Factor SEC-1 serialization into elliptic-curve crate

elliptic-curve-crate/src/weierstrass/point.rs

@@ -7,8 +7,12 @@
 //! <https://www.secg.org/sec1-v2.pdf>
 
 use super::Curve;
+use crate::ScalarBytes;
 use core::ops::Add;
-use generic_array::{typenum::U1, ArrayLength, GenericArray};
+use generic_array::{
+    typenum::{Unsigned, U1},
+    ArrayLength, GenericArray,
+};
 
 /// Size of a compressed elliptic curve point for the given curve when
 /// serialized using `Elliptic-Curve-Point-to-Octet-String` encoding
@@ -39,6 +43,19 @@ impl<C: Curve> CompressedPoint<C>
 where
     CompressedPointSize<C::ScalarSize>: ArrayLength<u8>,
 {
+    /// Compress and serialize an elliptic curve point from its affine coordinates
+    pub fn from_affine_coords(
+        x: &ScalarBytes<C::ScalarSize>,
+        y: &ScalarBytes<C::ScalarSize>,
+    ) -> Self {
+        // Is the y-coordinate odd in the SEC-1 sense: `self mod 2 == 1`?
+        let is_y_odd = y.as_ref().last().expect("last byte") & 1 == 1;
+        let mut bytes = GenericArray::default();
+        bytes[0] = if is_y_odd { 0x03 } else { 0x02 };
+        bytes[1..].copy_from_slice(x);
+        Self { bytes }
+    }
+
     /// Create a new compressed elliptic curve point
     pub fn from_bytes<B>(into_bytes: B) -> Option<Self>
     where
@@ -114,6 +131,19 @@ where
     <C::ScalarSize as Add>::Output: Add<U1>,
     UncompressedPointSize<C::ScalarSize>: ArrayLength<u8>,
 {
+    /// Serialize an elliptic curve point from its affine coordinates
+    pub fn from_affine_coords(
+        x: &ScalarBytes<C::ScalarSize>,
+        y: &ScalarBytes<C::ScalarSize>,
+    ) -> Self {
+        let scalar_size = C::ScalarSize::to_usize();
+        let mut bytes = GenericArray::default();
+        bytes[0] = 0x04;
+        bytes[1..(scalar_size + 1)].copy_from_slice(x);
+        bytes[(scalar_size + 1)..].copy_from_slice(y);
+        Self { bytes }
+    }
+
     /// Create a new uncompressed elliptic curve point
     pub fn from_bytes<B>(into_bytes: B) -> Option<Self>
     where

k256/src/arithmetic.rs

@@ -12,7 +12,6 @@ pub use self::scalar::Scalar;
 use core::convert::TryInto;
 use core::ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign};
 use elliptic_curve::{
-    generic_array::GenericArray,
     subtle::{Choice, ConditionallySelectable, ConstantTimeEq, CtOption},
     weierstrass::FixedBaseScalarMul,
 };
@@ -141,29 +140,20 @@ impl AffinePoint {
 impl From<AffinePoint> for CompressedPoint {
     /// Returns the SEC-1 compressed encoding of this point.
     fn from(affine_point: AffinePoint) -> CompressedPoint {
-        let mut encoded = [0; 33];
-        encoded[0] = if affine_point.y.is_odd().into() {
-            0x03
-        } else {
-            0x02
-        };
-        encoded[1..33].copy_from_slice(&affine_point.x.to_bytes());
-
-        CompressedPoint::from_bytes(GenericArray::clone_from_slice(&encoded[..]))
-            .expect("we encoded it correctly")
+        CompressedPoint::from_affine_coords(
+            &affine_point.x.to_bytes().into(),
+            &affine_point.y.to_bytes().into(),
+        )
     }
 }
 
 impl From<AffinePoint> for UncompressedPoint {
     /// Returns the SEC-1 uncompressed encoding of this point.
     fn from(affine_point: AffinePoint) -> UncompressedPoint {
-        let mut encoded = [0; 65];
-        encoded[0] = 0x04;
-        encoded[1..33].copy_from_slice(&affine_point.x.to_bytes());
-        encoded[33..65].copy_from_slice(&affine_point.y.to_bytes());
-
-        UncompressedPoint::from_bytes(GenericArray::clone_from_slice(&encoded[..]))
-            .expect("we encoded it correctly")
+        UncompressedPoint::from_affine_coords(
+            &affine_point.x.to_bytes().into(),
+            &affine_point.y.to_bytes().into(),
+        )
     }
 }
 

p256/src/arithmetic.rs

@@ -12,7 +12,6 @@ pub use self::scalar::Scalar;
 use core::convert::TryInto;
 use core::ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign};
 use elliptic_curve::{
-    generic_array::GenericArray,
     subtle::{Choice, ConditionallySelectable, ConstantTimeEq, CtOption},
     weierstrass::FixedBaseScalarMul,
 };
@@ -147,29 +146,20 @@ impl AffinePoint {
 impl From<AffinePoint> for CompressedPoint {
     /// Returns the SEC-1 compressed encoding of this point.
     fn from(affine_point: AffinePoint) -> CompressedPoint {
-        let mut encoded = [0; 33];
-        encoded[0] = if affine_point.y.is_odd().into() {
-            0x03
-        } else {
-            0x02
-        };
-        encoded[1..33].copy_from_slice(&affine_point.x.to_bytes());
-
-        CompressedPoint::from_bytes(GenericArray::clone_from_slice(&encoded[..]))
-            .expect("we encoded it correctly")
+        CompressedPoint::from_affine_coords(
+            &affine_point.x.to_bytes().into(),
+            &affine_point.y.to_bytes().into(),
+        )
     }
 }
 
 impl From<AffinePoint> for UncompressedPoint {
     /// Returns the SEC-1 uncompressed encoding of this point.
     fn from(affine_point: AffinePoint) -> UncompressedPoint {
-        let mut encoded = [0; 65];
-        encoded[0] = 0x04;
-        encoded[1..33].copy_from_slice(&affine_point.x.to_bytes());
-        encoded[33..65].copy_from_slice(&affine_point.y.to_bytes());
-
-        UncompressedPoint::from_bytes(GenericArray::clone_from_slice(&encoded[..]))
-            .expect("we encoded it correctly")
+        UncompressedPoint::from_affine_coords(
+            &affine_point.x.to_bytes().into(),
+            &affine_point.y.to_bytes().into(),
+        )
     }
 }