feat(secret): add BIP-0032 secp256k1 derivation for Ethereum keys behind feature flag

Fix derive_ethereum_key to use BIP-0032 instead of SLIP-0010, which
incorrectly handled unhardened indices in the Ethereum path m/44'/60'/0'/0/0.

crates/alknet-secret/src/derivation.rs

@@ -156,12 +156,12 @@ fn derive_master_key(seed: &[u8]) -> Result<XPrv, DerivationError> {
     Ok(XPrv::from_nonextended_force(&priv_bytes, &cc_bytes))
 }
 
-/// Parse a derivation path string into hardened child indices.
+/// Parse a derivation path string into child indices.
 ///
 /// Path format: `m/74'/0'/0'/0'`
-/// Each component must be a hardened index (with `'` or `h` suffix).
-/// Unhardened indices are allowed for BIP-0032 paths (e.g., Ethereum `m/44'/60'/0'/0/0`).
-fn parse_derivation_path(path: &str) -> Result<Vec<u32>, DerivationError> {
+/// Hardened indices have `'` or `h` suffix. Unhardened indices are allowed
+/// for BIP-0032 paths (e.g., Ethereum `m/44'/60'/0'/0/0`).
+pub fn parse_derivation_path(path: &str) -> Result<Vec<u32>, DerivationError> {
     if !path.starts_with('m') {
         return Err(DerivationError::InvalidPath(
             "path must start with 'm'".to_string(),
@@ -199,6 +199,10 @@ pub enum DerivationError {
     KeyDerivation(String),
     #[error("seed is not unlocked")]
     Locked,
+    #[error("secp256k1 error: {0}")]
+    Secp256k1(String),
+    #[error("unsupported key type")]
+    UnsupportedKeyType,
 }
 
 #[cfg(test)]

crates/alknet-secret/src/ethereum.rs

@@ -0,0 +1,246 @@
+//! BIP-0032 secp256k1 HD key derivation for Ethereum keys.
+//!
+//! This module implements hierarchical deterministic key derivation following
+//! BIP-0032 for secp256k1 curves. It is gated behind the `secp256k1` feature flag.
+//!
+//! Unlike SLIP-0010 (Ed25519), BIP-0032 supports both hardened and unhardened
+//! child derivation and uses HMAC-SHA512 with the key "Bitcoin seed" (not
+//! "ed25519 seed").
+//!
+//! # Ethereum Path
+//!
+//! The standard Ethereum derivation path is `m/44'/60'/0'/0/0` (EIP-84).
+//! The last two indices (`0/0`) are unhardened, which SLIP-0010 cannot handle.
+
+use hmac::{Hmac, Mac};
+use secp256k1::{PublicKey, Secp256k1, SecretKey};
+use sha2::Sha512;
+use zeroize::Zeroize;
+
+use crate::derivation::{parse_derivation_path, DerivationError};
+
+type HmacSha512 = Hmac<Sha512>;
+
+const HARDENED_OFFSET: u32 = 0x80000000;
+
+/// An extended private key for BIP-0032 secp256k1 derivation.
+///
+/// Contains the private key, compressed public key (33 bytes), and chain code
+/// for further child derivation.
+#[derive(Zeroize)]
+#[zeroize(drop)]
+pub struct Secp256k1ExtendedPrivKey {
+    /// The secp256k1 private key bytes (32 bytes).
+    #[zeroize]
+    private_key: Vec<u8>,
+    /// The compressed public key bytes (33 bytes).
+    public_key: Vec<u8>,
+    /// The chain code for child derivation (32 bytes).
+    chain_code: Vec<u8>,
+}
+
+impl Secp256k1ExtendedPrivKey {
+    /// Returns the private key bytes (32 bytes).
+    pub fn private_key(&self) -> &[u8] {
+        &self.private_key
+    }
+
+    /// Returns the compressed public key bytes (33 bytes).
+    pub fn public_key(&self) -> &[u8] {
+        &self.public_key
+    }
+
+    /// Returns the chain code bytes (32 bytes).
+    pub fn chain_code(&self) -> &[u8] {
+        &self.chain_code
+    }
+}
+
+/// Derive the BIP-0032 secp256k1 master key from a seed.
+///
+/// Uses HMAC-SHA512 with key "Bitcoin seed" over the seed bytes,
+/// following the BIP-0032 specification.
+pub fn derive_secp256k1_master_key(
+    seed: &[u8],
+) -> Result<Secp256k1ExtendedPrivKey, DerivationError> {
+    let mut mac = HmacSha512::new_from_slice(b"Bitcoin seed")
+        .map_err(|e| DerivationError::Hmac(e.to_string()))?;
+    mac.update(seed);
+    let result = mac.finalize().into_bytes();
+
+    let private_key_bytes = &result[..32];
+    let chain_code_bytes = &result[32..];
+
+    let secp = Secp256k1::new();
+    let secret_key = SecretKey::from_slice(private_key_bytes)
+        .map_err(|e| DerivationError::Secp256k1(e.to_string()))?;
+    let public_key = PublicKey::from_secret_key(&secp, &secret_key);
+
+    Ok(Secp256k1ExtendedPrivKey {
+        private_key: secret_key.secret_bytes().to_vec(),
+        public_key: public_key.serialize().to_vec(),
+        chain_code: chain_code_bytes.to_vec(),
+    })
+}
+
+/// Derive a child extended private key from a parent key at the given index.
+///
+/// For hardened indices (>= 0x80000000), uses the parent private key in the HMAC.
+/// For unhardened indices (< 0x80000000), uses the parent public key in the HMAC.
+fn derive_child(
+    parent: &Secp256k1ExtendedPrivKey,
+    index: u32,
+) -> Result<Secp256k1ExtendedPrivKey, DerivationError> {
+    let secp = Secp256k1::new();
+
+    let mut mac = HmacSha512::new_from_slice(parent.chain_code())
+        .map_err(|e| DerivationError::Hmac(e.to_string()))?;
+
+    if index >= HARDENED_OFFSET {
+        // Hardened child: HMAC-SHA512(Key = parent chain code, Data = 0x00 || parent private key || index)
+        mac.update(&[0x00]);
+        mac.update(parent.private_key());
+    } else {
+        // Unhardened child: HMAC-SHA512(Key = parent chain code, Data = parent public key || index)
+        mac.update(parent.public_key());
+    }
+    mac.update(&index.to_be_bytes());
+
+    let result = mac.finalize().into_bytes();
+    let child_key_bytes = &result[..32];
+    let child_chain_code = &result[32..];
+
+    // Add parent private key to child key bytes (mod n, the curve order)
+    let parent_secret = SecretKey::from_slice(parent.private_key())
+        .map_err(|e| DerivationError::Secp256k1(e.to_string()))?;
+    let child_key_raw = SecretKey::from_slice(child_key_bytes)
+        .map_err(|e| DerivationError::Secp256k1(e.to_string()))?;
+
+    // Tweak: child_key = (parent_key + tweak) mod n
+    let child_secret = parent_secret
+        .add_tweak(&child_key_raw.into())
+        .map_err(|e| DerivationError::Secp256k1(e.to_string()))?;
+
+    let child_public = PublicKey::from_secret_key(&secp, &child_secret);
+
+    Ok(Secp256k1ExtendedPrivKey {
+        private_key: child_secret.secret_bytes().to_vec(),
+        public_key: child_public.serialize().to_vec(),
+        chain_code: child_chain_code.to_vec(),
+    })
+}
+
+/// Derive a secp256k1 extended private key from a seed and derivation path.
+///
+/// This is the primary entry point for BIP-0032 secp256k1 derivation.
+/// Supports both hardened and unhardened indices.
+///
+/// # Example
+///
+/// ```ignore
+/// use alknet_secret::ethereum::derive_secp256k1_path;
+/// use alknet_secret::derivation::PATHS;
+///
+/// let key = derive_secp256k1_path(seed, PATHS::ETHEREUM).unwrap();
+/// assert_eq!(key.private_key().len(), 32);
+/// assert_eq!(key.public_key().len(), 33); // compressed
+/// ```
+pub fn derive_secp256k1_path(
+    seed: &[u8],
+    path: &str,
+) -> Result<Secp256k1ExtendedPrivKey, DerivationError> {
+    let indices = parse_derivation_path(path)?;
+    let master = derive_secp256k1_master_key(seed)?;
+
+    let mut current = master;
+    for index in indices {
+        current = derive_child(&current, index)?;
+    }
+
+    Ok(current)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::PATHS;
+
+    #[test]
+    fn test_bip32_master_key_vector() {
+        // BIP-0032 test vector 1: seed "000102030405060708090a0b0c0d0e0f"
+        let seed = hex::decode("000102030405060708090a0b0c0d0e0f").unwrap();
+        let master = derive_secp256k1_master_key(&seed).unwrap();
+
+        // Expected master private key from BIP-0032 test vector 1
+        let expected_priv =
+            hex::decode("e8f32e723decf4051aefac8e2c93c9c5b214313817cdb01a1494b917c8436b35")
+                .unwrap();
+        assert_eq!(master.private_key(), expected_priv.as_slice());
+
+        // Expected master public key (compressed) from BIP-0032 test vector 1
+        let expected_pub =
+            hex::decode("0339a36013301597daef41fbe593a02cc513d0b55527ec2df1050e2e8ff49c85c2")
+                .unwrap();
+        assert_eq!(master.public_key(), expected_pub.as_slice());
+
+        // Expected chain code from BIP-0032 test vector 1
+        let expected_cc =
+            hex::decode("873dff81c02f525623fd1fe5167eac3a55a049de3d314bb42ee227ffed37d508")
+                .unwrap();
+        assert_eq!(master.chain_code(), expected_cc.as_slice());
+    }
+
+    #[test]
+    fn test_bip32_derive_m_44h_60h_0h_0_0() {
+        let seed = hex::decode("000102030405060708090a0b0c0d0e0f").unwrap();
+        let key = derive_secp256k1_path(&seed, "m/44'/60'/0'/0/0").unwrap();
+        assert_eq!(key.private_key().len(), 32);
+        assert_eq!(key.public_key().len(), 33);
+    }
+
+    #[test]
+    fn test_ethereum_keypair_is_valid() {
+        let mnemonic = crate::mnemonic::Mnemonic::generate(24).unwrap();
+        let seed = mnemonic.to_seed(None);
+        let key = derive_secp256k1_path(seed.as_bytes(), PATHS::ETHEREUM).unwrap();
+
+        let secp = Secp256k1::new();
+        let secret_key = SecretKey::from_slice(key.private_key()).unwrap();
+        let public_key = PublicKey::from_secret_key(&secp, &secret_key);
+        assert_eq!(key.public_key(), public_key.serialize().as_slice());
+    }
+
+    #[test]
+    fn test_ethereum_differs_from_ed25519() {
+        let mnemonic = crate::mnemonic::Mnemonic::generate(24).unwrap();
+        let seed = mnemonic.to_seed(None);
+
+        let eth_key = derive_secp256k1_path(seed.as_bytes(), PATHS::ETHEREUM).unwrap();
+        let ed_key =
+            crate::derivation::derive_path_from_seed(seed.as_bytes(), PATHS::ETHEREUM).unwrap();
+
+        assert_ne!(eth_key.private_key(), ed_key.private_key());
+    }
+
+    #[test]
+    fn test_deterministic_derivation() {
+        let mnemonic = crate::mnemonic::Mnemonic::generate(24).unwrap();
+        let seed = mnemonic.to_seed(None);
+
+        let key1 = derive_secp256k1_path(seed.as_bytes(), PATHS::ETHEREUM).unwrap();
+        let key2 = derive_secp256k1_path(seed.as_bytes(), PATHS::ETHEREUM).unwrap();
+
+        assert_eq!(key1.private_key(), key2.private_key());
+        assert_eq!(key1.public_key(), key2.public_key());
+    }
+
+    #[test]
+    fn test_compressed_public_key_is_33_bytes() {
+        let mnemonic = crate::mnemonic::Mnemonic::generate(24).unwrap();
+        let seed = mnemonic.to_seed(None);
+        let key = derive_secp256k1_path(seed.as_bytes(), PATHS::ETHEREUM).unwrap();
+        assert_eq!(key.public_key().len(), 33);
+        // Compressed public key starts with 0x02 or 0x03
+        assert!(key.public_key()[0] == 0x02 || key.public_key()[0] == 0x03);
+    }
+}

crates/alknet-secret/src/lib.rs

@@ -26,6 +26,7 @@
 //! - [`encryption`] — AES-256-GCM encrypt/decrypt and `EncryptedData` type
 //! - [`protocol`] — `SecretProtocol` irpc service enum, `DerivedKey`, `KeyType`
 //! - [`service`] — `SecretService` implementation with Unlock/Lock lifecycle
+//! - [`ethereum`] — BIP-0032 secp256k1 HD key derivation (behind `secp256k1` feature)
 
 pub mod derivation;
 pub mod encryption;
@@ -33,9 +34,15 @@ pub mod mnemonic;
 pub mod protocol;
 pub mod service;
 
+#[cfg(feature = "secp256k1")]
+pub mod ethereum;
+
 // Re-export primary public API
-pub use derivation::{ExtendedPrivKey, PATHS};
+pub use derivation::{DerivationError, ExtendedPrivKey, PATHS};
 pub use encryption::{EncryptedData, EncryptionError};
 pub use mnemonic::{Language, Mnemonic, Seed};
 pub use protocol::{DerivedKey, KeyType, SecretMessage, SecretProtocol};
 pub use service::{SecretService, SecretServiceError, SecretServiceHandle};
+
+#[cfg(feature = "secp256k1")]
+pub use ethereum::Secp256k1ExtendedPrivKey;

crates/alknet-secret/src/service.rs

@@ -73,6 +73,8 @@ pub enum SecretServiceError {
     Encryption(String),
     #[error("invalid path: {0}")]
     InvalidPath(String),
+    #[error("unsupported key type")]
+    UnsupportedKeyType,
 }
 
 impl From<crate::mnemonic::MnemonicError> for SecretServiceError {
@@ -200,22 +202,35 @@ impl SecretServiceHandle {
     }
 
     /// Derive a secp256k1 (Ethereum) keypair at the given path.
+    ///
+    /// Uses BIP-0032 derivation (HMAC-SHA512 with "Bitcoin seed") when the
+    /// `secp256k1` feature is enabled. Returns `UnsupportedKeyType` when the
+    /// feature is disabled.
     pub fn derive_ethereum_key(&self, path: &str) -> Result<DerivedKey, SecretServiceError> {
-        let inner = self.inner.read().unwrap();
-        if !inner.unlocked {
-            return Err(SecretServiceError::ServiceLocked);
-        }
-        let seed = inner
-            .seed
-            .as_ref()
-            .ok_or(SecretServiceError::ServiceLocked)?;
+        #[cfg(feature = "secp256k1")]
+        {
+            let inner = self.inner.read().unwrap();
+            if !inner.unlocked {
+                return Err(SecretServiceError::ServiceLocked);
+            }
+            let seed = inner
+                .seed
+                .as_ref()
+                .ok_or(SecretServiceError::ServiceLocked)?;
 
-        let key = derivation::derive_path_from_seed(seed.as_bytes(), path)?;
-        Ok(DerivedKey {
-            key_type: KeyType::Secp256k1,
-            private_key: key.private_key().to_vec(),
-            public_key: key.public_key().to_vec(),
-        })
+            let key = crate::ethereum::derive_secp256k1_path(seed.as_bytes(), path)?;
+            Ok(DerivedKey {
+                key_type: KeyType::Secp256k1,
+                private_key: key.private_key().to_vec(),
+                public_key: key.public_key().to_vec(),
+            })
+        }
+
+        #[cfg(not(feature = "secp256k1"))]
+        {
+            let _ = path;
+            Err(SecretServiceError::UnsupportedKeyType)
+        }
     }
 
     /// Encrypt plaintext using the derived encryption key.
@@ -399,4 +414,41 @@ mod tests {
         service.lock();
         assert!(service.decrypt(&encrypted).is_err());
     }
+
+    #[cfg(feature = "secp256k1")]
+    #[test]
+    fn test_derive_ethereum_key_bip32() {
+        let service = SecretServiceHandle::new();
+        service.unlock_new(24).unwrap();
+
+        let key = service.derive_ethereum_key(PATHS::ETHEREUM).unwrap();
+        assert_eq!(key.key_type, KeyType::Secp256k1);
+        assert_eq!(key.private_key.len(), 32);
+        assert_eq!(key.public_key.len(), 33);
+    }
+
+    #[cfg(feature = "secp256k1")]
+    #[test]
+    fn test_ethereum_key_differs_from_ed25519() {
+        let service = SecretServiceHandle::new();
+        service.unlock_new(24).unwrap();
+
+        let eth_key = service.derive_ethereum_key(PATHS::ETHEREUM).unwrap();
+        let ed_key = service.derive_ed25519(PATHS::IDENTITY).unwrap();
+
+        assert_ne!(eth_key.private_key, ed_key.private_key);
+    }
+
+    #[cfg(not(feature = "secp256k1"))]
+    #[test]
+    fn test_derive_ethereum_key_unsupported_without_feature() {
+        let service = SecretServiceHandle::new();
+        service.unlock_new(24).unwrap();
+
+        let result = service.derive_ethereum_key(PATHS::ETHEREUM);
+        assert!(matches!(
+            result,
+            Err(SecretServiceError::UnsupportedKeyType)
+        ));
+    }
 }