Refine type for MAC (#577)

zebra-chain/src/transaction/joinsplit.rs

@@ -34,9 +34,7 @@ pub struct JoinSplit<P: ZkSnarkProof> {
     /// JoinSplit description.
     pub random_seed: [u8; 32],
     /// A message authentication tag.
-    ///
-    /// XXX refine type to [T; 2] -- there are two macs
-    pub vmacs: [[u8; 32]; 2],
+    pub vmacs: [crate::types::MAC; 2],
     /// A ZK JoinSplit proof, either a
     /// [`Groth16Proof`](crate::proofs::Groth16Proof) or a
     /// [`Bctv14Proof`](crate::proofs::Bctv14Proof).

zebra-chain/src/transaction/serialize.rs

@@ -242,8 +242,8 @@ impl<P: ZkSnarkProof> ZcashSerialize for JoinSplit<P> {
         writer.write_all(&self.commitments[1][..])?;
         writer.write_all(&self.ephemeral_key.as_bytes()[..])?;
         writer.write_all(&self.random_seed[..])?;
-        writer.write_all(&self.vmacs[0][..])?;
-        writer.write_all(&self.vmacs[1][..])?;
+        self.vmacs[0].zcash_serialize(&mut writer)?;
+        self.vmacs[1].zcash_serialize(&mut writer)?;
         self.zkproof.zcash_serialize(&mut writer)?;
         self.enc_ciphertexts[0].zcash_serialize(&mut writer)?;
         self.enc_ciphertexts[1].zcash_serialize(&mut writer)?;
@@ -261,7 +261,10 @@ impl<P: ZkSnarkProof> ZcashDeserialize for JoinSplit<P> {
             commitments: [reader.read_32_bytes()?, reader.read_32_bytes()?],
             ephemeral_key: x25519_dalek::PublicKey::from(reader.read_32_bytes()?),
             random_seed: reader.read_32_bytes()?,
-            vmacs: [reader.read_32_bytes()?, reader.read_32_bytes()?],
+            vmacs: [
+                crate::types::MAC::zcash_deserialize(&mut reader)?,
+                crate::types::MAC::zcash_deserialize(&mut reader)?,
+            ],
             zkproof: P::zcash_deserialize(&mut reader)?,
             enc_ciphertexts: [
                 notes::sprout::EncryptedCiphertext::zcash_deserialize(&mut reader)?,

zebra-chain/src/transaction/tests/arbitrary.rs

@@ -26,7 +26,7 @@ impl<P: ZkSnarkProof + Arbitrary + 'static> Arbitrary for JoinSplit<P> {
             array::uniform2(array::uniform32(any::<u8>())),
             array::uniform32(any::<u8>()),
             array::uniform32(any::<u8>()),
-            array::uniform2(array::uniform32(any::<u8>())),
+            array::uniform2(any::<crate::types::MAC>()),
             any::<P>(),
             array::uniform2(any::<sprout::EncryptedCiphertext>()),
         )

zebra-chain/src/types.rs

@@ -1,20 +1,15 @@
 //! Newtype wrappers for primitive data types with semantic meaning.
-
+#![allow(clippy::unit_arg)]
+use crate::serialization::{
+    ReadZcashExt, SerializationError, WriteZcashExt, ZcashDeserialize, ZcashSerialize,
+};
+use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
+use chrono::{DateTime, TimeZone, Utc};
 use std::{
     fmt,
     io::{self, Read},
 };
 
-use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
-use chrono::{DateTime, TimeZone, Utc};
-
-#[cfg(test)]
-use proptest_derive::Arbitrary;
-
-use crate::serialization::{
-    ReadZcashExt, SerializationError, WriteZcashExt, ZcashDeserialize, ZcashSerialize,
-};
-
 pub mod amount;
 
 /// A u32 which represents a block height value.
@@ -93,9 +88,30 @@ impl Arbitrary for LockTime {
     type Strategy = BoxedStrategy<Self>;
 }
 
+/// A sequence of message authentication tags ...
+///
+/// binding h_sig to each a_sk of the JoinSplit description, computed as
+/// described in § 4.10 ‘Non-malleability (Sprout)’ on p. 37
+#[derive(PartialEq, Clone, Debug, Serialize, Deserialize)]
+#[cfg_attr(test, derive(proptest_derive::Arbitrary))]
+pub struct MAC([u8; 32]);
+
+impl ZcashDeserialize for MAC {
+    fn zcash_deserialize<R: Read>(mut reader: R) -> Result<Self, SerializationError> {
+        let bytes = reader.read_32_bytes()?;
+
+        Ok(Self(bytes))
+    }
+}
+
+impl ZcashSerialize for MAC {
+    fn zcash_serialize<W: io::Write>(&self, mut writer: W) -> Result<(), io::Error> {
+        writer.write_all(&self.0[..])
+    }
+}
 /// An encoding of a Bitcoin script.
 #[derive(Clone, Eq, PartialEq, Serialize, Deserialize)]
-#[cfg_attr(test, derive(Arbitrary))]
+#[cfg_attr(test, derive(proptest_derive::Arbitrary))]
 pub struct Script(pub Vec<u8>);
 
 impl fmt::Debug for Script {