From d8476fe6cfcd422a0a7a2b588b93eb9e1897df01 Mon Sep 17 00:00:00 2001
From: Henry de Valence <hdevalence@hdevalence.ca>
Date: Fri, 9 Dec 2016 17:24:43 -0800
Subject: [PATCH 1/2] Add a Scalar::random() constructor

This adds a dependency on the `rand` crate, used to construct an
OS-backed CSPRNG.  The implementation in this commit is somewhat
inefficient as it constructs a new OsRng object every time; it might be
better to construct it once.  (Seems like a lot of overhead for a few
getrandom(2) calls...)
---
 Cargo.toml    |  1 +
 src/lib.rs    |  2 ++
 src/scalar.rs | 14 ++++++++++++++
 3 files changed, 17 insertions(+)

diff --git a/Cargo.toml b/Cargo.toml
index 7db21b5..1973fcd 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -15,6 +15,7 @@ exclude = [
 ]
 
 [dependencies]
+rand = "0.3"
 arrayref = "0.3.2"
 
 # The development profile, used for `cargo build`.
diff --git a/src/lib.rs b/src/lib.rs
index ddb79e6..9253c39 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -37,6 +37,8 @@ extern crate test;
 #[macro_use]
 extern crate arrayref;
 
+extern crate rand;
+
 // Modules for low-level operations directly on field elements and curve points.
 
 pub mod field;
diff --git a/src/scalar.rs b/src/scalar.rs
index 610c563..7f0adf0 100644
--- a/src/scalar.rs
+++ b/src/scalar.rs
@@ -29,6 +29,9 @@
 use std::clone::Clone;
 use std::ops::{Index, IndexMut};
 
+use rand::OsRng;
+use rand::Rng;
+
 use field::{load3, load4};
 
 /// The `Scalar` struct represents an element in ℤ/lℤ, where
@@ -60,6 +63,17 @@ impl IndexMut<usize> for Scalar {
 }
 
 impl Scalar {
+    /// Return a `Scalar` chosen uniformly at random using a CSPRNG.
+    /// Panics if the operating system's CSPRNG is unavailable.
+    pub fn random() -> Self {
+        // XXX is there a more efficient way than building
+        // the rng every time here?
+        let mut rng = OsRng::new().unwrap();
+        let mut scalar_bytes = [0u8; 64];
+        rng.fill_bytes(&mut scalar_bytes);
+        Scalar::reduce(&scalar_bytes)
+    }
+
     /// Construct the additive identity
     pub fn zero() -> Self {
         Scalar([0u8; 32])

From c6b1b497b05e7270c773a35e168e80686e01fbfa Mon Sep 17 00:00:00 2001
From: Henry de Valence <hdevalence@hdevalence.ca>
Date: Fri, 9 Dec 2016 17:45:54 -0800
Subject: [PATCH 2/2] Add benchmark for Scalar::random()

---
 src/scalar.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/scalar.rs b/src/scalar.rs
index 7f0adf0..8a575fd 100644
--- a/src/scalar.rs
+++ b/src/scalar.rs
@@ -431,6 +431,11 @@ mod test {
     use super::*;
     use test::Bencher;
 
+    #[bench]
+    fn bench_scalar_random(b: &mut Bencher) {
+        b.iter(|| Scalar::random());
+    }
+
     #[bench]
     fn bench_scalar_multiply_add(b: &mut Bencher) {
         b.iter(|| Scalar::multiply_add(&X, &Y, &Z) );