From c44d70299ba9a2b3116e733ecc10fc805ff4e9da Mon Sep 17 00:00:00 2001
From: Jay Graber <jay@z.cash>
Date: Mon, 14 May 2018 20:40:35 -0700
Subject: [PATCH] Add ask_to_ak and nsk_to_nk

---
 include/librustzcash.h |  4 ++++
 src/rustzcash.rs       | 41 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/include/librustzcash.h b/include/librustzcash.h
index 6aa388a..794662b 100644
--- a/include/librustzcash.h
+++ b/include/librustzcash.h
@@ -8,6 +8,10 @@ extern "C" {
 
     void librustzcash_to_scalar(const unsigned char *input, unsigned char *result);
 
+    void librustzcash_ask_to_ak(const unsigned char *ask, unsigned char *result);
+
+    void librustzcash_nsk_to_nk(const unsigned char *nsk, unsigned char *result);
+
     /// Loads the zk-SNARK parameters into memory and saves
     /// paths as necessary. Only called once.
     void librustzcash_init_zksnark_params(
diff --git a/src/rustzcash.rs b/src/rustzcash.rs
index 74fc115..b9f7b6b 100644
--- a/src/rustzcash.rs
+++ b/src/rustzcash.rs
@@ -13,7 +13,7 @@ use pairing::{BitIterator, Field, PrimeField, PrimeFieldRepr, bls12_381::{Bls12,
 
 use sapling_crypto::{circuit::multipack,
                      jubjub::{edwards, FixedGenerators, JubjubBls12, JubjubEngine, JubjubParams,
-                              ToUniform, Unknown, fs::FsRepr},
+                              PrimeOrder, ToUniform, Unknown, fs::FsRepr},
                      pedersen_hash::{pedersen_hash, Personalization}, redjubjub::{self, Signature}};
 
 use sapling_crypto::circuit::sprout::{self, TREE_DEPTH as SPROUT_TREE_DEPTH};
@@ -71,6 +71,19 @@ fn read_le(from: &[u8]) -> FrRepr {
     f
 }
 
+/// Reads an FsRepr from [u8] of length 32
+/// and multiplies it by the given base.
+/// This will panic (abort) if length provided is
+/// not correct
+fn fixed_scalar_mult(from: &[u8], p_g: FixedGenerators) -> edwards::Point<Bls12, PrimeOrder> {
+    assert_eq!(from.len(), 32);
+
+    let mut f = <<Bls12 as JubjubEngine>::Fs as PrimeField>::Repr::default();
+    f.read_le(from).expect("length is 32 bytes");
+
+    JUBJUB.generator(p_g).mul(f, &JUBJUB)
+}
+
 #[no_mangle]
 pub extern "system" fn librustzcash_init_zksnark_params(
     spend_path: *const c_char,
@@ -202,6 +215,32 @@ pub extern "system" fn librustzcash_to_scalar(
         .expect("length is 32 bytes");
 }
 
+#[no_mangle]
+pub extern "system" fn librustzcash_ask_to_ak(
+    ask: *const [c_uchar; 32],
+    result: *mut [c_uchar; 32],
+) {
+    let ask = unsafe { &*ask };
+    let ak = fixed_scalar_mult(ask, FixedGenerators::SpendingKeyGenerator);
+
+    let result = unsafe { &mut *result };
+
+    ak.write(&mut result[..]).expect("length is 32 bytes");
+}
+
+#[no_mangle]
+pub extern "system" fn librustzcash_nsk_to_nk(
+    nsk: *const [c_uchar; 32],
+    result: *mut [c_uchar; 32],
+) {
+    let nsk = unsafe { &*nsk };
+    let nk = fixed_scalar_mult(nsk, FixedGenerators::ProofGenerationKey);
+
+    let result = unsafe { &mut *result };
+
+    nk.write(&mut result[..]).expect("length is 32 bytes");
+}
+
 /// XOR two uint64_t values and return the result, used
 /// as a temporary mechanism for introducing Rust into
 /// Zcash.