k256: add expose-field feature; fix/CI benches (#161)

The benchmarks want direct access to `FieldElement` for benchmarking
purposes. This is a legitimate use case, but breaks encapsulation in
that we'd like to generally prevent exposure of `FieldElement` except
for "hazmat"-style use cases.

This is a well-recognized problem with Rust in general:
rust-lang/cargo#2911

This commit adds a semi-hidden `expose-field` feature in order to fix
the benchmark. Perhaps it will be abused, but the alternative is not
having a benchmark, which seems bad too.

It also adds a CI build step to ensure the benchmarks compile to prevent
this sort of regression in the future.

.github/workflows/k256.yml

@@ -62,3 +62,4 @@ jobs:
     - run: cargo test --all-features
     - run: cargo test --features field-montgomery,rand
     - run: cargo test --features force-32-bit,rand
+    - run: cargo build --all-features --benches

k256/Cargo.toml

@@ -39,6 +39,7 @@ digest = ["elliptic-curve/digest", "ecdsa-core/digest"]
 ecdh = ["elliptic-curve/ecdh", "rand", "zeroize"]
 ecdsa = ["arithmetic", "digest", "ecdsa-core/rand", "ecdsa-core/sign", "ecdsa-core/verify", "rand", "zeroize"]
 endomorphism-mul = []
+expose-field = ["arithmetic"]
 field-montgomery = []
 force-32-bit = []
 keccak256 = ["digest", "sha3"]
@@ -50,10 +51,11 @@ std = ["elliptic-curve/std"]
 zeroize = ["elliptic-curve/zeroize"]
 
 [package.metadata.docs.rs]
-all-features = true
+features = ["ecdh", "ecdsa", "sha256", "keccak256"]
 rustdoc-args = ["--cfg", "docsrs"]
 
 [[bench]]
 name = "bench"
 path = "bench/bench.rs"
 harness = false
+required-features = ["expose-field"]

k256/bench/bench.rs

@@ -1,7 +1,8 @@
-use core::convert::TryInto;
+//! secp256k1 benchmarks
+
 use criterion::measurement::Measurement;
 use criterion::{criterion_group, criterion_main, BenchmarkGroup, Criterion};
-
+use hex_literal::hex;
 use k256::{elliptic_curve::FromBytes, FieldElement, ProjectivePoint, Scalar};
 
 fn test_scalar_x() -> Scalar {
@@ -30,10 +31,8 @@ fn test_scalar_y() -> Scalar {
 
 fn bench_point_mul<'a, M: Measurement>(group: &mut BenchmarkGroup<'a, M>) {
     let p = ProjectivePoint::generator();
-    let ms = "AA5E28D6A97A2479A65527F7290311A3624D4CC0FA1578598EE3C2613BF99522";
-    let m = hex::decode(&ms).unwrap();
-    let s = Scalar::from_bytes(m[..].try_into().unwrap()).unwrap();
-
+    let m = hex!("AA5E28D6A97A2479A65527F7290311A3624D4CC0FA1578598EE3C2613BF99522");
+    let s = Scalar::from_bytes(&m.into()).unwrap();
     group.bench_function("point-scalar mul", |b| b.iter(|| &p * &s));
 }
 
@@ -82,20 +81,26 @@ fn bench_scalar(c: &mut Criterion) {
 }
 
 fn test_field_element_x() -> FieldElement {
-    FieldElement::from_bytes(&[
-        0xbb, 0x48, 0x8a, 0xef, 0x41, 0x6a, 0x41, 0xd7, 0x68, 0x0d, 0x1c, 0xf0, 0x1d, 0x70, 0xf5,
-        0x9b, 0x60, 0xd7, 0xf5, 0xf7, 0x7e, 0x30, 0xe7, 0x8b, 0x8b, 0xf9, 0xd2, 0xd8, 0x82, 0xf1,
-        0x56, 0xa6,
-    ])
+    FieldElement::from_bytes(
+        &[
+            0xbb, 0x48, 0x8a, 0xef, 0x41, 0x6a, 0x41, 0xd7, 0x68, 0x0d, 0x1c, 0xf0, 0x1d, 0x70,
+            0xf5, 0x9b, 0x60, 0xd7, 0xf5, 0xf7, 0x7e, 0x30, 0xe7, 0x8b, 0x8b, 0xf9, 0xd2, 0xd8,
+            0x82, 0xf1, 0x56, 0xa6,
+        ]
+        .into(),
+    )
     .unwrap()
 }
 
 fn test_field_element_y() -> FieldElement {
-    FieldElement::from_bytes(&[
-        0x67, 0xe2, 0xf6, 0x80, 0x71, 0xed, 0x82, 0x81, 0xe8, 0xae, 0xd6, 0xbc, 0xf1, 0xc5, 0x20,
-        0x7c, 0x5e, 0x63, 0x37, 0x22, 0xd9, 0x20, 0xaf, 0xd6, 0xae, 0x22, 0xd0, 0x6e, 0xeb, 0x80,
-        0x35, 0xe3,
-    ])
+    FieldElement::from_bytes(
+        &[
+            0x67, 0xe2, 0xf6, 0x80, 0x71, 0xed, 0x82, 0x81, 0xe8, 0xae, 0xd6, 0xbc, 0xf1, 0xc5,
+            0x20, 0x7c, 0x5e, 0x63, 0x37, 0x22, 0xd9, 0x20, 0xaf, 0xd6, 0xae, 0x22, 0xd0, 0x6e,
+            0xeb, 0x80, 0x35, 0xe3,
+        ]
+        .into(),
+    )
     .unwrap()
 }
 

k256/src/arithmetic.rs

@@ -9,7 +9,7 @@ pub(crate) mod scalar;
 #[cfg(test)]
 mod dev;
 
-pub(crate) use field::FieldElement;
+pub use field::FieldElement;
 
 use crate::Secp256k1;
 use affine::AffinePoint;

k256/src/lib.rs

@@ -68,6 +68,9 @@ pub use arithmetic::{
     scalar::{NonZeroScalar, Scalar},
 };
 
+#[cfg(feature = "expose-field")]
+pub use arithmetic::FieldElement;
+
 use elliptic_curve::consts::U32;
 
 #[cfg(feature = "oid")]