From 4ef2e9ae685875c433ad2cdce6358a5c70c00fbd Mon Sep 17 00:00:00 2001
From: Simon <simon@bitcartel.com>
Date: Mon, 21 May 2018 14:46:31 -0700
Subject: [PATCH] Add ffi and computation for Sapling note commitment.

---
 include/librustzcash.h | 21 ++++++++++++
 src/rustzcash.rs       | 76 ++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 95 insertions(+), 2 deletions(-)

diff --git a/include/librustzcash.h b/include/librustzcash.h
index b6e1184..876e727 100644
--- a/include/librustzcash.h
+++ b/include/librustzcash.h
@@ -102,6 +102,27 @@ extern "C" {
     /// `librustzcash_sapling_verification_ctx_init`.
     void librustzcash_sapling_verification_ctx_free(void *);
 
+    /// Compute a Sapling commitment.
+    ///
+    /// The `diversifier` parameter must be 11 bytes in length.
+    /// The `pk_d` and `r` parameters must be of length 32.
+    /// The result is also of length 32 and placed in `result`.
+    /// Returns false if the diversifier or pk_d is not valid
+    bool librustzcash_sapling_compute_cm(
+        const unsigned char *diversifier,
+        const unsigned char *pk_d,
+        const uint64_t value,
+        const unsigned char *r,
+        unsigned char *result
+    );
+
+    /// Generate uniform Sapling commitment randomness `r`.
+    /// The result is of length 32.
+    /// Returns false if there was an error.
+    bool librustzcash_sapling_generate_commitment_randomness(
+        unsigned char *result
+    );
+
     /// Sprout JoinSplit proof generation.
     void librustzcash_sprout_prove(
         unsigned char *proof_out,
diff --git a/src/rustzcash.rs b/src/rustzcash.rs
index 08fe1f3..f43bfb1 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, constants::CRH_IVK_PERSONALIZATION,
                      jubjub::{edwards, FixedGenerators, JubjubBls12, JubjubEngine, JubjubParams,
-                              PrimeOrder, ToUniform, Unknown, fs::FsRepr},
+                              PrimeOrder, ToUniform, Unknown, fs::{Fs, FsRepr}},
                      pedersen_hash::{pedersen_hash, Personalization}, redjubjub::{self, Signature}};
 
 use sapling_crypto::circuit::sprout::{self, TREE_DEPTH as SPROUT_TREE_DEPTH};
@@ -25,7 +25,7 @@ use blake2_rfc::blake2s::Blake2s;
 
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
 
-use rand::OsRng;
+use rand::{OsRng, Rng};
 use std::io::BufReader;
 
 use libc::{c_char, c_uchar, size_t, int64_t, uint32_t, uint64_t};
@@ -301,6 +301,78 @@ pub extern "system" fn librustzcash_ivk_to_pkd(
     }
 }
 
+/// Return 32 byte randomness, uniform, to be used for a Sapling commitment.
+#[no_mangle]
+pub extern "system" fn librustzcash_sapling_generate_commitment_randomness(
+    result: *mut [c_uchar; 32],
+) -> bool {
+    // create random 64 byte buffer
+    let mut rng = OsRng::new().expect("should be able to construct RNG");
+    let mut buffer = [0u8; 64];
+    for i in 0..buffer.len() {
+        buffer[i] = rng.gen();
+    }
+
+    // TODO: Remove this debug statement
+    println!("buffer of random bytes: {:?}", &buffer[..]);
+
+    // reduce to uniform value
+    let r = <Bls12 as JubjubEngine>::Fs::to_uniform(&buffer[..]);
+    let result = unsafe { &mut *result };
+    r.into_repr()
+        .write_le(&mut result[..])
+        .expect("result must be 32 bytes");
+
+    true
+}
+
+/// Compute Sapling note commitment.
+#[no_mangle]
+pub extern "system" fn librustzcash_sapling_compute_cm(
+    diversifier: *const [c_uchar; 11],
+    pk_d: *const [c_uchar; 32],
+    value: uint64_t,
+    r: *const [c_uchar; 32],
+    result: *mut [c_uchar; 32],
+) -> bool {
+    let diversifier = sapling_crypto::primitives::Diversifier(unsafe { *diversifier });
+    let g_d = match diversifier.g_d::<Bls12>(&JUBJUB) {
+        Some(g_d) => g_d,
+        None => return false,
+    };
+
+    let pk_d = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*pk_d })[..], &JUBJUB) {
+        Ok(p) => p,
+        Err(_) => return false,
+    };
+
+    let pk_d = match pk_d.as_prime_order(&JUBJUB) {
+        Some(pk_d) => pk_d,
+        None => return false,
+    };
+
+    // Deserialize randomness
+    let r = unsafe { *r };
+    let mut repr = FsRepr::default();
+    repr.read_le(&r[..]).expect("length is not 32 bytes");
+    let r = match Fs::from_repr(repr) {
+        Ok(p) => p,
+        Err(_) => return false,
+    };
+
+    let note = sapling_crypto::primitives::Note {
+        value,
+        g_d,
+        pk_d,
+        r,
+    };
+
+    let result = unsafe { &mut *result };
+    write_le(note.cm(&JUBJUB).into_repr(), &mut result[..]);
+
+    true
+}
+
 /// XOR two uint64_t values and return the result, used
 /// as a temporary mechanism for introducing Rust into
 /// Zcash.