Add Message Signing page with examples

Signed-off-by: Iker Ruiz de Infante Gonzalez <iker@irzinfante.dev>

Author: irzinfante

docs/advanced/message_signing.md

@@ -0,0 +1,37 @@
+Message Signing
+===============
+
+Web3j can sign arbitrary-sized messages in the `personal_sign` style (see [EIP-191](https://eips.ethereum.org/EIPS/eip-191)), via the `signPrefixedMessage` method of the `Sign` class. This allows you to sign a message, obtain the signature as a `SignatureData` type object (containing the `r`, `s` and `v` components of the signature), and recover the signer's public key or address from the signature and the original message.
+
+## Usage
+
+Here is a complete usage example:
+
+```java
+// Create a signer from a private key
+Credentials credentials = Credentials.create("<private key>");
+
+// Sign the message
+String message = "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks";
+SignatureData signatureData = Sign.signPrefixedMessage(message.getBytes(StandardCharsets.UTF_8), credentials.getEcKeyPair());
+
+// Recover the signer's public key and convert it to address
+BigInteger ecPubKey = Sign.signedPrefixedMessageToKey(message.getBytes(), signatureData);
+String address = Numeric.prependHexPrefix(Keys.getAddress(ecPubKey));
+
+// Check if the signer’s address matches the recovered address
+address.equals(credentials.getAddress()); // true
+```
+
+### Obtain signature hex string
+
+By concatenating the components of the signature from the `signatureData` object, you can obtain the signature as a hexadecimal string as follows:
+
+```java
+byte[] signature = new byte[65];
+System.arraycopy(signatureData.getR(), 0, signature, 0, 32);
+System.arraycopy(signatureData.getS(), 0, signature, 32, 32);
+System.arraycopy(signatureData.getV(), 0, signature, 64, 1);
+
+Numeric.toHexString(signature); // "0x..."
+```

mkdocs.yml

@@ -65,6 +65,7 @@ nav:
       - Ethereum Name Service: advanced/ethereum_name_service.md
       - TLS over HTTP and Websockets: advanced/tls_http_websockets.md
       - HSM Transaction Signing: advanced/HSM_transaction_signing.md
+      - Message Signing: advanced/message_signing.md
       - Web3j Core Modules: advanced/web3j_core_modules.md
       - Json Web Key (JWK) support: advanced/jwk_support.md
   - Command Line Tools: command_line_tools.md