feat(session): support Signer trait through a Session

`signature` is a widely used "standard" trait offering high level APIs to manipulate
signers and verifiers.

This implements a `Signer` trait on a returned `SignatureRequest` through `Session` which is
a prepared "signature request" with filled mechanism and key.

This makes it possible to wire cryptoki further with other APIs of the Rust ecosystem.

cryptoki/Cargo.toml

@@ -19,6 +19,7 @@ psa-crypto = { version = "0.9.0", default-features = false, optional = true }
 cryptoki-sys = { path = "../cryptoki-sys", version = "0.1.4" }
 paste = "1.0.6"
 secrecy = "0.8.0"
+signature = { version = "2.1.0", optional = true, features = [ "std" ] }
 
 [dev-dependencies]
 num-traits = "0.2.14"
@@ -27,6 +28,8 @@ serial_test = "0.5.1"
 testresult = "0.2.0"
 
 [features]
+default = [ "signature-traits" ]
 psa-crypto-conversions = ["psa-crypto"]
+signature-traits = ["signature"]
 generate-bindings = ["cryptoki-sys/generate-bindings"]
 serde = ["secrecy/serde"]

cryptoki/src/session/signing_macing.rs

@@ -9,7 +9,16 @@ use crate::session::Session;
 use cryptoki_sys::*;
 use std::convert::TryInto;
 
+#[cfg(feature = "signature-traits")]
+use signature::Signer;
+
 impl Session {
+    #[cfg(feature = "signature-traits")]
+    /// Prepare a signature request which implements the signature::Signer trait.
+    pub fn prepare_signature<'a>(&'a self, mechanism: &'a Mechanism<'a>, key: ObjectHandle) -> SignatureRequest {
+        SignatureRequest::new(mechanism, key, self)
+    }
+
     /// Sign data in single-part
     pub fn sign(&self, mechanism: &Mechanism, key: ObjectHandle, data: &[u8]) -> Result<Vec<u8>> {
         let mut mechanism: CK_MECHANISM = mechanism.into();
@@ -86,3 +95,30 @@ impl Session {
         }
     }
 }
+
+#[cfg(feature = "signature-traits")]
+#[derive(Debug)]
+pub struct SignatureRequest<'sess: 'a, 'a, 'b> {
+    mechanism: &'a Mechanism<'b>,
+    key: ObjectHandle,
+    session: &'sess Session
+}
+
+#[cfg(feature = "signature-traits")]
+impl<'sess, 'a, 'b> SignatureRequest<'sess, 'a, 'b> {
+    pub fn new(mechanism: &'a Mechanism<'b>, key: ObjectHandle, session: &'sess Session) -> Self {
+        SignatureRequest {
+            mechanism,
+            key,
+            session
+        }
+    }
+}
+
+
+#[cfg(feature = "signature-traits")]
+impl<'sess, 'a, 'b> Signer<Vec<u8>> for SignatureRequest<'sess, 'a, 'b> {
+    fn try_sign(&self, msg: &[u8]) -> core::result::Result<Vec<u8>, signature::Error> {
+        self.session.sign(self.mechanism, self.key, msg).map_err(signature::Error::from_source)
+    }
+}