add derive child key for Private Keys

src/crypto.rs

@@ -468,7 +468,7 @@ impl SecretKey {
     ///
     /// Mnemonics generated by the Android and iOS wallets will work
     ///
-    /// Returned key will support deriving child keys with `.derive_child()`
+    /// Returned key will support deriving child keys with `.derive_child_key(index)`
     pub fn from_mnemonic_with_passphrase(mnemonic: &str, passphrase: &str) -> Result<Self, Error> {
         let mnemonic = Mnemonic::from_phrase(mnemonic, Language::English)?;
 
@@ -495,6 +495,22 @@ impl SecretKey {
         Ok(secret_key)
     }
 
+    /// Derive a new private key at the given wallet index.
+    ///
+    /// Currently fails if the key was not generated with `generate_mnemonic` or `generated_mnemonic_with_passphrase`
+    /// or reconstructed with `from_mnemonic` or `from_mnemonic_with_passphrase`
+    pub fn derive_child_key(&self, index: u32) -> Result<Self, Error> {
+        let chain_code = self.chain_code
+            .map_or(Err(err_msg("this Ed25519 private key does not support key derivation")), |cc| {Ok(cc)})?;
+
+        let (key_bytes, chain_code) = Self::derive_child_key_bytes(self.as_bytes(), &chain_code, &index)?;
+
+        Ok(SecretKey{
+            value: ed25519_dalek::SecretKey::from_bytes(&key_bytes)?,
+            chain_code: Some(chain_code)
+        })
+    }
+
     /// Format a `SecretKey` as a vec of bytes in ASN.1 format.
     pub fn to_encoded_bytes(&self) -> Vec<u8> {
         der_encode(&PrivateKeyInfo {