add clear_on_drop to toml & add secret key type

Cargo.toml

@@ -27,6 +27,9 @@ default-features = false
 default-features = false
 version = "0.2"
 
+[dependencies.clear_on_drop]
+version = "0.2"
+
 [dev-dependencies]
 criterion = "0.2"
 rand = "0.5"
@@ -38,6 +41,6 @@ harness = false
 [features]
 default = ["std", "nightly", "u64_backend"]
 std = ["curve25519-dalek/std"]
-nightly = ["curve25519-dalek/nightly"]
+nightly = ["curve25519-dalek/nightly", "clear_on_drop/nightly"]
 u64_backend = ["curve25519-dalek/u64_backend"]
 u32_backend = ["curve25519-dalek/u32_backend"]

src/lib.rs

@@ -126,6 +126,8 @@
 #![cfg_attr(feature = "bench", feature(test))]
 #![deny(missing_docs)]
 
+extern crate clear_on_drop;
+
 extern crate curve25519_dalek;
 
 extern crate rand_core;

src/x25519.rs

@@ -12,6 +12,10 @@
 //! This implements x25519 key exchange as specified by Mike Hamburg
 //! and Adam Langley in [RFC7748](https://tools.ietf.org/html/rfc7748).
 
+use core::fmt::{Debug};
+
+use clear_on_drop::clear::Clear;
+
 use curve25519_dalek::constants::ED25519_BASEPOINT_TABLE;
 use curve25519_dalek::montgomery::MontgomeryPoint;
 use curve25519_dalek::scalar::Scalar;
@@ -19,6 +23,26 @@ use curve25519_dalek::scalar::Scalar;
 use rand_core::RngCore;
 use rand_core::CryptoRng;
 
+/// The length of a curve25519 EdDSA `SecretKey`, in bytes.
+pub const SECRET_KEY_LENGTH: usize = 32;
+
+/// An EdDSA secret key.
+#[repr(C)]
+#[derive(Default)] // we derive Default in order to use the clear() method in Drop
+pub struct SecretKey(pub (crate) [u8; SECRET_KEY_LENGTH]);
+
+impl Debug for SecretKey {
+    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
+        write!(f, "SecretKey: {:?}", &self.0[..])
+    }
+}
+
+/// Overwrite secret key material with null bytes when it goes out of scope.
+impl Drop for SecretKey {
+    fn drop(&mut self) {
+        self.0.clear();
+    }
+}
 /// "Decode" a scalar from a 32-byte array.
 ///
 /// By "decode" here, what is really meant is applying key clamping by twiddling