From a93798d209294e3ea018a8960518523e7eb0e4b6 Mon Sep 17 00:00:00 2001
From: Henry de Valence <hdevalence@hdevalence.ca>
Date: Mon, 27 Feb 2017 11:19:06 -0800
Subject: [PATCH 1/2] Add a function to hash a byte slice to a scalar.

---
 Cargo.toml    | 10 ++++++++++
 src/lib.rs    |  5 +++++
 src/scalar.rs | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 48 insertions(+)

diff --git a/Cargo.toml b/Cargo.toml
index da90912..691f464 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -22,6 +22,16 @@ version = "0.3.3"
 optional = true
 version = "0.3"
 
+[dependencies.digest]
+version = "0.4"
+
+[dependencies.generic-array]
+# same version that digest depends on
+version = "^0.6"
+
+[dev-dependencies.sha2]
+version = "0.4"
+
 [features]
 default = ["std"]
 std = ["rand"]
diff --git a/src/lib.rs b/src/lib.rs
index fe26b70..13fec92 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -38,10 +38,15 @@ extern crate std;
 
 #[cfg(test)]
 extern crate test;
+#[cfg(test)]
+extern crate sha2;
 
 #[macro_use]
 extern crate arrayref;
 
+extern crate generic_array;
+extern crate digest;
+
 #[cfg(feature = "std")]
 extern crate rand;
 
diff --git a/src/scalar.rs b/src/scalar.rs
index 85c77be..db19a9e 100644
--- a/src/scalar.rs
+++ b/src/scalar.rs
@@ -36,6 +36,10 @@ use core::ops::{Neg};
 #[cfg(feature = "std")]
 use rand::Rng;
 
+use digest::Digest;
+use generic_array::GenericArray;
+use generic_array::typenum::U64;
+
 use constants;
 use utils::{load3, load4};
 use subtle::CTAssignable;
@@ -159,6 +163,35 @@ impl Scalar {
         Scalar::reduce(&scalar_bytes)
     }
 
+    /// Hash a slice of bytes into a scalar.
+    ///
+    /// Takes a type parameter `D`, which is any `Digest` producing 64
+    /// bytes (512 bits) of output.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate curve25519_dalek;
+    /// # use curve25519_dalek::scalar::Scalar;
+    /// extern crate sha2;
+    /// use sha2::Sha512;
+    ///
+    /// # fn main() {
+    /// let msg = "To really appreciate architecture, you may even need to commit a murder";
+    /// let s = Scalar::hash_from_bytes::<Sha512>(msg.as_bytes());
+    /// # }
+    /// ```
+    ///
+    pub fn hash_from_bytes<D>(input: &[u8]) -> Scalar
+            where D: Digest<OutputSize=U64> + Default {
+        let mut hash = D::default();
+        hash.input(input);
+        // XXX this seems clumsy
+        let mut output = [0u8;64];
+        output.copy_from_slice(hash.result().as_slice());
+        Scalar::reduce(&output)
+    }
+
     /// Construct the additive identity
     pub fn zero() -> Self {
         Scalar([0u8; 32])

From fe15bc1d0b676f3b3d3ae21d2a41edcf1fc6276d Mon Sep 17 00:00:00 2001
From: Henry de Valence <hdevalence@hdevalence.ca>
Date: Mon, 27 Feb 2017 11:22:07 -0800
Subject: [PATCH 2/2] Add note on fn main() in doctest

---
 src/scalar.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/scalar.rs b/src/scalar.rs
index db19a9e..60d92ba 100644
--- a/src/scalar.rs
+++ b/src/scalar.rs
@@ -176,6 +176,8 @@ impl Scalar {
     /// extern crate sha2;
     /// use sha2::Sha512;
     ///
+    /// # // Need fn main() here in comment so the doctest compiles
+    /// # // See https://doc.rust-lang.org/book/documentation.html#documentation-as-tests
     /// # fn main() {
     /// let msg = "To really appreciate architecture, you may even need to commit a murder";
     /// let s = Scalar::hash_from_bytes::<Sha512>(msg.as_bytes());