impl ConditionallySelectable for Field

Cargo.toml

@@ -21,6 +21,7 @@ crossbeam = { version = "0.7", optional = true }
 pairing = { version = "0.16", path = "../pairing", optional = true }
 rand_core = "0.5"
 byteorder = "1"
+subtle = "2.2.1"
 
 [dev-dependencies]
 hex-literal = "0.2"

src/groth16/tests/dummy_engine.rs

@@ -10,6 +10,7 @@ use std::cmp::Ordering;
 use std::fmt;
 use std::num::Wrapping;
 use std::ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign};
+use subtle::{Choice, ConditionallySelectable};
 
 const MODULUS_R: Wrapping<u32> = Wrapping(64513);
 
@@ -22,6 +23,16 @@ impl fmt::Display for Fr {
     }
 }
 
+impl ConditionallySelectable for Fr {
+    fn conditional_select(a: &Self, b: &Self, choice: Choice) -> Self {
+        Fr(Wrapping(u32::conditional_select(
+            &(a.0).0,
+            &(b.0).0,
+            choice,
+        )))
+    }
+}
+
 impl Neg for Fr {
     type Output = Self;