elliptic-curve: add hex serde (de)serializer for PublicKey (#967)

This commit changes the `Serializer` and `Deserializer` impls on the
`PublicKey` type to work more like the ones on other types in this crate
and its downstream dependencies:

- Use binary DER serialization with binary formats (what it does today)
- Use an upper-case hex serialization of the DER in conjunction with
  "human readable" formats

Though richer formats exist for this use case (such as `JwkEcKey` which
is an existing implementation of JWK), hex-serialized DER is relatively
easy to work with, and can be decoded easily with online tools such as
https://lapo.it/asn1js/

Note that the `Deserialize` impl for `PublicKey` never worked with
human-readable formats as it was attempting to deserialize to `&[u8]`,
which doesn't work with formats like JSON or TOML because they need an
intermediate decoding step and therefore require deserialization to
`Vec<u8>`. So this isn't a breaking change from the `Deserialize` side.

Fixes RustCrypto/elliptic-curves#536

elliptic-curve/Cargo.toml

@@ -42,7 +42,7 @@ sha3 = "0.10"
 
 [features]
 default = ["arithmetic"]
-alloc = ["der/alloc", "sec1/alloc", "zeroize/alloc"] # todo: use weak activation for `group`/`sec1` alloc when available
+alloc = ["base16ct/alloc", "der/alloc", "sec1/alloc", "zeroize/alloc"] # todo: use weak activation for `group`/`sec1` alloc when available
 arithmetic = ["ff", "group"]
 bits = ["arithmetic", "ff/bits"]
 dev = ["arithmetic", "hex-literal", "pem", "pkcs8"]

elliptic-curve/src/public_key.rs

@@ -32,7 +32,6 @@ use {
 use alloc::string::{String, ToString};
 
 #[cfg(all(feature = "pem", feature = "serde"))]
-#[cfg_attr(docsrs, doc(cfg(all(feature = "pem", feature = "serde"))))]
 use serde::{de, ser, Deserialize, Serialize};
 
 /// Elliptic curve public keys.
@@ -353,8 +352,8 @@ where
     }
 }
 
-#[cfg(all(feature = "pem", feature = "serde"))]
-#[cfg_attr(docsrs, doc(cfg(all(feature = "pem", feature = "serde"))))]
+#[cfg(all(feature = "alloc", feature = "serde"))]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "alloc", feature = "serde"))))]
 impl<C> Serialize for PublicKey<C>
 where
     C: Curve + AlgorithmParameters + ProjectiveArithmetic,
@@ -365,15 +364,18 @@ where
     where
         S: ser::Serializer,
     {
-        self.to_public_key_der()
-            .map_err(ser::Error::custom)?
-            .as_ref()
-            .serialize(serializer)
+        let der = self.to_public_key_der().map_err(ser::Error::custom)?;
+
+        if serializer.is_human_readable() {
+            base16ct::upper::encode_string(der.as_ref()).serialize(serializer)
+        } else {
+            der.as_ref().serialize(serializer)
+        }
     }
 }
 
-#[cfg(all(feature = "pem", feature = "serde"))]
-#[cfg_attr(docsrs, doc(cfg(all(feature = "pem", feature = "serde"))))]
+#[cfg(all(feature = "alloc", feature = "serde"))]
+#[cfg_attr(docsrs, doc(cfg(all(feature = "alloc", feature = "serde"))))]
 impl<'de, C> Deserialize<'de> for PublicKey<C>
 where
     C: Curve + AlgorithmParameters + ProjectiveArithmetic,
@@ -386,8 +388,15 @@ where
     {
         use de::Error;
 
-        <&[u8]>::deserialize(deserializer)
-            .and_then(|bytes| Self::from_public_key_der(bytes).map_err(D::Error::custom))
+        if deserializer.is_human_readable() {
+            let der_bytes = base16ct::mixed::decode_vec(<&str>::deserialize(deserializer)?)
+                .map_err(D::Error::custom)?;
+            Self::from_public_key_der(&der_bytes)
+        } else {
+            let der_bytes = <&[u8]>::deserialize(deserializer)?;
+            Self::from_public_key_der(der_bytes)
+        }
+        .map_err(D::Error::custom)
     }
 }