keytree: add serialization for xpub, xprv (#228)

keytree/src/lib.rs

@@ -50,6 +50,38 @@ impl Xprv {
             precompressed_pubkey: self.precompressed_pubkey,
         }
     }
+
+    /// Serializes this Xprv to a sequence of bytes.
+    pub fn to_bytes(&self) -> [u8; 64] {
+        let mut buf = [0u8; 64];
+        buf[..32].copy_from_slice(&self.scalar.to_bytes());
+        buf[32..].copy_from_slice(&self.dk);
+        buf
+    }
+
+    /// Decodes an Xprv from a 64-byte array, and fails if the provided array is not
+    /// exactly 64 bytes.
+    pub fn from_bytes(bytes: &[u8]) -> Option<Self> {
+        if bytes.len() != 64 {
+            return None;
+        }
+
+        let mut scalar_bytes = [0u8; 32];
+        scalar_bytes.copy_from_slice(&bytes[..32]);
+        let scalar = match Scalar::from_canonical_bytes(scalar_bytes) {
+            Some(x) => x,
+            None => return None,
+        };
+        let mut dk = [0u8; 32];
+        dk.copy_from_slice(&bytes[32..]);
+        let precompressed_pubkey = (scalar * &constants::RISTRETTO_BASEPOINT_POINT).compress();
+
+        return Some(Xprv {
+            scalar,
+            dk,
+            precompressed_pubkey,
+        });
+    }
 }
 
 impl Xpub {
@@ -78,6 +110,36 @@ impl Xpub {
             precompressed_pubkey: child_point.compress(),
         }
     }
+
+    /// Serializes this Xpub to a sequence of bytes.
+    pub fn to_bytes(&self) -> [u8; 64] {
+        let mut buf = [0u8; 64];
+        buf[..32].copy_from_slice(self.precompressed_pubkey.as_bytes());
+        buf[32..].copy_from_slice(&self.dk);
+        buf
+    }
+
+    /// Decodes an Xpub from a 64-byte array, and fails if the provided array is not
+    /// exactly 64 bytes, or if the compressed point fails to decompress.
+    pub fn from_bytes(bytes: &[u8]) -> Option<Self> {
+        if bytes.len() != 64 {
+            return None;
+        }
+
+        let precompressed_pubkey = CompressedRistretto::from_slice(&bytes[..32]);
+        let mut dk = [0u8; 32];
+        dk.copy_from_slice(&bytes[32..]);
+
+        let point = match precompressed_pubkey.decompress() {
+            Some(p) => p,
+            None => return None,
+        };
+        Some(Xpub {
+            point,
+            dk,
+            precompressed_pubkey,
+        })
+    }
 }
 
 #[cfg(test)]
@@ -106,6 +168,42 @@ mod tests {
         assert_eq!(expected_scalar, xprv.scalar);
     }
 
+    #[test]
+    fn serialize_xprv_test() {
+        let seed = [0u8; 32];
+        let mut rng = ChaChaRng::from_seed(seed);
+        let xprv = Xprv::random(&mut rng);
+        let xprv_bytes = xprv.to_bytes();
+
+        // hardcoded, but happens to be expected_scalar concatenated with expected_dk
+        let expected_bytes = [
+            74, 83, 195, 251, 188, 89, 151, 14, 229, 248, 90, 248, 19, 135, 93, 255, 193, 58, 144,
+            74, 46, 83, 174, 126, 101, 250, 13, 234, 110, 98, 201, 1, 159, 7, 231, 190, 85, 81, 56,
+            122, 152, 186, 151, 124, 115, 45, 8, 13, 203, 15, 41, 160, 72, 227, 101, 105, 18, 198,
+            83, 62, 50, 238, 122, 237,
+        ];
+        assert_eq!(&xprv_bytes[..], &expected_bytes[..]);
+    }
+
+    #[test]
+    fn deserialize_xprv_test() {
+        let xprv_bytes = [
+            74, 83, 195, 251, 188, 89, 151, 14, 229, 248, 90, 248, 19, 135, 93, 255, 193, 58, 144,
+            74, 46, 83, 174, 126, 101, 250, 13, 234, 110, 98, 201, 1, 159, 7, 231, 190, 85, 81, 56,
+            122, 152, 186, 151, 124, 115, 45, 8, 13, 203, 15, 41, 160, 72, 227, 101, 105, 18, 198,
+            83, 62, 50, 238, 122, 237,
+        ];
+
+        let xprv = Xprv::from_bytes(&xprv_bytes).unwrap();
+
+        let seed = [0u8; 32];
+        let mut rng = ChaChaRng::from_seed(seed);
+        let expected_xprv = Xprv::random(&mut rng);
+
+        assert_eq!(xprv.dk, expected_xprv.dk);
+        assert_eq!(xprv.scalar, expected_xprv.scalar);
+    }
+
     #[test]
     fn random_xpub_test() {
         let seed = [0u8; 32];
@@ -129,6 +227,48 @@ mod tests {
         assert_eq!(xpub.precompressed_pubkey, expected_compressed_point);
     }
 
+    #[test]
+    fn serialize_xpub_test() {
+        let seed = [0u8; 32];
+        let mut rng = ChaChaRng::from_seed(seed);
+        let xprv = Xprv::random(&mut rng);
+        let xpub = xprv.to_xpub();
+
+        let xpub_bytes = xpub.to_bytes();
+
+        // hardcoded, but happens to be expected_scalar concatenated with expected_compressed_point
+        let expected_bytes = [
+            156, 102, 163, 57, 200, 52, 79, 146, 47, 195, 32, 108, 181, 218, 232, 20, 165, 148,
+            192, 23, 125, 211, 35, 92, 37, 77, 156, 64, 154, 101, 184, 8, 159, 7, 231, 190, 85, 81,
+            56, 122, 152, 186, 151, 124, 115, 45, 8, 13, 203, 15, 41, 160, 72, 227, 101, 105, 18,
+            198, 83, 62, 50, 238, 122, 237,
+        ];
+        assert_eq!(&xpub_bytes[..], &expected_bytes[..]);
+    }
+
+    #[test]
+    fn deserialize_xpub_test() {
+        let xpub_bytes = [
+            156, 102, 163, 57, 200, 52, 79, 146, 47, 195, 32, 108, 181, 218, 232, 20, 165, 148,
+            192, 23, 125, 211, 35, 92, 37, 77, 156, 64, 154, 101, 184, 8, 159, 7, 231, 190, 85, 81,
+            56, 122, 152, 186, 151, 124, 115, 45, 8, 13, 203, 15, 41, 160, 72, 227, 101, 105, 18,
+            198, 83, 62, 50, 238, 122, 237,
+        ];
+        let xpub = Xpub::from_bytes(&xpub_bytes).unwrap();
+
+        let seed = [0u8; 32];
+        let mut rng = ChaChaRng::from_seed(seed);
+        let expected_xprv = Xprv::random(&mut rng);
+        let expected_xpub = expected_xprv.to_xpub();
+
+        assert_eq!(xpub.dk, expected_xpub.dk);
+        assert_eq!(xpub.point, expected_xpub.point);
+        assert_eq!(
+            xpub.precompressed_pubkey,
+            expected_xpub.precompressed_pubkey
+        );
+    }
+
     #[test]
     fn random_xpub_derivation_test() {
         let seed = [0u8; 32];