feat: add message signing module

Cargo.toml

@@ -17,9 +17,17 @@ rayon = "1.10.0"
 futures = "0.3"
 log = "0.4"
 async-trait = "0.1"
-reqwest = { version = "0.12.4", features = ["rustls-tls", "gzip", "json"], default-features = false, optional = true }
+reqwest = { version = "0.12.4", features = [
+  "rustls-tls",
+  "gzip",
+  "json",
+], default-features = false, optional = true }
 hex = { version = "0.4.3", features = ["serde"], optional = true }
 bdk_coin_select = "0.4.0"
+bip322 = "0.0.7"
+
+[dev-dependencies]
+rand = "0.9.0"
 
 [features]
 blindbit-backend = ["reqwest", "hex"]

src/client/mod.rs

@@ -1,4 +1,5 @@
 mod client;
+mod signing;
 mod spend;
 mod structs;
 

src/client/signing.rs

@@ -0,0 +1,92 @@
+use bitcoin::key::Secp256k1;
+use bitcoin::{Address, Network, PrivateKey, Witness};
+use silentpayments::SilentPaymentAddress;
+
+use crate::SpClient;
+
+impl SpClient {
+    /// Signs a message using the client's spend key.
+    pub fn sign_message(&self, msg: &[u8]) -> anyhow::Result<Witness> {
+        let network = Network::Bitcoin;
+        let secp = Secp256k1::new();
+
+        let spend_sk = self.try_get_secret_spend_key()?;
+        let spend_pk = spend_sk.public_key(&secp);
+        let spend_privkey = PrivateKey::new(spend_sk, network);
+
+        let xonly = spend_pk.x_only_public_key().0;
+
+        let address = Address::p2tr(&secp, xonly, None, network);
+
+        Ok(bip322::sign_simple(&address, msg, spend_privkey)?)
+    }
+}
+
+pub fn verify_message(
+    address: SilentPaymentAddress,
+    msg: &[u8],
+    signature: Witness,
+) -> anyhow::Result<()> {
+    let spend_pk = address.get_spend_key();
+
+    let xonly = spend_pk.x_only_public_key().0;
+
+    let network = Network::Bitcoin;
+    let secp = Secp256k1::new();
+    let taproot_address = Address::p2tr(&secp, xonly, None, network);
+
+    Ok(bip322::verify_simple(&taproot_address, msg, signature)?)
+}
+
+#[cfg(test)]
+mod test {
+    use bitcoin::{Network, Witness};
+    use silentpayments::secp256k1::SecretKey;
+
+    use crate::{client::signing::verify_message, SpClient, SpendKey};
+
+    fn create_random_client() -> SpClient {
+        use rand::prelude::*;
+        let mut rng = rand::rng();
+
+        let scan_sk_bytes: [u8; 32] = rng.random();
+        let spend_sk_bytes: [u8; 32] = rng.random();
+
+        let network = Network::Bitcoin;
+        let scan_sk = SecretKey::from_slice(&scan_sk_bytes).unwrap();
+        let spend_sk = SecretKey::from_slice(&spend_sk_bytes).unwrap();
+
+        SpClient::new(scan_sk, SpendKey::Secret(spend_sk), network).unwrap()
+    }
+
+    #[test]
+    fn sign_and_verify() {
+        // message to sign
+        let message = b"random message to sign";
+
+        // different, unrelated message
+        let unrelated_message = b"wrong message";
+
+        // create a random sp-client with an sp-address
+        let client = create_random_client();
+        let sp_address = client.get_receiving_address();
+
+        // create a different, unrelated client
+        let unrelated_client = create_random_client();
+        let unrelated_sp_address = unrelated_client.get_receiving_address();
+
+        // sign message and verify it is correct with the client's sp-address
+        let witness = client.sign_message(message).unwrap();
+        assert!(verify_message(sp_address, message, witness.clone()).is_ok());
+
+        // different message should be an error
+        assert!(verify_message(sp_address, unrelated_message, witness.clone()).is_err());
+
+        // different sp_address should be an error
+        assert!(verify_message(unrelated_sp_address, message, witness).is_err());
+
+        // different witness should be an error
+        let random_witness = Witness::from_slice(&[[]]);
+        assert!(verify_message(sp_address, message, random_witness).is_err());
+    }
+}