zcash_note_encryption: Add API to prepare epk.

Co-authored-by: Jack Grigg <jack@electriccoin.co>
Signed-off-by: Daira Hopwood <daira@jacaranda.org>


Extracted from: 515b0a40ec

CHANGELOG.md

@@ -6,7 +6,17 @@ and this library adheres to Rust's notion of
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- `zcash_note_encryption::Domain`:
+  - `Domain::PreparedEphemeralPublicKey` associated type.
+  - `Domain::prepare_epk` method, which produces the above type.
 
+### Changed
+- MSRV is now 1.56.1.
+- Migrated to `group 0.13`.
+- `zcash_note_encryption::Domain` now requires `epk` to be converted to
+  `Domain::PreparedEphemeralPublicKey` before being passed to
+  `Domain::ka_agree_dec`.
 - Changes to batch decryption APIs:
   - The return types of `batch::try_note_decryption` and
     `batch::try_compact_note_decryption` have changed. Now, instead of
@@ -16,8 +26,5 @@ and this library adheres to Rust's notion of
     argument to the function. Each successful result includes the index of the
     entry in `ivks` used to decrypt the value.
 
-### Changed
-- MSRV is now 1.56.1.
-
 ## [0.1.0] - 2021-12-17
 Initial release.

Cargo.toml

@@ -21,6 +21,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 [dependencies]
 chacha20 = { version = "0.8", default-features = false }
 chacha20poly1305 = { version = "0.9", default-features = false }
+group = "0.12"
 rand_core = { version = "0.6", default-features = false }
 subtle = { version = "2.2.3", default-features = false }
 

src/batch.rs

@@ -51,7 +51,7 @@ where
         return (0..outputs.len()).map(|_| None).collect();
     };
 
-    // Fetch the ephemeral keys for each output and batch-parse them.
+    // Fetch the ephemeral keys for each output, and batch-parse and prepare them.
     let ephemeral_keys = D::batch_epk(outputs.iter().map(|(_, output)| output.ephemeral_key()));
 
     // Derive the shared secrets for all combinations of (ivk, output).

src/lib.rs

@@ -113,6 +113,7 @@ enum NoteValidity {
 pub trait Domain {
     type EphemeralSecretKey: ConstantTimeEq;
     type EphemeralPublicKey;
+    type PreparedEphemeralPublicKey;
     type SharedSecret;
     type SymmetricKey: AsRef<[u8]>;
     type Note;
@@ -136,6 +137,9 @@ pub trait Domain {
     /// Extracts the `DiversifiedTransmissionKey` from the note.
     fn get_pk_d(note: &Self::Note) -> Self::DiversifiedTransmissionKey;
 
+    /// Prepare an ephemeral public key for more efficient scalar multiplication.
+    fn prepare_epk(epk: Self::EphemeralPublicKey) -> Self::PreparedEphemeralPublicKey;
+
     /// Derives `EphemeralPublicKey` from `esk` and the note's diversifier.
     fn ka_derive_public(
         note: &Self::Note,
@@ -152,7 +156,7 @@ pub trait Domain {
     /// decryption.
     fn ka_agree_dec(
         ivk: &Self::IncomingViewingKey,
-        epk: &Self::EphemeralPublicKey,
+        epk: &Self::PreparedEphemeralPublicKey,
     ) -> Self::SharedSecret;
 
     /// Derives the `SymmetricKey` used to encrypt the note plaintext.
@@ -306,10 +310,15 @@ pub trait BatchDomain: Domain {
     /// them.
     fn batch_epk(
         ephemeral_keys: impl Iterator<Item = EphemeralKeyBytes>,
-    ) -> Vec<(Option<Self::EphemeralPublicKey>, EphemeralKeyBytes)> {
+    ) -> Vec<(Option<Self::PreparedEphemeralPublicKey>, EphemeralKeyBytes)> {
         // Default implementation: do the non-batched thing.
         ephemeral_keys
-            .map(|ephemeral_key| (Self::epk(&ephemeral_key), ephemeral_key))
+            .map(|ephemeral_key| {
+                (
+                    Self::epk(&ephemeral_key).map(Self::prepare_epk),
+                    ephemeral_key,
+                )
+            })
             .collect()
     }
 }
@@ -514,7 +523,7 @@ pub fn try_note_decryption<D: Domain, Output: ShieldedOutput<D, ENC_CIPHERTEXT_S
 ) -> Option<(D::Note, D::Recipient, D::Memo)> {
     let ephemeral_key = output.ephemeral_key();
 
-    let epk = D::epk(&ephemeral_key)?;
+    let epk = D::prepare_epk(D::epk(&ephemeral_key)?);
     let shared_secret = D::ka_agree_dec(ivk, &epk);
     let key = D::kdf(shared_secret, &ephemeral_key);
 
@@ -611,7 +620,7 @@ pub fn try_compact_note_decryption<D: Domain, Output: ShieldedOutput<D, COMPACT_
 ) -> Option<(D::Note, D::Recipient)> {
     let ephemeral_key = output.ephemeral_key();
 
-    let epk = D::epk(&ephemeral_key)?;
+    let epk = D::prepare_epk(D::epk(&ephemeral_key)?);
     let shared_secret = D::ka_agree_dec(ivk, &epk);
     let key = D::kdf(shared_secret, &ephemeral_key);