ctap2-proto: Simplify authenticator::client_pin::auth_protocol traits

Rather than use manual lifetime management for PinUvAuthProtocol session
keys, change the auth_protocol::platform::Session trait to represent a
single Session, which maintains its own platform key agreement key, and
can be managed with the lifetime of the value itself.

crates/ctap2-proto/src/authenticator/client_pin/auth_protocol.rs

@@ -1,8 +1,43 @@
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
-pub trait Authenticator {
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
+#[cfg_attr(
+    feature = "serde",
+    derive(Serialize, Deserialize),
+    serde(into = "u8", try_from = "u8")
+)]
+pub enum Version {
+    One = 1,
+    Two = 2,
+}
+
+impl From<Version> for u8 {
+    fn from(value: Version) -> Self {
+        value as u8
+    }
+}
+
+impl TryFrom<u8> for Version {
+    type Error = super::Error;
+
+    fn try_from(value: u8) -> Result<Self, Self::Error> {
+        match value {
+            1 => Ok(Version::One),
+            2 => Ok(Version::Two),
+            _ => Err(super::Error::InvalidParameter),
+        }
+    }
+}
+
+/// The AES block size, in bytes.
+pub const BLOCK_SIZE: usize = 16;
+
+pub mod authenticator {
+    use super::Version;
+    pub trait Authenticator {
         type Error; // TODO: Can the error cases be enumerated here?
+        const VERSION: Version;
 
         /// This process is run by the authenticator at power-on.
         fn initialize(&mut self) -> Result<(), Self::Error>;
@@ -16,73 +51,53 @@ pub trait Authenticator {
         /// Returns the authenticator’s public key as a COSE_Key structure.
         fn get_public_key(&self) -> Result<cosey::PublicKey, Self::Error>;
 
-    /// Processes the output of encapsulate from the peer and produces a shared
-    /// secret, known to both platform and authenticator.
+        /// Processes the output of encapsulate from the peer and produces a
+        /// shared secret, known to both platform and authenticator.
         fn decapsulate(&self, peer_cose_key: cosey::PublicKey) -> Result<Vec<u8>, Self::Error>;
 
         /// Decrypts a ciphertext, using sharedSecret as a key, and returns the
         /// plaintext.
         fn decrypt(&self, ciphertext: &[u8]) -> Result<Vec<u8>, Self::Error>;
 
-    /// Verifies that the signature is a valid MAC for the given message. If the
-    /// key parameter value is the current pinUvAuthToken, it also checks
-    /// whether the pinUvAuthToken is in use or not.
+        /// Verifies that the signature is a valid MAC for the given message. If
+        /// the key parameter value is the current pinUvAuthToken, it
+        /// also checks whether the pinUvAuthToken is in use or not.
         fn verify(&self, key: &[u8], message: &[u8], signature: &[u8]) -> Result<(), Self::Error>;
+    }
 }
 
-pub trait Platform {
+pub mod platform {
+    use super::{Version, BLOCK_SIZE};
+
+    pub trait Session<const VERSION: Version>: Sized {
         type Error; // TODO: Can the error cases be enumerated here?
-    /// This is run by the platform when starting a series of transactions
-    /// with a specific authenticator.
-    fn initialize(&self) -> Result<(), Self::Error>;
 
-    /// Generates an encapsulation for the authenticator’s public key and
-    /// returns the message to transmit and the shared secret.
-    fn encapsulate(
-        &self,
-        peer_cose_key: cosey::PublicKey,
-    ) -> Result<(cosey::PublicKey, Vec<u8>), Self::Error>; // TODO: Return a struct for OK variant?
+        /// Encompasses both the `initialize` and `encapsulate` functions in the
+        /// platform interface of the spec. This is done to guard against
+        /// private key reuse by the platform. As a consequence, a new
+        /// session must be initialized for each transaction with a
+        /// freshly generated private key.
+        fn initialize(peer_cose_key: cosey::PublicKey) -> Result<Self, Self::Error>;
+
+        fn platform_key_agreement_key(&self) -> &cosey::PublicKey;
 
         /// Encrypts a plaintext to produce a ciphertext, which may be longer
         /// than the plaintext. The plaintext is restricted to being a
         /// multiple of the AES block size (16 bytes) in length.
-    // TODO: Return a specific type instead of raw bytes?
-    fn encrypt(&self, key: &[u8], plaintext: &[u8]) -> Result<Vec<u8>, Self::Error>;
+        fn encrypt<const N: usize>(
+            &self,
+            plaintext: &[[u8; BLOCK_SIZE]; N],
+        ) -> Result<[[u8; BLOCK_SIZE]; N], Self::Error>;
 
         /// Decrypts a ciphertext and returns the plaintext.
         // TODO: Return a specific type instead of raw bytes?
-    fn decrypt(&self, key: &[u8], ciphertext: &[u8]) -> Result<Vec<u8>, Self::Error>;
+        fn decrypt<const N: usize>(
+            &self,
+            ciphertext: &[[u8; BLOCK_SIZE]; N],
+        ) -> [[u8; BLOCK_SIZE]; N];
 
         /// Computes a MAC of the given message.
         // TODO: Return a specific type instead of raw bytes?
-    fn authenticate(&self, key: &[u8], message: &[u8]) -> Result<Vec<u8>, Self::Error>;
-}
-
-#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
-#[cfg_attr(
-    feature = "serde",
-    derive(Serialize, Deserialize),
-    serde(into = "u8", try_from = "u8")
-)]
-pub enum Version {
-    One = 1,
-    Two = 2,
-}
-
-impl From<Version> for u8 {
-    fn from(value: Version) -> Self {
-        value as u8
-    }
-}
-
-impl TryFrom<u8> for Version {
-    type Error = super::Error;
-
-    fn try_from(value: u8) -> Result<Self, Self::Error> {
-        match value {
-            1 => Ok(Version::One),
-            2 => Ok(Version::Two),
-            _ => Err(super::Error::InvalidParameter),
-        }
+        fn authenticate(&self, message: &[u8]) -> Result<[u8; 16], Self::Error>;
     }
 }