pczt: Encode as header-prefixed Postcard

pczt/Cargo.toml

@@ -18,6 +18,11 @@ zcash_protocol = { workspace = true, optional = true }
 blake2b_simd = { workspace = true, optional = true }
 rand_core = { workspace = true, optional = true }
 
+# Encoding
+postcard = { version = "1", features = ["alloc"] }
+serde.workspace = true
+serde_with = "3"
+
 # Payment protocols
 # - Transparent
 secp256k1 = { workspace = true, optional = true  }

pczt/src/common.rs

@@ -1,9 +1,11 @@
 use std::collections::BTreeMap;
 
+use serde::{Deserialize, Serialize};
+
 use crate::roles::combiner::merge_map;
 
 /// Global fields that are relevant to the transaction as a whole.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub(crate) struct Global {
     //
     // Transaction effecting data.
@@ -128,7 +130,7 @@ impl Global {
     }
 }
 
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub(crate) struct Zip32Derivation {
     /// The [ZIP 32 seed fingerprint](https://zips.z.cash/zip-0032#seed-fingerprints).
     pub(crate) seed_fingerprint: [u8; 32],

pczt/src/lib.rs

@@ -52,20 +52,25 @@
 //! - Transaction Extractor (anyone can execute)
 //!   - Creates bindingSig and extracts the final transaction.
 
+use serde::{Deserialize, Serialize};
+
 pub mod roles;
 
 mod common;
 mod orchard;
 mod sapling;
 mod transparent;
 
+const MAGIC_BYTES: &[u8] = b"PCZT";
+const PCZT_VERSION_1: u32 = 1;
+
 #[cfg(feature = "zcp-builder")]
 const SAPLING_TX_VERSION: u32 = 4;
 const V5_TX_VERSION: u32 = 5;
 const V5_VERSION_GROUP_ID: u32 = 0x26A7270A;
 
 /// A partially-created Zcash transaction.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct Pczt {
     /// Global fields that are relevant to the transaction as a whole.
     global: common::Global,
@@ -82,3 +87,43 @@ pub struct Pczt {
     sapling: sapling::Bundle,
     orchard: orchard::Bundle,
 }
+
+impl Pczt {
+    /// Parses a PCZT from its encoding.
+    pub fn parse(bytes: &[u8]) -> Result<Self, ParseError> {
+        if bytes.len() < 8 {
+            return Err(ParseError::TooShort);
+        }
+        if &bytes[..4] != MAGIC_BYTES {
+            return Err(ParseError::NotPczt);
+        }
+        let version = u32::from_le_bytes(bytes[4..8].try_into().unwrap());
+        if version != PCZT_VERSION_1 {
+            return Err(ParseError::UnknownVersion(version));
+        }
+
+        // This is a v1 PCZT.
+        postcard::from_bytes(&bytes[8..]).map_err(ParseError::Postcard)
+    }
+
+    /// Serializes this PCZT.
+    pub fn serialize(&self) -> Vec<u8> {
+        let mut bytes = vec![];
+        bytes.extend_from_slice(MAGIC_BYTES);
+        bytes.extend_from_slice(&PCZT_VERSION_1.to_le_bytes());
+        postcard::to_extend(self, bytes).expect("can serialize into memory")
+    }
+}
+
+/// Errors that can occur while parsing a PCZT.
+#[derive(Debug)]
+pub enum ParseError {
+    /// The bytes do not contain a PCZT.
+    NotPczt,
+    /// The PCZT encoding was invalid.
+    Postcard(postcard::Error),
+    /// The bytes are too short to contain a PCZT.
+    TooShort,
+    /// The PCZT has an unknown version.
+    UnknownVersion(u32),
+}

pczt/src/orchard.rs

@@ -2,14 +2,16 @@ use std::collections::BTreeMap;
 
 #[cfg(feature = "orchard")]
 use ff::PrimeField;
+use serde::{Deserialize, Serialize};
+use serde_with::serde_as;
 
 use crate::{
     common::Zip32Derivation,
     roles::combiner::{merge_map, merge_optional},
 };
 
 /// PCZT fields that are specific to producing the transaction's Orchard bundle (if any).
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub(crate) struct Bundle {
     /// The Orchard actions in this bundle.
     ///
@@ -52,7 +54,7 @@ pub(crate) struct Bundle {
     pub(crate) bsk: Option<[u8; 32]>,
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub(crate) struct Action {
     //
     // Action effecting data.
@@ -77,7 +79,8 @@ pub(crate) struct Action {
 }
 
 /// Information about a Sapling spend within a transaction.
-#[derive(Clone, Debug)]
+#[serde_as]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub(crate) struct Spend {
     //
     // Spend-specific Action effecting data.
@@ -91,12 +94,14 @@ pub(crate) struct Spend {
     /// The spend authorization signature.
     ///
     /// This is set by the Signer.
+    #[serde_as(as = "Option<[_; 64]>")]
     pub(crate) spend_auth_sig: Option<[u8; 64]>,
 
     /// The address that received the note being spent.
     ///
     /// - This is set by the Constructor (or Updater?).
     /// - This is required by the Prover.
+    #[serde_as(as = "Option<[_; 43]>")]
     pub(crate) recipient: Option<[u8; 43]>,
 
     /// The value of the input being spent.
@@ -128,6 +133,7 @@ pub(crate) struct Spend {
     ///
     /// - This is set by the Updater.
     /// - This is required by the Prover.
+    #[serde_as(as = "Option<[_; 96]>")]
     pub(crate) fvk: Option<[u8; 96]>,
 
     /// A witness from the note to the bundle's anchor.
@@ -160,7 +166,8 @@ pub(crate) struct Spend {
 }
 
 /// Information about an Orchard output within a transaction.
-#[derive(Clone, Debug)]
+#[serde_as]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub(crate) struct Output {
     //
     // Output-specific Action effecting data.
@@ -186,6 +193,7 @@ pub(crate) struct Output {
     ///
     /// - This is set by the Constructor.
     /// - This is required by the Prover.
+    #[serde_as(as = "Option<[_; 43]>")]
     pub(crate) recipient: Option<[u8; 43]>,
 
     /// The value of the output.

pczt/src/sapling.rs

@@ -1,5 +1,8 @@
 use std::collections::BTreeMap;
 
+use serde::{Deserialize, Serialize};
+use serde_with::serde_as;
+
 use crate::{
     common::Zip32Derivation,
     roles::combiner::{merge_map, merge_optional},
@@ -8,7 +11,7 @@ use crate::{
 const GROTH_PROOF_SIZE: usize = 48 + 96 + 48;
 
 /// PCZT fields that are specific to producing the transaction's Sapling bundle (if any).
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub(crate) struct Bundle {
     pub(crate) spends: Vec<Spend>,
     pub(crate) outputs: Vec<Output>,
@@ -33,7 +36,8 @@ pub(crate) struct Bundle {
 }
 
 /// Information about a Sapling spend within a transaction.
-#[derive(Clone, Debug)]
+#[serde_as]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub(crate) struct Spend {
     //
     // SpendDescription effecting data.
@@ -48,17 +52,20 @@ pub(crate) struct Spend {
     /// The Spend proof.
     ///
     /// This is set by the Prover.
+    #[serde_as(as = "Option<[_; GROTH_PROOF_SIZE]>")]
     pub(crate) zkproof: Option<[u8; GROTH_PROOF_SIZE]>,
 
     /// The spend authorization signature.
     ///
     /// This is set by the Signer.
+    #[serde_as(as = "Option<[_; 64]>")]
     pub(crate) spend_auth_sig: Option<[u8; 64]>,
 
     /// The address that received the note being spent.
     ///
     /// - This is set by the Constructor (or Updater?).
     /// - This is required by the Prover.
+    #[serde_as(as = "Option<[_; 43]>")]
     pub(crate) recipient: Option<[u8; 43]>,
 
     /// The value of the input being spent.
@@ -136,7 +143,8 @@ pub(crate) struct Spend {
 }
 
 /// Information about a Sapling output within a transaction.
-#[derive(Clone, Debug)]
+#[serde_as]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub(crate) struct Output {
     //
     // OutputDescription effecting data.
@@ -162,12 +170,14 @@ pub(crate) struct Output {
     /// The Output proof.
     ///
     /// This is set by the Prover.
+    #[serde_as(as = "Option<[_; GROTH_PROOF_SIZE]>")]
     pub(crate) zkproof: Option<[u8; GROTH_PROOF_SIZE]>,
 
     /// The address that will receive the output.
     ///
     /// - This is set by the Constructor.
     /// - This is required by the Prover.
+    #[serde_as(as = "Option<[_; 43]>")]
     pub(crate) recipient: Option<[u8; 43]>,
 
     /// The value of the output.