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Signing messages with Nethereum

Ethereum signing basics

Nethereum provides methods to sign messages in an Ethereum compatible format. The following is a quick guide to signing a string with Nethereum and verifying a signature using various methods.

In the Ethereum context, signing a message allows us to verify that a piece of data was signed by a specific account, in other terms, it’s a way to prove to a smart contract/human that an account approved a message.

Signing a message with a private key does not require interacting with the Ethereum network. It can be done completely offline, hence the following code can be run without a testchain.

Nethereum provides with a class that can be used to sign or verify messages: EthereumMessageSigner. Let’s now explore how to use EthereumMessageSigner with two very common scenarios in the Ethereum context.

Signing messages and verifying signatures with Nethereum

Let’s first reference our assemblies and namespaces:

using Nethereum.Web3;
using Nethereum.Util;
using System.Collections.Generic;
using System.Text;
using Nethereum.Signer;
using Nethereum.Hex.HexConvertors.Extensions;
using Nethereum.ABI.Encoders;

Now let’s declare elements that we will in every of our examples:

address declares the signer’s account address:

var address = "0x12890d2cce102216644c59dae5baed380d84830c";

msg1 declares the content of the message itself, here is a simple string:

var msg1 = "wee test message 18/09/2017 02:55PM";

privatekey declares the private key of the signer’s account:

var privateKey = "0xb5b1870957d373ef0eeffecc6e4812c0fd08f554b37b233526acc331bf1544f7";

signer1 creates an instance of the EthereumMessageSigner object:

var signer1 = new EthereumMessageSigner();

1-Encoding and signing a message using EncodeUTF8AndSign:

The most common scenario when signing a message goes as follows:

A message needs to be signed, it’s most likely a string and hence can be encoded in UTF8 and then signed, therefore we will use EncodeUTF8AndSign

EncodeUTF8AndSign requires two arguments:

  • The message itself

  • The signing account’s private key

var signature1 = signer1.EncodeUTF8AndSign(msg1, new EthECKey(privateKey));

2- Verifying a signed message encoded in UTF8 using EncodeUTF8AndEcRecover:

The Ethereum signature verification process is a bit different from classical digital signatures, here the output of a signature verification is not the message (or the message hash) but the signer’s address, since the address is a part of the public key hash. Verification is successful if the recovered address is equal to the provided address, which can only happen if the signer is the owner of the account’s private key.

In this case the EncodeUTF8AndEcRecover method is used to verify the signer’s address of a message encoded in UTF8:

addressRec1 evaluates to the signer’s address, thus proving the validity of the message.

var addressRec1 = signer1.EncodeUTF8AndEcRecover(msg1, signature1);

3-Hashing and signing a message using HashAndSign:

In some cases, hashing data and then signing it might be more relevant, i.e. when dealing with a large file.

HashAndSign enables you to do this in one go:

var msg2 = "test";
var signer2 = new EthereumMessageSigner();
var signature2 = signer2.HashAndSign(msg2,

4-Verifying a hashed message using HashAndEcRecover:

When receiving a signature that has been made with a hashed file it’s necessary to start by hashing the file we want to verify and then recover the address that signed it.

HashAndEcRecover enables you to do this in one single step:

var addressRec2 = signer2.HashAndEcRecover(msg2, signature2);
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