Add legacy feature with AffineCoordinates trait

elliptic-curve-crate/Cargo.toml

@@ -37,10 +37,10 @@ default-features = false
 hex = "0.4"
 
 [features]
-default = []
+legacy = []
 weierstrass = []
 std = []
 
 [package.metadata.docs.rs]
-all-features = true
+features = ["weierstrass", "std"]
 rustdoc-args = ["--cfg", "docsrs"]

elliptic-curve-crate/src/legacy.rs

@@ -0,0 +1,26 @@
+//! Support for implementing legacy protocols that require direct access to
+//! coordinates of affine points on elliptic curves.
+//!
+//! These APIs violate the group abstraction and expose coordinate field
+//! elements which could be potentially misused when designing new protocols
+//! based on elliptic curve groups.
+//!
+//! For that reason, we strongly suggest they aren't used in new protocols, but
+//! only as needed when implementing legacy protocols which require them.
+
+use generic_array::ArrayLength;
+
+/// Byte array containing a serialized field element
+pub type FieldElementBytes<Size> = generic_array::GenericArray<u8, Size>;
+
+/// Access to the coordinates of an affine point
+pub trait AffineCoordinates {
+    /// Size of a field element representing an affine coordinate
+    type FieldElementSize: ArrayLength<u8>;
+
+    /// x-coordinate (field element)
+    fn x(&self) -> FieldElementBytes<Self::FieldElementSize>;
+
+    /// y-coordinate (field element)
+    fn y(&self) -> FieldElementBytes<Self::FieldElementSize>;
+}

elliptic-curve-crate/src/lib.rs

@@ -27,15 +27,18 @@ pub use rand_core;
 pub mod error;
 pub mod secret_key;
 
-pub use generic_array::{self, typenum::consts};
-pub use subtle;
+#[cfg(feature = "legacy")]
+#[cfg_attr(docsrs, doc(cfg(feature = "legacy")))]
+pub mod legacy;
 
 // TODO(tarcieri): other curve forms
 #[cfg(feature = "weierstrass")]
 #[cfg_attr(docsrs, doc(cfg(feature = "weierstrass")))]
 pub mod weierstrass;
 
 pub use self::{error::Error, secret_key::SecretKey};
+pub use generic_array::{self, typenum::consts};
+pub use subtle;
 
 /// Byte array containing a serialized scalar value (i.e. an integer)
 pub type ScalarBytes<Size> = generic_array::GenericArray<u8, Size>;

k256/Cargo.toml

@@ -30,10 +30,11 @@ proptest = "0.10"
 [features]
 default = ["arithmetic", "std"]
 arithmetic = []
+legacy = ["arithmetic", "elliptic-curve/legacy"]
 rand = ["elliptic-curve/rand_core"]
 test-vectors = []
 std = ["elliptic-curve/std"]
 
 [package.metadata.docs.rs]
-all-features = true
+features = ["arithmetic", "rand", "test-vectors", "std"]
 rustdoc-args = ["--cfg", "docsrs"]

k256/src/arithmetic.rs

@@ -15,6 +15,9 @@ use crate::{CompressedPoint, PublicKey, ScalarBytes, Secp256k1, UncompressedPoin
 use field::{FieldElement, MODULUS};
 use scalar::Scalar;
 
+#[cfg(feature = "legacy")]
+use elliptic_curve::{consts::U32, legacy};
+
 #[cfg(feature = "rand")]
 use crate::SecretKey;
 
@@ -186,6 +189,19 @@ impl Neg for AffinePoint {
     }
 }
 
+#[cfg(feature = "legacy")]
+impl legacy::AffineCoordinates for AffinePoint {
+    type FieldElementSize = U32;
+
+    fn x(&self) -> legacy::FieldElementBytes<U32> {
+        self.x.to_bytes().into()
+    }
+
+    fn y(&self) -> legacy::FieldElementBytes<U32> {
+        self.y.to_bytes().into()
+    }
+}
+
 /// A point on the secp256k1 curve in projective coordinates.
 #[derive(Clone, Copy, Debug)]
 #[cfg_attr(docsrs, doc(cfg(feature = "arithmetic")))]

p256/Cargo.toml

@@ -29,10 +29,11 @@ proptest = "0.10"
 [features]
 default = ["arithmetic", "std"]
 arithmetic = []
+legacy = ["arithmetic", "elliptic-curve/legacy"]
 rand = ["elliptic-curve/rand_core"]
 test-vectors = []
 std = ["elliptic-curve/std"]
 
 [package.metadata.docs.rs]
-all-features = true
+features = ["arithmetic", "rand", "test-vectors", "std"]
 rustdoc-args = ["--cfg", "docsrs"]

p256/src/arithmetic.rs

@@ -15,6 +15,9 @@ use crate::{CompressedPoint, NistP256, PublicKey, ScalarBytes, UncompressedPoint
 use field::{FieldElement, MODULUS};
 use scalar::Scalar;
 
+#[cfg(feature = "legacy")]
+use elliptic_curve::{consts::U32, legacy};
+
 #[cfg(feature = "rand")]
 use crate::SecretKey;
 
@@ -192,6 +195,19 @@ impl Neg for AffinePoint {
     }
 }
 
+#[cfg(feature = "legacy")]
+impl legacy::AffineCoordinates for AffinePoint {
+    type FieldElementSize = U32;
+
+    fn x(&self) -> legacy::FieldElementBytes<U32> {
+        self.x.to_bytes().into()
+    }
+
+    fn y(&self) -> legacy::FieldElementBytes<U32> {
+        self.y.to_bytes().into()
+    }
+}
+
 /// A point on the secp256r1 curve in projective coordinates.
 #[derive(Clone, Copy, Debug)]
 #[cfg_attr(docsrs, doc(cfg(feature = "arithmetic")))]