[#1477] Allow Orchard-only UFVK be created from an Orchard FVK

zcash_keys/CHANGELOG.md

@@ -6,8 +6,12 @@ and this library adheres to Rust's notion of
 
 ## [Unreleased]
 ### Notable changes
-- `zcash_keys` now supports TEX (transparent-source-only) addresses as specified
+- `zcash_keys`:
+  - Now supports TEX (transparent-source-only) addresses as specified
   in [ZIP 320](https://zips.z.cash/zip-0320).
+  - Has an `unstable-frost` feature for FROST implementors to use when
+  API changes are needed specifically for the FROST threshold signatures 
+  use case. 
 
 ### Added
 - `zcash_keys::address::Address::try_from_zcash_address`

zcash_keys/Cargo.toml

@@ -93,6 +93,9 @@ test-dependencies = [
 
 #! ### Experimental features
 
+## Exposes unstable APIs that are compatible with FROST key management
+unstable-frost = ["orchard"]
+
 ## Exposes unstable APIs. Their behaviour may change at any time.
 unstable = ["dep:byteorder"]
 

zcash_keys/src/keys.rs

@@ -658,6 +658,22 @@ impl UnifiedFullViewingKey {
         )
     }
 
+    #[cfg(feature = "unstable-frost")]
+    pub fn from_orchard_fvk(
+        orchard: orchard::keys::FullViewingKey,
+    ) -> Result<UnifiedFullViewingKey, DerivationError> {
+        Self::from_checked_parts(
+            #[cfg(feature = "transparent-inputs")]
+            None,
+            #[cfg(feature = "sapling")]
+            None,
+            #[cfg(feature = "orchard")]
+            Some(orchard),
+            // We don't currently allow constructing new UFVKs with unknown items, but we store
+            // this to allow parsing such UFVKs.
+            vec![],
+        )
+    }
     /// Construct a UFVK from its constituent parts, after verifying that UIVK derivation can
     /// succeed.
     fn from_checked_parts(