ctutils: subtle interop (#1262)

Adds bidirectional `From` conversions for the following:
- `ctutils::Choice` <=> `subtle::Choice`
- `ctutils::CtOption` <=> `subtle::CtOption`

Support is gated under an optional `subtle` feature.

This will simplify using `ctutils` in codebases that are already using
`subtle` such as `crypto-bigint` (the main place it seems interesting to
actually attempt to use it).

Author: tarcieri

.github/workflows/ctutils.yml

@@ -64,6 +64,8 @@ jobs:
         with:
           toolchain: ${{ matrix.rust }}
       - run: cargo test
+      - run: cargo test --all-features
+      - run: cargo test --all-features --release
 
   # Test using `cargo careful`
   test-careful:

Cargo.lock

@@ -18,3 +18,6 @@ rust-version = "1.85"
 
 [dependencies]
 cmov = "0.4"
+
+# optional dependencies
+subtle = { version = "2", optional = true, default-features = false }

ctutils/Cargo.toml

@@ -159,6 +159,22 @@ impl From<Choice> for bool {
     }
 }
 
+#[cfg(feature = "subtle")]
+impl From<subtle::Choice> for Choice {
+    #[inline]
+    fn from(choice: subtle::Choice) -> Choice {
+        Choice(choice.unwrap_u8())
+    }
+}
+
+#[cfg(feature = "subtle")]
+impl From<Choice> for subtle::Choice {
+    #[inline]
+    fn from(choice: Choice) -> subtle::Choice {
+        subtle::Choice::from(choice.0)
+    }
+}
+
 impl Not for Choice {
     type Output = Choice;
 

ctutils/src/choice.rs

@@ -385,6 +385,31 @@ impl<T> From<CtOption<T>> for Option<T> {
     }
 }
 
+/// NOTE: in order to be able to unwrap the `subtle::CtOption` we rely on a `Default` bound in
+/// order to have a placeholder value, and `ConditionallySelectable` to be able to use `unwrap_or`.
+#[cfg(feature = "subtle")]
+impl<T> From<subtle::CtOption<T>> for CtOption<T>
+where
+    T: subtle::ConditionallySelectable + Default,
+{
+    #[inline]
+    fn from(src: subtle::CtOption<T>) -> CtOption<T> {
+        let is_some = src.is_some();
+        CtOption {
+            value: src.unwrap_or(Default::default()),
+            is_some: is_some.into(),
+        }
+    }
+}
+
+#[cfg(feature = "subtle")]
+impl<T> From<CtOption<T>> for subtle::CtOption<T> {
+    #[inline]
+    fn from(src: CtOption<T>) -> subtle::CtOption<T> {
+        subtle::CtOption::new(src.value, src.is_some.into())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use crate::{Choice, CtEq, CtOption, CtSelect};