Add PrimeFieldBits support to Scalar (dalek-cryptography#579)

Co-authored-by: Michael Rosenberg <[email protected]> Co-authored-by: pinkforest(she/her) <[email protected]>
dzizazda · Sep 20, 2023 · 76a8b2a · 76a8b2a
1 parent 533b53a
commit 76a8b2a
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diff --git a/curve25519-dalek/CHANGELOG.md b/curve25519-dalek/CHANGELOG.md
@@ -5,6 +5,10 @@ major series.
 
 ## 4.x series
 
+### Unreleased
+
+* Add implementation for `PrimeFieldBits`, behind the `group-bits` feature flag.
+
 ### 4.1.1
 
 * Mark `constants::BASEPOINT_ORDER` deprecated from pub API

diff --git a/curve25519-dalek/Cargo.toml b/curve25519-dalek/Cargo.toml
@@ -27,7 +27,7 @@ rustdoc-args = [
     "--html-in-header", "docs/assets/rustdoc-include-katex-header.html",
     "--cfg", "docsrs",
 ]
-features = ["serde", "rand_core", "digest", "legacy_compatibility", "group"]
+features = ["serde", "rand_core", "digest", "legacy_compatibility", "group-bits"]
 
 [dev-dependencies]
 sha2 = { version = "0.10", default-features = false }
@@ -48,6 +48,7 @@ required-features = ["alloc", "rand_core"]
 
 [dependencies]
 cfg-if = "1"
+ff = { version = "0.13", default-features = false, optional = true }
 group = { version = "0.13", default-features = false, optional = true }
 rand_core = { version = "0.6.4", default-features = false, optional = true }
 digest = { version = "0.10", default-features = false, optional = true }
@@ -67,6 +68,7 @@ alloc = ["zeroize?/alloc"]
 precomputed-tables = []
 legacy_compatibility = []
 group = ["dep:group", "rand_core"]
+group-bits = ["group", "ff/bits"]
 
 [target.'cfg(all(not(curve25519_dalek_backend = "fiat"), not(curve25519_dalek_backend = "serial"), target_arch = "x86_64"))'.dependencies]
 curve25519-dalek-derive = { version = "0.1", path = "../curve25519-dalek-derive" }
diff --git a/curve25519-dalek/src/scalar.rs b/curve25519-dalek/src/scalar.rs
@@ -124,6 +124,8 @@ use core::ops::{Sub, SubAssign};
 
 use cfg_if::cfg_if;
 
+#[cfg(feature = "group-bits")]
+use group::ff::{FieldBits, PrimeFieldBits};
 #[cfg(feature = "group")]
 use {
     group::ff::{Field, FromUniformBytes, PrimeField},
@@ -1321,6 +1323,19 @@ impl PrimeField for Scalar {
     };
 }
 
+#[cfg(feature = "group-bits")]
+impl PrimeFieldBits for Scalar {
+    type ReprBits = [u8; 32];
+
+    fn to_le_bits(&self) -> FieldBits<Self::ReprBits> {
+        self.to_repr().into()
+    }
+
+    fn char_le_bits() -> FieldBits<Self::ReprBits> {
+        constants::BASEPOINT_ORDER_PRIVATE.to_bytes().into()
+    }
+}
+
 #[cfg(feature = "group")]
 impl FromUniformBytes<64> for Scalar {
     fn from_uniform_bytes(bytes: &[u8; 64]) -> Self {