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.

PR suggestions:
	Make "frost" feature be under the "unstable" set of features
	correct the unstable feature definition
	use visibility crate

CHANGELOG.md

@@ -6,6 +6,12 @@ and this project adheres to Rust's notion of
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### added
+- Added [Visibility crate](https://crates.io/crates/visibility) to modify
+visibility of methods and struct for the `unstable-frost` feature.
+- Added `SpendValidatingKey` serialization and deserialization from bytes
+visibility under the `unstable-frost` feature
+  - `orchard::keys::SpendValidatingKey`
 
 ## [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 +186,17 @@ 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)
     }
 
+    /// 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_attr(feature = "unstable-frost", visibility::make(pub))]
     pub(crate) fn from_bytes(bytes: &[u8]) -> Option<Self> {
         <[u8; 32]>::try_from(bytes)
             .ok()