p521: serde feature (#962)

Adds `serde::{Serialize, Deserialize}` impls for `AffinePoint` and
`Scalar`.

p521/Cargo.toml

@@ -24,6 +24,7 @@ ecdsa-core = { version = "0.16.5", package = "ecdsa", optional = true, default-f
 hex-literal = { version = "0.4", optional = true }
 primeorder = { version = "0.13.3", optional = true, path = "../primeorder" }
 rand_core = { version = "0.6", optional = true, default-features = false }
+serdect = { version = "0.2", optional = true, default-features = false }
 sha2 = { version = "0.10", optional = true, default-features = false }
 
 [dev-dependencies]
@@ -45,7 +46,8 @@ ecdsa = ["arithmetic", "ecdsa-core/signing", "ecdsa-core/verifying", "sha512"]
 getrandom = ["rand_core/getrandom"]
 jwk = ["elliptic-curve/jwk"]
 pem = ["elliptic-curve/pem", "pkcs8"]
-pkcs8 = ["elliptic-curve/pkcs8"]
+pkcs8 = ["ecdsa-core?/pkcs8", "elliptic-curve/pkcs8"]
+serde = ["ecdsa-core?/serde", "elliptic-curve/serde", "primeorder?/serde", "serdect"]
 sha512 = ["digest", "dep:sha2"]
 test-vectors = ["dep:hex-literal"]
 voprf = ["elliptic-curve/voprf", "dep:sha2"]

p521/src/arithmetic/scalar.rs

@@ -654,6 +654,26 @@ impl TryFrom<U576> for Scalar {
     }
 }
 
+#[cfg(feature = "serde")]
+impl Serialize for Scalar {
+    fn serialize<S>(&self, serializer: S) -> core::result::Result<S::Ok, S::Error>
+    where
+        S: ser::Serializer,
+    {
+        ScalarPrimitive::from(self).serialize(serializer)
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<'de> Deserialize<'de> for Scalar {
+    fn deserialize<D>(deserializer: D) -> core::result::Result<Self, D::Error>
+    where
+        D: de::Deserializer<'de>,
+    {
+        Ok(ScalarPrimitive::deserialize(deserializer)?.into())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::Scalar;

p521/src/lib.rs

@@ -15,6 +15,16 @@
     unused_qualifications
 )]
 
+//! ## `serde` support
+//!
+//! When the `serde` feature of this crate is enabled, `Serialize` and
+//! `Deserialize` are impl'd for the following types:
+//!
+//! - [`AffinePoint`]
+//! - [`Scalar`]
+//!
+//! Please see type-specific documentation for more information.
+
 #[cfg(feature = "arithmetic")]
 mod arithmetic;