add derive child key for Private Keys

Author: QuestofIranon

src/crypto.rs

@@ -313,8 +313,7 @@ impl PublicKey {
     }
 }
 
-/// Construct a `PublicKey` from a hex representation of a raw or ASN.1 encoded
-/// key.
+/// Construct a `PublicKey` from a hex representation of a raw or ASN.1 encoded key.
 impl FromStr for PublicKey {
     type Err = Error;
 
@@ -468,7 +467,7 @@ 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 +494,22 @@ 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 {
@@ -532,7 +547,7 @@ impl SecretKey {
 
         input[0] = 0u8;
         input[1..33].copy_from_slice(parent_key);
-        input[34..37].copy_from_slice(&index.to_be_bytes());
+        input[33..37].copy_from_slice(&index.to_be_bytes());
 
         input[33] = input[33] | 128u8;
 
@@ -767,6 +782,7 @@ mod tests {
         Ok(())
     }
 
+
     #[test]
     fn test_reconstruct() -> Result<(), Error> {
         let (secret1, mnemonic) = SecretKey::generate_mnemonic();
@@ -776,4 +792,16 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn test_reconstruct_with_passphrase() -> Result<(), Error> {
+        let passphrase = "HelloHedera!";
+
+        let (secret1, mnemonic) = SecretKey::generate_mnemonic_with_passphrase(passphrase);
+        let secret2 = SecretKey::from_mnemonic_with_passphrase(&mnemonic, passphrase)?;
+
+        assert_eq!(secret1.as_bytes(), secret2.as_bytes());
+
+        Ok(())
+    }
 }