Implement constant time comparison and selection of Fp2 elements.

src/fp2.rs

@@ -1,5 +1,7 @@
 //! This module implements arithmetic over the quadratic extension field Fp2.
 
+use subtle::{Choice, ConditionallySelectable, ConstantTimeEq};
+
 use crate::fp::Fp;
 
 #[derive(Copy, Clone, Debug)]
@@ -7,3 +9,92 @@ pub struct Fp2 {
     pub c0: Fp,
     pub c1: Fp,
 }
+
+impl ConstantTimeEq for Fp2 {
+    fn ct_eq(&self, other: &Self) -> Choice {
+        self.c0.ct_eq(&other.c0) & self.c1.ct_eq(&other.c1)
+    }
+}
+
+impl Eq for Fp2 {}
+impl PartialEq for Fp2 {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.ct_eq(other).unwrap_u8() == 1
+    }
+}
+
+impl ConditionallySelectable for Fp2 {
+    fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
+        Fp2 {
+            c0: Fp::conditional_select(&a.c0, &b.c0, choice),
+            c1: Fp::conditional_select(&a.c1, &b.c1, choice),
+        }
+    }
+}
+
+#[test]
+fn test_conditional_selection() {
+    let a = Fp2 {
+        c0: Fp::from_raw_unchecked([1, 2, 3, 4, 5, 6]),
+        c1: Fp::from_raw_unchecked([7, 8, 9, 10, 11, 12]),
+    };
+    let b = Fp2 {
+        c0: Fp::from_raw_unchecked([13, 14, 15, 16, 17, 18]),
+        c1: Fp::from_raw_unchecked([19, 20, 21, 22, 23, 24]),
+    };
+
+    assert_eq!(
+        ConditionallySelectable::conditional_select(&a, &b, Choice::from(0u8)),
+        a
+    );
+    assert_eq!(
+        ConditionallySelectable::conditional_select(&a, &b, Choice::from(1u8)),
+        b
+    );
+}
+
+#[test]
+fn test_equality() {
+    fn is_equal(a: &Fp2, b: &Fp2) -> bool {
+        let eq = a == b;
+        let ct_eq = a.ct_eq(&b);
+
+        assert_eq!(eq, ct_eq.unwrap_u8() == 1);
+
+        eq
+    }
+
+    assert!(is_equal(
+        &Fp2 {
+            c0: Fp::from_raw_unchecked([1, 2, 3, 4, 5, 6]),
+            c1: Fp::from_raw_unchecked([7, 8, 9, 10, 11, 12]),
+        },
+        &Fp2 {
+            c0: Fp::from_raw_unchecked([1, 2, 3, 4, 5, 6]),
+            c1: Fp::from_raw_unchecked([7, 8, 9, 10, 11, 12]),
+        }
+    ));
+
+    assert!(!is_equal(
+        &Fp2 {
+            c0: Fp::from_raw_unchecked([2, 2, 3, 4, 5, 6]),
+            c1: Fp::from_raw_unchecked([7, 8, 9, 10, 11, 12]),
+        },
+        &Fp2 {
+            c0: Fp::from_raw_unchecked([1, 2, 3, 4, 5, 6]),
+            c1: Fp::from_raw_unchecked([7, 8, 9, 10, 11, 12]),
+        }
+    ));
+
+    assert!(!is_equal(
+        &Fp2 {
+            c0: Fp::from_raw_unchecked([1, 2, 3, 4, 5, 6]),
+            c1: Fp::from_raw_unchecked([2, 8, 9, 10, 11, 12]),
+        },
+        &Fp2 {
+            c0: Fp::from_raw_unchecked([1, 2, 3, 4, 5, 6]),
+            c1: Fp::from_raw_unchecked([7, 8, 9, 10, 11, 12]),
+        }
+    ));
+}