feat(tee): add support for recoverable signatures

Signatures produced by the TEE Prover are now compatible with the
on-chain verifier that uses the `ecrecover` precompile.

Until now, we've been using _non-recoverable_ signatures in the TEE
prover with a compressed ECDSA public key in each attestation -- it was
compressed because there are only 64 bytes available in the report
attestation quote. That worked fine for off-chain proof verification,
but for on-chain verification, it's better to use the Ethereum address
derived from the public key so we can call ecrecover in Solidity to
verify the signature.

This PR goes hand in hand with matter-labs/teepot#228

core/bin/zksync_tee_prover/Cargo.toml

@@ -16,7 +16,11 @@ anyhow.workspace = true
 async-trait.workspace = true
 envy.workspace = true
 reqwest = { workspace = true, features = ["zstd"] }
-secp256k1 = { workspace = true, features = ["serde"] }
+secp256k1 = { workspace = true, features = [
+    "global-context",
+    "recovery",
+    "serde",
+] }
 serde = { workspace = true, features = ["derive"] }
 thiserror.workspace = true
 tokio = { workspace = true, features = ["full"] }
@@ -31,3 +35,7 @@ zksync_prover_interface.workspace = true
 zksync_tee_verifier.workspace = true
 zksync_types.workspace = true
 zksync_vlog.workspace = true
+
+[dev-dependencies]
+hex.workspace = true
+sha3.workspace = true

core/bin/zksync_tee_prover/src/api_client.rs

@@ -1,5 +1,5 @@
 use reqwest::{Client, Response, StatusCode};
-use secp256k1::{ecdsa::Signature, PublicKey};
+use secp256k1::PublicKey;
 use serde::Serialize;
 use url::Url;
 use zksync_basic_types::H256;
@@ -87,13 +87,13 @@ impl TeeApiClient {
     pub async fn submit_proof(
         &self,
         batch_number: L1BatchNumber,
-        signature: Signature,
+        signature: [u8; 65],
         pubkey: &PublicKey,
         root_hash: H256,
         tee_type: TeeType,
     ) -> Result<(), TeeProverError> {
         let request = SubmitTeeProofRequest(Box::new(L1BatchTeeProofForL1 {
-            signature: signature.serialize_compact().into(),
+            signature: signature.into(),
             pubkey: pubkey.serialize().into(),
             proof: root_hash.as_bytes().into(),
             tee_type,

core/bin/zksync_tee_prover/src/tee_prover.rs

@@ -1,6 +1,6 @@
 use std::fmt;
 
-use secp256k1::{ecdsa::Signature, Message, PublicKey, Secp256k1};
+use secp256k1::{Message, PublicKey, Secp256k1, SecretKey, SECP256K1};
 use zksync_basic_types::H256;
 use zksync_node_framework::{
     service::StopReceiver,
@@ -67,10 +67,24 @@ impl fmt::Debug for TeeProver {
 }
 
 impl TeeProver {
+    /// Signs the message in Ethereum-compatible format for on-chain verification.
+    fn sign_message(&self, sec: &SecretKey, message: Message) -> Result<[u8; 65], TeeProverError> {
+        let s = SECP256K1.sign_ecdsa_recoverable(&message, sec);
+        let (rec_id, data) = s.serialize_compact();
+
+        let mut signature = [0u8; 65];
+        signature[..64].copy_from_slice(&data);
+        // as defined in the Ethereum Yellow Paper (Appendix F)
+        // https://ethereum.github.io/yellowpaper/paper.pdf
+        signature[64] = 27 + rec_id.to_i32() as u8;
+
+        Ok(signature)
+    }
+
     fn verify(
         &self,
         tvi: TeeVerifierInput,
-    ) -> Result<(Signature, L1BatchNumber, H256), TeeProverError> {
+    ) -> Result<([u8; 65], L1BatchNumber, H256), TeeProverError> {
         match tvi {
             TeeVerifierInput::V1(tvi) => {
                 let observer = METRICS.proof_generation_time.start();
@@ -79,7 +93,7 @@ impl TeeProver {
                 let batch_number = verification_result.batch_number;
                 let msg_to_sign = Message::from_slice(root_hash_bytes)
                     .map_err(|e| TeeProverError::Verification(e.into()))?;
-                let signature = self.config.signing_key.sign_ecdsa(msg_to_sign);
+                let signature = self.sign_message(&self.config.signing_key, msg_to_sign)?;
                 let duration = observer.observe();
                 tracing::info!(
                     proof_generation_time = duration.as_secs_f64(),
@@ -182,3 +196,55 @@ impl Task for TeeProver {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use anyhow::Result;
+    use secp256k1::ecdsa::{RecoverableSignature, RecoveryId};
+    use sha3::{Digest, Keccak256};
+
+    /// Converts a public key into an Ethereum address by hashing the encoded public key with Keccak256.
+    pub fn public_key_to_address(public: &PublicKey) -> [u8; 20] {
+        let public_key_bytes = public.serialize_uncompressed();
+
+        // Skip the first byte (0x04) which indicates uncompressed key
+        let hash: [u8; 32] = Keccak256::digest(&public_key_bytes[1..]).into();
+
+        // Take the last 20 bytes of the hash to get the Ethereum address
+        let mut address = [0u8; 20];
+        address.copy_from_slice(&hash[12..]);
+        address
+    }
+
+    /// Equivalent to the ecrecover precompile.
+    pub fn recover_signer_unchecked(sig: &[u8; 65], msg: &Message) -> Result<[u8; 20]> {
+        let sig = RecoverableSignature::from_compact(
+            &sig[0..64],
+            RecoveryId::from_i32(sig[64] as i32 - 27)?,
+        )?;
+        let public = SECP256K1.recover_ecdsa(msg, &sig)?;
+        Ok(public_key_to_address(&public))
+    }
+
+    #[test]
+    /// Credit to:
+    /// https://github.com/taikoxyz/raiko/blob/ddba6b0add73f05778617e3950ffafc371b9293d/provers/sgx/guest/src/signature.rs#L67
+    fn recover() {
+        let proof = "01000000c13bd882edb37ffbabc9f9e34a0d9789633b850fe55e625b768cc8e5feed7d9f7ab536cbc210c2fcc1385aaf88d8a91d8adc2740245f9deee5fd3d61dd2a71662fb6639515f1e2f3354361a82d86c1952352c1a81b";
+        let proof_bytes = hex::decode(proof).unwrap();
+        let msg = "216ac5cd5a5e13b0c9a81efb1ad04526b9f4ddd2fe6ebc02819c5097dfb0958c";
+        let msg_bytes = hex::decode(msg).unwrap();
+        let proof_addr = recover_signer_unchecked(
+            &proof_bytes[24..].try_into().unwrap(),
+            &Message::from_slice(&msg_bytes).unwrap(),
+        )
+        .unwrap();
+        let priv_key = "324b5d1744ec27d6ac458350ce6a6248680bb0209521b2c730c1fe82a433eb54";
+        let priv_key_bytes = hex::decode(priv_key).unwrap();
+        let priv_key = SecretKey::from_slice(&priv_key_bytes).unwrap();
+        let pubkey = PublicKey::from_secret_key_global(&priv_key);
+        let pub_addr = public_key_to_address(&pubkey);
+        assert_eq!(pub_addr, proof_addr);
+    }
+}