How to verify a signature from the Phantom wallet?

Question

I am able to sign a message on the client side using something like this:

const encodedMessage = new TextEncoder().encode("some message");
const signedMessage = await provider.signMessage(encodedMessage, "utf8");

Now I want to verify the signature on the back end side using Kotlin (or Java).

Problem:

1. I cannot find the information about the algorithm that is used for signing messages in the documentation.
2. It seems like there are no Java/Kotlin libraries that work with Phantom and Solana (found a few which are not updated for a year).

Question: How can I verify a signature from the Phantom wallet using Java? Or generally how can I do that in any language? (I will port it to Java then).

Answer 1

The easiest way to do it in Java or Kotlin is using sol4k.

First, add it to your project (Gradle as an example):

implementation 'org.sol4k:sol4k:0.3.2'

Then call it from your Java code:

import org.sol4k.Base58;
import org.sol4k.PublicKey;
// ...
boolean verifySignature(String signature, String walletAddress) {
    String message = "signing message";
    byte[] messageBytes = message.getBytes();
    PublicKey publicKey = new PublicKey(walletAddress);
    byte[] signatureBytes = Base58.decode(signature);
    return publicKey.verify(signatureBytes, messageBytes);
}

Here, signature and walletAddress are Base58 encoded strings.

Frontend example how to get wallet address and the signature (Phantom docs):

const provider = window.phantom.solana;
const encodedMessage = new TextEncoder().encode('signing message');
const signedMessage = await provider.signMessage(encodedMessage, "utf8");
const base58 = await import("bs58");
const walletAddress = provider.publicKey.toString();
const signature = base58.encode(signedMessage.signature);

Here is a blogpost on the subject.