Allow SpendValidatingKey to be constructed from bytes

closes #427

This adds a FROST feature flag so that public interface of the
crate is not altered in its meaning for non-FROST applications.

Key derivation is intended to be done on a specific way for usual
applications. Interface changes introduce by FROST can pose a risk
for the rest of the use cases that don't involve the assumptions
of the FROST signature scheme protocol. we don't want non-FROST
cases to make use the helper functions FROST needs to derive the
needed key elements.

CHANGELOG.md

@@ -6,6 +6,10 @@ and this project adheres to Rust's notion of
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### added
+- Added `SpendValidatingKey` serialization and deserialization from bytes
+visibility under the `FROST` feature
+  - `orchard::keys::SpendValidatingKey`
 
 ## [0.8.0] - 2024-03-25
 

Cargo.toml

@@ -73,6 +73,7 @@ bench = false
 default = ["multicore"]
 multicore = ["halo2_proofs/multicore"]
 dev-graph = ["halo2_proofs/dev-graph", "image", "plotters"]
+FROST = []
 test-dependencies = ["proptest"]
 
 [[bench]]

src/keys.rs

@@ -191,7 +191,16 @@ impl SpendValidatingKey {
         // so the point repr is the same as I2LEOSP of its x-coordinate.
         <[u8; 32]>::from(&self.0)
     }
+    /// Converts this spend validating key to its serialized form,
+    /// I2LEOSP_256(ak).
+    #[cfg(feature = "FROST")]
+    pub fn into_bytes(&self) -> [u8; 32] {
+        self.to_bytes()
+    }
 
+    /// Attempts to convert these bytes into a spend validating key
+    /// from its serialized form, I2LEOSP_256(ak). Returns None if
+    /// it can't be created.
     pub(crate) fn from_bytes(bytes: &[u8]) -> Option<Self> {
         <[u8; 32]>::try_from(bytes)
             .ok()
@@ -208,6 +217,14 @@ impl SpendValidatingKey {
             })
             .map(SpendValidatingKey)
     }
+
+    /// Attempts to convert these bytes into a spend validating key
+    /// from its serialized form, I2LEOSP_256(ak). Returns None if
+    /// it can't be created.
+    #[cfg(feature = "FROST")]
+    pub fn with_bytes(bytes: &[u8]) -> Option<Self> {
+        SpendValidatingKey::from_bytes(bytes)
+    }
 }
 
 /// A key used to derive [`Nullifier`]s from [`Note`]s.