impl ConditionallySelectable for Field

ff/Cargo.toml

@@ -14,6 +14,7 @@ edition = "2018"
 byteorder = "1"
 ff_derive = { version = "0.4.0", path = "ff_derive", optional = true }
 rand_core = "0.5"
+subtle = "2.2.1"
 
 [features]
 default = []

ff/ff_derive/src/lib.rs

@@ -833,6 +833,16 @@ fn prime_field_impl(
             }
         }
 
+        impl ::subtle::ConditionallySelectable for #name {
+            fn conditional_select(a: &#name, b: &#name, choice: ::subtle::Choice) -> #name {
+                let mut res = [0u64; #limbs];
+                for i in 0..#limbs {
+                    res[i] = u64::conditional_select(&(a.0).0[i], &(b.0).0[i], choice);
+                }
+                #name(#repr(res))
+            }
+        }
+
         impl ::std::ops::Neg for #name {
             type Output = #name;
 

ff/src/lib.rs

@@ -12,6 +12,7 @@ use std::error::Error;
 use std::fmt;
 use std::io::{self, Read, Write};
 use std::ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign};
+use subtle::ConditionallySelectable;
 
 /// This trait represents an element of a field.
 pub trait Field:
@@ -24,6 +25,7 @@ pub trait Field:
     + fmt::Debug
     + fmt::Display
     + 'static
+    + ConditionallySelectable
     + Add<Output = Self>
     + Sub<Output = Self>
     + Mul<Output = Self>