Add keygen.

Cargo.toml

@@ -5,6 +5,7 @@ authors = ["Henry de Valence <hdevalence@hdevalence.ca>"]
 edition = "2018"
 
 [dependencies]
+rand_core = "0.5"
 thiserror = "1.0"
 blake2b_simd = "0.5"
 jubjub = { git = "https://github.com/zkcrypto/jubjub", rev = "e83f7d2bd136498a27f9d943fea635d8682bf2c6" }

src/secret_key.rs

@@ -2,6 +2,8 @@ use std::{convert::TryFrom, marker::PhantomData};
 
 use crate::{Binding, Error, PublicKey, Randomizer, Scalar, SigType, Signature, SpendAuth};
 
+use rand_core::{CryptoRng, RngCore};
+
 /// A refinement type indicating that the inner `[u8; 32]` represents an
 /// encoding of a RedJubJub secret key.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
@@ -63,6 +65,21 @@ impl<T: SigType> TryFrom<SecretKeyBytes<T>> for SecretKey<T> {
     }
 }
 
+impl<R, T> From<R> for SecretKey<T>
+where
+    R: RngCore + CryptoRng,
+    T: SigType,
+{
+    fn from(mut rng: R) -> SecretKey<T> {
+        let mut bytes = [0; 64];
+        rng.fill_bytes(&mut bytes);
+        SecretKey {
+            sk: Scalar::from_bytes_wide(&bytes),
+            _marker: PhantomData,
+        }
+    }
+}
+
 impl<'a> From<&'a SecretKey<SpendAuth>> for PublicKey<SpendAuth> {
     fn from(sk: &'a SecretKey<SpendAuth>) -> PublicKey<SpendAuth> {
         // XXX-jubjub: this is pretty baroque