, not a transaction. This is the same format used by MetaMask and other wallets (personal_sign).

Verify a Signature​

Recover the signer address from a message and its signature, then compare it to the expected address:

var signer = new EthereumMessageSigner();
var recoveredAddress = signer.EncodeUTF8AndEcRecover(message, signature);

if (expectedAddress.IsTheSameAddress(recoveredAddress))
{
    Console.WriteLine("Signature is valid -- the message was signed by " + recoveredAddress);
}
else
{
    Console.WriteLine("Signature mismatch -- expected " + expectedAddress + " but recovered " + recoveredAddress);
}

IsTheSameAddress performs a case-insensitive comparison, handling mixed-case checksummed addresses.

Sign Raw Bytes​

When you already have the raw byte payload rather than a string:

var signer = new EthereumMessageSigner();
var data = new byte[] { 0x48, 0x65, 0x6c, 0x6c, 0x6f };
var signature = signer.Sign(data, privateKey);
var recoveredAddress = signer.EcRecover(data, signature);

Console.WriteLine("Recovered: " + recoveredAddress);

Hash and Sign Shortcuts​

These methods accept the private key as a hex string directly, without needing to construct an EthECKey first:

var signer = new EthereumMessageSigner();
var signature = signer.HashAndSign(message, privateKey);
var recovered = signer.HashAndEcRecover(message, signature);

Console.WriteLine("Recovered: " + recovered);

Next Steps​

note

Ethereum enforces low-S signatures (EIP-2) to prevent transaction malleability. Nethereum handles this automatically -- you only need to be aware of it if you are implementing custom signature verification. Use EthECKey.VerifyAllowingOnlyLowS if you need to explicitly reject high-S signatures.