fido-common: Implement serde for credential::public_key::Descriptor

Adds serde_with as a dependency in order to serialize Descriptor::id as
bytes instead of as a sequence.

Enables cfg_eval feature to enable conditional usage of serde_with
behind the "serde" feature flag for the fido-common crate.

crates/fido-common/Cargo.toml

@@ -9,7 +9,8 @@ edition = "2021"
 ciborium = { version = "0.2.1", default-features = false, optional = true }
 coset = { version = "0.3.4", default-features = false }
 serde = { version = "1", features = ["derive"], optional = true }
+serde_with = { path = "../../../serde_with/serde_with", optional = true }
 bitflags = { version = "2.2.1", default-features = false, optional = true }
 
 [features]
-serde = ["dep:serde", "dep:bitflags", "dep:ciborium"]
+serde = ["dep:serde", "dep:serde_with", "dep:bitflags", "dep:ciborium"]

crates/fido-common/src/credential/public_key.rs

@@ -3,6 +3,8 @@ use std::collections::BTreeSet;
 
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
+#[cfg(feature = "serde")]
+use serde_with::{serde_as, skip_serializing_none, Bytes};
 
 #[cfg(feature = "serde")]
 pub(crate) mod algorithm {
@@ -48,8 +50,14 @@ pub struct Parameters {
 }
 
 /// > This dictionary identifies a specific public key credential.
-#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
-#[derive(Debug)]
+#[cfg_eval]
+#[derive(Debug, Clone)]
+#[cfg_attr(
+    feature = "serde",
+    serde_as,
+    skip_serializing_none,
+    derive(Serialize, Deserialize)
+)]
 pub struct Descriptor {
     /// > This member contains the type of the public key credential
     /// > the caller is referring to.
@@ -58,6 +66,8 @@ pub struct Descriptor {
     /// > A probabilistically-unique byte sequence identifying a
     /// > public key credential source and its authentication
     /// > assertions.
+    // Bounds: [16, 1023] bytes
+    #[cfg_attr(feature = "serde", serde_as(as = "Bytes"))]
     pub id: Vec<u8>,
     /// > This... member contains a hint as to how the client might
     /// > communicate with the managing authenticator of the public

crates/fido-common/src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(split_array, slice_take)]
+#![feature(cfg_eval, split_array, slice_take)]
 
 pub mod attestation;
 pub mod authenticator;