dsa: introduce serde

This commits introduces the dependency `serde` to `dsa`. It is gated
by two features: `serde` and `serde_secrets`. The first only derives
`Deserialize` and `Serialize` for non-secret types like `Signature`.
The latter additionally for `SignatureKey`, which should be used with
caution, because it could allow accidentally leaking secrets.

dsa/Cargo.toml

@@ -21,6 +21,7 @@ num-bigint = { package = "num-bigint-dig", version = "0.8", default-features = f
 num-traits = { version = "0.2", default-features = false }
 pkcs8 = { version = "=0.11.0-rc.1", default-features = false, features = ["alloc"] }
 rfc6979 = { version = "=0.5.0-pre.4", path = "../rfc6979" }
+serde = { version = "1.0", default-features = false, features = ["derive"], optional = true }
 sha2 = { version = "=0.11.0-pre.4", default-features = false }
 signature = { version = "=2.3.0-pre.4", default-features = false, features = ["alloc", "digest", "rand_core"] }
 zeroize = { version = "1", default-features = false }
@@ -32,4 +33,12 @@ rand_chacha = "0.3"
 sha1 = "=0.11.0-pre.4"
 
 [features]
+serde = [
+    "dep:serde",
+    "num-bigint/serde",
+]
+serde_secrets = [
+    "serde",
+    "zeroize/serde",
+]
 std = []

dsa/src/components.rs

@@ -11,10 +11,14 @@ use pkcs8::der::{
 };
 use signature::rand_core::CryptoRngCore;
 
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
 /// The common components of an DSA keypair
 ///
 /// (the prime p, quotient q and generator g)
 #[derive(Clone, Debug, PartialEq, PartialOrd)]
+#[cfg_attr(feature = "serde", derive(Deserialize, Serialize))]
 #[must_use]
 pub struct Components {
     /// Prime p

dsa/src/lib.rs

@@ -75,8 +75,12 @@ use pkcs8::der::{
 };
 use signature::SignatureEncoding;
 
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
 /// Container of the DSA signature
 #[derive(Clone, Debug)]
+#[cfg_attr(feature = "serde", derive(Deserialize, Serialize))]
 #[must_use]
 pub struct Signature {
     /// Signature part r

dsa/src/signing_key.rs

@@ -24,11 +24,15 @@ use signature::{
 };
 use zeroize::{Zeroize, Zeroizing};
 
+#[cfg(feature = "serde_secrets")]
+use serde::{Deserialize, Serialize};
+
 /// DSA private key.
 ///
 /// The [`(try_)sign_digest_with_rng`](::signature::RandomizedDigestSigner) API uses regular non-deterministic signatures,
 /// while the [`(try_)sign_digest`](::signature::DigestSigner) API uses deterministic signatures as described in RFC 6979
 #[derive(Clone, PartialEq)]
+#[cfg_attr(feature = "serde_secrets", derive(Deserialize, Serialize))]
 #[must_use]
 pub struct SigningKey {
     /// Public key

dsa/src/verifying_key.rs

@@ -16,8 +16,12 @@ use pkcs8::{
 };
 use signature::{hazmat::PrehashVerifier, DigestVerifier, Verifier};
 
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
 /// DSA public key.
 #[derive(Clone, Debug, PartialEq, PartialOrd)]
+#[cfg_attr(feature = "serde", derive(Deserialize, Serialize))]
 #[must_use]
 pub struct VerifyingKey {
     /// common components