Allow SpendValidatingKey to be constructed from bytes

CHANGELOG.md

@@ -6,6 +6,11 @@ and this project adheres to Rust's notion of
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- `orchard::keys::SpendValidatingKey::{from_bytes, to_bytes}` behind the
+  `unstable-frost` feature flag. These are temporary APIs exposed for development
+  purposes, and will be replaced by type-safe FROST APIs once ZIP 312 key
+  generation is specified (https://github.com/zcash/zips/pull/883).
 
 ## [0.8.0] - 2024-03-25
 

Cargo.toml

@@ -45,6 +45,7 @@ zcash_note_encryption = "0.4"
 incrementalmerkletree = "0.5"
 zcash_spec = "0.1"
 zip32 = "0.1"
+visibility = "0.1.0"
 
 # Logging
 tracing = "0.1"
@@ -71,6 +72,7 @@ bench = false
 
 [features]
 default = ["multicore"]
+unstable-frost = []
 multicore = ["halo2_proofs/multicore"]
 dev-graph = ["halo2_proofs/dev-graph", "image", "plotters"]
 test-dependencies = ["proptest"]

src/keys.rs

@@ -186,12 +186,19 @@ impl SpendValidatingKey {
 
     /// Converts this spend validating key to its serialized form,
     /// I2LEOSP_256(ak).
+    #[cfg_attr(feature = "unstable-frost", visibility::make(pub))]
     pub(crate) fn to_bytes(&self) -> [u8; 32] {
         // This is correct because the wrapped point must have ỹ = 0, and
         // so the point repr is the same as I2LEOSP of its x-coordinate.
-        <[u8; 32]>::from(&self.0)
+        let b = <[u8; 32]>::from(&self.0);
+        assert!(b[31] & 0x80 == 0);
+        b
     }
 
+    /// Attempts to parse a byte slice as a spend validating key, `I2LEOSP_256(ak)`.
+    ///
+    /// Returns `None` if the given slice does not contain a valid spend validating key.
+    #[cfg_attr(feature = "unstable-frost", visibility::make(pub))]
     pub(crate) fn from_bytes(bytes: &[u8]) -> Option<Self> {
         <[u8; 32]>::try_from(bytes)
             .ok()