zip316::transparent: Implement ZIP 316 transparent internal ovk.

zcash_primitives/src/transparent.rs

@@ -1,6 +1,7 @@
-use crate::legacy::TransparentAddress;
+use crate::{legacy::TransparentAddress, sapling::keys::prf_expand_vec};
 use hdwallet::{ExtendedPrivKey, ExtendedPubKey};
 use sha2::{Digest, Sha256};
+use std::convert::TryInto;
 
 /// A type representing a private key at the BIP-44 external child
 /// level `m/44'/<coin_type>'/<account>'/0/<child_index>
@@ -42,4 +43,54 @@ impl ExternalPubKey {
     pub fn public_key(&self) -> &secp256k1::key::PublicKey {
         &self.0.public_key
     }
+
+    /// Returns the chain code component of this public key.
+    pub fn chain_code(&self) -> &[u8] {
+        &self.0.chain_code
+    }
+
+    /// Derives the internal ovk and external ovk corresponding to this
+    /// transparent fvk. As specified in [ZIP 316][transparent-ovk].
+    ///
+    /// [transparent-ovk]: https://zips.z.cash/zip-0316#deriving-internal-keys
+    fn ovk_for_shielding(&self) -> (InternalOvk, ExternalOvk) {
+        let i_ovk = prf_expand_vec(
+            &self.chain_code(),
+            &[&[0xd0], &self.public_key().serialize()],
+        );
+        let i_ovk = i_ovk.as_bytes();
+        let ovk_internal = InternalOvk(i_ovk[..32].try_into().unwrap());
+        let ovk_external = ExternalOvk(i_ovk[32..].try_into().unwrap());
+
+        (ovk_internal, ovk_external)
+    }
+
+    /// Derives the internal ovk corresponding to this transparent fvk.
+    pub fn internal_ovk(&self) -> InternalOvk {
+        self.ovk_for_shielding().0
+    }
+
+    /// Derives the external ovk corresponding to this transparent fvk.
+    pub fn external_ovk(&self) -> ExternalOvk {
+        self.ovk_for_shielding().1
+    }
+}
+
+/// Internal ovk used for autoshielding.
+pub struct InternalOvk([u8; 32]);
+
+impl InternalOvk {
+    pub fn as_bytes(&self) -> [u8; 32] {
+        self.0
+    }
+}
+
+/// External ovk used by zcashd for transparent -> shielded spends to
+/// external receivers.
+pub struct ExternalOvk([u8; 32]);
+
+impl ExternalOvk {
+    pub fn as_bytes(&self) -> [u8; 32] {
+        self.0
+    }
 }