Update zcash_primitives/CHANGELOG.md with change key derivation methods.

zcash_primitives/CHANGELOG.md

@@ -31,8 +31,8 @@ and this library adheres to Rust's notion of
   Orchard). This type makes it possible to encode a type-safe state machine
   for the application of authorizing data to a transaction; implementations of
   this trait represent different states of the authorization process.
-- New bundle types under the `zcash_primitives::transaction` submodules, one for 
-  each Zcash sub-protocol. These are now used instead of bare fields 
+- New bundle types under the `zcash_primitives::transaction` submodules, one for
+  each Zcash sub-protocol. These are now used instead of bare fields
   within the `TransactionData` type.
   - `components::sapling::Bundle` bundle of
     Sapling transaction elements. This new struct is parameterized by a
@@ -58,7 +58,7 @@ and this library adheres to Rust's notion of
   diversifier index space, whereas `sapling_diversifier` just attempts to use the
   provided diversifier index and returns `None` if it does not produce a valid
   diversifier.
-- `zcash_primitives::zip32::DiversifierKey::diversifier` has been renamed to 
+- `zcash_primitives::zip32::DiversifierKey::diversifier` has been renamed to
   `find_diversifier` and the `diversifier` method has new semantics.
   `find_diversifier` searches the diversifier index space to find a diversifier
   index which produces a valid diversifier, whereas `diversifier` just attempts
@@ -71,6 +71,23 @@ and this library adheres to Rust's notion of
   just attempts to create an address corresponding to the diversifier derived
   from the provided diversifier index and returns `None` if the provided index
   does not produce a valid diversifier.
+- `zcash_primitives::zip32::ExtendedSpendingKey.derive_internal` has been
+  added to facilitate the derivation of an internal (change) spending key.
+  This spending key can be used to spend change sent to an internal address
+  corresponding to the associated full viewing key as specified in
+  [ZIP 316](https://zips.z.cash/zip-0316#encoding-of-unified-full-incoming-viewing-keys)..
+- `zcash_primitives::zip32::ExtendedFullViewingKey.derive_internal` has been
+  added to facilitate the derivation of an internal (change) spending key.
+  This spending key can be used to spend change sent to an internal address
+  corresponding to the associated full viewing key as specified in
+  [ZIP 32](https://zips.z.cash/zip-0032#deriving-a-sapling-internal-spending-key).
+- `zcash_primitives::zip32::sapling_derive_internal_fvk` provides the
+  internal implementation of `ExtendedFullViewingKey.derive_internal`
+  but does not require a complete extended full viewing key, just
+  the full viewing key and the diversifier key. In the future, this
+  function will likely be refactored to become a member function of
+  a new `DiversifiableFullViewingKey` type, which represents the ability
+  to derive IVKs, OVKs, and addresses, but not child viewing keys.
 
 ### Changed
 - MSRV is now 1.51.0.