Allow creating a FullViewingKey from a given SpendValidatingKey

Allows creating a FullViewingKey from a given SpendValidatingKey and
from the other components which can be randomly generated, but there
may be some utility in specifying them too like supporting FROST backup
schemes that don't centralize spend authority

closes #431

see related #430

PR Suggestions

CHANGELOG.md

@@ -6,7 +6,16 @@ and this project adheres to Rust's notion of
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
-### Added
+### 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`
+- Added `orchard::keys::FullViewingKey::from_sk_and_ak` under the
+`unstable-frost` feature flag
+- Added `orchard::keys::FullViewingKey::from_checked_parts` under the
+`unstable-frost` feature flag
 - `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

src/keys.rs

@@ -225,6 +225,7 @@ impl SpendValidatingKey {
 /// [`Note`]: crate::note::Note
 /// [orchardkeycomponents]: https://zips.z.cash/protocol/nu5.pdf#orchardkeycomponents
 #[derive(Copy, Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
+#[cfg_attr(feature = "unstable-frost", visibility::make(pub))]
 pub(crate) struct NullifierDerivingKey(pallas::Base);
 
 impl NullifierDerivingKey {
@@ -266,6 +267,7 @@ impl NullifierDerivingKey {
 ///
 /// [orchardkeycomponents]: https://zips.z.cash/protocol/nu5.pdf#orchardkeycomponents
 #[derive(Copy, Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
+#[cfg_attr(feature = "unstable-frost", visibility::make(pub))]
 pub(crate) struct CommitIvkRandomness(pallas::Scalar);
 
 impl From<&SpendingKey> for CommitIvkRandomness {
@@ -333,6 +335,32 @@ impl From<FullViewingKey> for SpendValidatingKey {
 }
 
 impl FullViewingKey {
+    /// Creates a `FullViewingKey` from a `SpendingKey` and `SpendValidatingKey`.
+    /// This is necessary for FROST key management.
+    ///
+    /// Note: See [FROST Book - Technical details](https://frost.zfnd.org/zcash/technical-details.html)
+    #[cfg(feature = "unstable-frost")]
+    pub fn from_sk_and_ak(sk: &SpendingKey, ak: SpendValidatingKey) -> FullViewingKey {
+        FullViewingKey {
+            ak,
+            nk: NullifierDerivingKey::from(sk),
+            rivk: CommitIvkRandomness::from(sk),
+        }
+    }
+
+    /// Creates a `FullViewingKey` from its checked parts. This is necessary for FROST
+    /// key management in order to avoid centralizing spend authority in a backup scheme.
+    ///
+    /// Note: See [FROST Book - Technical details - Backing Up Key Shares](https://frost.zfnd.org/zcash/technical-details.html)
+    #[cfg(feature = "unstable-frost")]
+    pub fn from_checked_parts(
+        ak: SpendValidatingKey,
+        nk: NullifierDerivingKey,
+        rivk: CommitIvkRandomness,
+    ) -> FullViewingKey {
+        FullViewingKey { ak, nk, rivk }
+    }
+
     pub(crate) fn nk(&self) -> &NullifierDerivingKey {
         &self.nk
     }