elliptic-curve: add sec1::Coordinates::tag method (#407)

Obtain the `Tag` needed to encode a given set of `sec1::Coordinates`

elliptic-curve/src/sec1.rs

@@ -353,36 +353,21 @@ pub enum Coordinates<'a, C: Curve> {
  },
 }
 
-/// Trait for deserializing a value from a SEC1 encoded curve point.
-///
-/// This is intended for use with the `AffinePoint` type for a given elliptic curve.
-pub trait FromEncodedPoint<C>
-where
- C: Curve,
- UntaggedPointSize<C>: Add<U1> + ArrayLength<u8>,
- UncompressedPointSize<C>: ArrayLength<u8>,
- Self: Sized,
-{
- /// Deserialize the type this trait is impl'd on from an [`EncodedPoint`].
- ///
- /// # Returns
- ///
- /// `None` if the [`EncodedPoint`] is invalid.
- fn from_encoded_point(public_key: &EncodedPoint<C>) -> Option<Self>;
-}
-
-/// Trait for serializing a value to a SEC1 encoded curve point.
-///
-/// This is intended for use with the `AffinePoint` type for a given elliptic curve.
-pub trait ToEncodedPoint<C>
-where
- C: Curve,
- UntaggedPointSize<C>: Add<U1> + ArrayLength<u8>,
- UncompressedPointSize<C>: ArrayLength<u8>,
-{
- /// Serialize this value as a SEC1 [`EncodedPoint`], optionally applying
- /// point compression.
- fn to_encoded_point(&self, compress: bool) -> EncodedPoint<C>;
+impl<'a, C: Curve> Coordinates<'a, C> {
+ /// Get the tag value needed to encode this set of [`Coordinates`]
+ pub fn tag(&self) -> Tag {
+ match self {
+ Coordinates::Identity => Tag::Identity,
+ Coordinates::Compressed { y_is_odd, .. } => {
+ if *y_is_odd {
+ Tag::CompressedOddY
+ } else {
+ Tag::CompressedEvenY
+ }
+ }
+ Coordinates::Uncompressed { .. } => Tag::Uncompressed,
+ }
+ }
 }
 
 /// Tag byte used by the `Elliptic-Curve-Point-to-Octet-String` encoding.
@@ -454,6 +439,38 @@ impl From<Tag> for u8 {
  }
 }
 
+/// Trait for deserializing a value from a SEC1 encoded curve point.
+///
+/// This is intended for use with the `AffinePoint` type for a given elliptic curve.
+pub trait FromEncodedPoint<C>
+where
+ C: Curve,
+ UntaggedPointSize<C>: Add<U1> + ArrayLength<u8>,
+ UncompressedPointSize<C>: ArrayLength<u8>,
+ Self: Sized,
+{
+ /// Deserialize the type this trait is impl'd on from an [`EncodedPoint`].
+ ///
+ /// # Returns
+ ///
+ /// `None` if the [`EncodedPoint`] is invalid.
+ fn from_encoded_point(public_key: &EncodedPoint<C>) -> Option<Self>;
+}
+
+/// Trait for serializing a value to a SEC1 encoded curve point.
+///
+/// This is intended for use with the `AffinePoint` type for a given elliptic curve.
+pub trait ToEncodedPoint<C>
+where
+ C: Curve,
+ UntaggedPointSize<C>: Add<U1> + ArrayLength<u8>,
+ UncompressedPointSize<C>: ArrayLength<u8>,
+{
+ /// Serialize this value as a SEC1 [`EncodedPoint`], optionally applying
+ /// point compression.
+ fn to_encoded_point(&self, compress: bool) -> EncodedPoint<C>;
+}
+
 #[cfg(test)]
 mod tests {
  use super::{Coordinates, Tag};