Introduce a trait to abstract the CTAP environment

The end goal is to provide users with:
- the Env trait that they should implement
- the Ctap struct that they can use

src/env/mod.rs

@@ -0,0 +1,18 @@
+use crate::ctap::hid::ChannelID;
+use crate::ctap::status_code::Ctap2StatusCode;
+use crypto::rng256::Rng256;
+
+#[cfg(feature = "std")]
+pub mod test;
+
+pub trait UserPresence {
+    fn check(&self, cid: ChannelID) -> Result<(), Ctap2StatusCode>;
+}
+
+pub trait Env {
+    type Rng: Rng256;
+    type UserPresence: UserPresence;
+
+    fn rng(&mut self) -> &mut Self::Rng;
+    fn user_presence(&mut self) -> &mut Self::UserPresence;
+}

src/env/test.rs

@@ -0,0 +1,48 @@
+use crate::ctap::hid::ChannelID;
+use crate::ctap::status_code::Ctap2StatusCode;
+use crate::env::{Env, UserPresence};
+use crypto::rng256::ThreadRng256;
+
+pub struct TestEnv {
+    rng: ThreadRng256,
+    user_presence: TestUserPresence,
+}
+
+pub struct TestUserPresence {
+    check: Box<dyn Fn(ChannelID) -> Result<(), Ctap2StatusCode>>,
+}
+
+impl TestEnv {
+    pub fn new() -> Self {
+        let rng = ThreadRng256 {};
+        let user_presence = TestUserPresence {
+            check: Box::new(|_| Ok(())),
+        };
+        TestEnv { rng, user_presence }
+    }
+}
+
+impl TestUserPresence {
+    pub fn set(&mut self, check: impl Fn(ChannelID) -> Result<(), Ctap2StatusCode> + 'static) {
+        self.check = Box::new(check);
+    }
+}
+
+impl UserPresence for TestUserPresence {
+    fn check(&self, cid: ChannelID) -> Result<(), Ctap2StatusCode> {
+        (self.check)(cid)
+    }
+}
+
+impl Env for TestEnv {
+    type Rng = ThreadRng256;
+    type UserPresence = TestUserPresence;
+
+    fn rng(&mut self) -> &mut Self::Rng {
+        &mut self.rng
+    }
+
+    fn user_presence(&mut self) -> &mut Self::UserPresence {
+        &mut self.user_presence
+    }
+}