Add PrfExpand<([u8; 32], [u8; 4])::with_tag (for tagged ZIP 32 child

derivation).

Signed-off-by: Daira-Emma Hopwood <daira@jacaranda.org>

CHANGELOG.md

@@ -6,6 +6,8 @@ and this library adheres to Rust's notion of
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- `PrfExpand<([u8; 32], [u8; 4])::with_tag` (for tagged ZIP 32 child derivation).
 
 ## [0.1.2] - 2024-10-01
 ### Added

src/prf_expand.rs

@@ -96,6 +96,12 @@ impl PrfExpand<([u8; 32], [u8; 4])> {
     pub const SPROUT_ZIP32_CHILD: Self = Self::new(0x80);
     pub const ORCHARD_ZIP32_CHILD: Self = Self::new(0x81);
     pub const ARBITRARY_ZIP32_CHILD: Self = Self::new(0xAB);
+
+    /// Expands the given secret key in this domain, with an additional
+    /// `tag` input.
+    pub fn with_tag(self, c_par: &[u8], sk_par: &[u8; 32], i: &[u8; 4], tag: &[u8]) -> [u8; 64] {
+        self.apply(c_par, &[sk_par, i, tag])
+    }
 }
 impl PrfExpand<([u8; 32], [u8; 32])> {
     pub const ORCHARD_DK_OVK: Self = Self::new(0x82);