pirate-librustzcash/src/rustzcash.rs

extern crate bellman;
extern crate blake2_rfc;
extern crate byteorder;
extern crate libc;
extern crate pairing;
extern crate rand;
extern crate sapling_crypto;

#[macro_use]
extern crate lazy_static;

use pairing::{
    bls12_381::{Bls12, Fr, FrRepr}, BitIterator, Field, PrimeField, PrimeFieldRepr,
};

use sapling_crypto::{
    circuit::multipack, constants::CRH_IVK_PERSONALIZATION,
    jubjub::{
        edwards, fs::{Fs, FsRepr}, FixedGenerators, JubjubBls12, JubjubEngine, JubjubParams,
        PrimeOrder, ToUniform, Unknown,
    },
    pedersen_hash::{pedersen_hash, Personalization}, redjubjub::{self, Signature},
};

use sapling_crypto::circuit::sprout::{self, TREE_DEPTH as SPROUT_TREE_DEPTH};
// TODO: make these consistent
const SAPLING_TREE_DEPTH: usize = 32;

use bellman::groth16::{
    create_random_proof, prepare_verifying_key, verify_proof, Parameters, PreparedVerifyingKey,
    Proof, VerifyingKey,
};

use blake2_rfc::blake2s::Blake2s;

use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};

use rand::{OsRng, Rand, Rng};
use std::io::BufReader;

use libc::{c_char, c_uchar, int64_t, size_t, uint32_t, uint64_t};
use std::ffi::CStr;
use std::fs::File;
use std::slice;

use sapling_crypto::primitives::{ProofGenerationKey, ValueCommitment, ViewingKey};

pub mod equihash;

#[cfg(test)]
mod tests;

lazy_static! {
    static ref JUBJUB: JubjubBls12 = { JubjubBls12::new() };
}

static mut SAPLING_SPEND_VK: Option<PreparedVerifyingKey<Bls12>> = None;
static mut SAPLING_OUTPUT_VK: Option<PreparedVerifyingKey<Bls12>> = None;
static mut SPROUT_GROTH16_VK: Option<PreparedVerifyingKey<Bls12>> = None;

static mut SAPLING_SPEND_PARAMS: Option<Parameters<Bls12>> = None;
static mut SAPLING_OUTPUT_PARAMS: Option<Parameters<Bls12>> = None;
static mut SPROUT_GROTH16_PARAMS_PATH: Option<String> = None;

fn is_small_order<Order>(p: &edwards::Point<Bls12, Order>) -> bool {
    p.double(&JUBJUB).double(&JUBJUB).double(&JUBJUB) == edwards::Point::zero()
}

/// Writes an FrRepr to [u8] of length 32
fn write_le(f: FrRepr, to: &mut [u8]) {
    assert_eq!(to.len(), 32);

    f.write_le(to).expect("length is 32 bytes");
}

/// Reads an FrRepr from a [u8] of length 32.
/// This will panic (abort) if length provided is
/// not correct.
fn read_le(from: &[u8]) -> FrRepr {
    assert_eq!(from.len(), 32);

    let mut f = FrRepr::default();
    f.read_le(from).expect("length is 32 bytes");

    f
}

/// Reads an FsRepr from [u8] of length 32
/// This will panic (abort) if length provided is
/// not correct
fn read_fs(from: &[u8]) -> FsRepr {
    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");

    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> {
    let f = read_fs(from);

    JUBJUB.generator(p_g).mul(f, &JUBJUB)
}

#[no_mangle]
pub extern "system" fn librustzcash_init_zksnark_params(
    spend_path: *const c_char,
    output_path: *const c_char,
    sprout_path: *const c_char,
) {
    // Initialize jubjub parameters here
    lazy_static::initialize(&JUBJUB);

    // These should be valid CStr's, but the decoding may fail on Windows
    // so we may need to use OSStr or something.
    let spend_path = unsafe { CStr::from_ptr(spend_path) }
        .to_str()
        .expect("parameter path encoding error")
        .to_string();
    let output_path = unsafe { CStr::from_ptr(output_path) }
        .to_str()
        .expect("parameter path encoding error")
        .to_string();
    let sprout_path = unsafe { CStr::from_ptr(sprout_path) }
        .to_str()
        .expect("parameter path encoding error")
        .to_string();

    // Load from each of the paths
    let mut spend_fs = File::open(spend_path).expect("couldn't load Sapling spend parameters file");
    let mut output_fs =
        File::open(output_path).expect("couldn't load Sapling output parameters file");
    let mut sprout_fs =
        File::open(&sprout_path).expect("couldn't load Sprout groth16 parameters file");

    // Deserialize params
    let spend_params = Parameters::<Bls12>::read(&mut spend_fs, false)
        .expect("couldn't deserialize Sapling spend parameters file");
    let output_params = Parameters::<Bls12>::read(&mut output_fs, false)
        .expect("couldn't deserialize Sapling spend parameters file");
    let sprout_vk = VerifyingKey::<Bls12>::read(&mut sprout_fs)
        .expect("couldn't deserialize Sprout Groth16 verifying key");

    // Prepare verifying keys
    let spend_vk = prepare_verifying_key(&spend_params.vk);
    let output_vk = prepare_verifying_key(&output_params.vk);
    let sprout_vk = prepare_verifying_key(&sprout_vk);

    // Caller is responsible for calling this function once, so
    // these global mutations are safe.
    unsafe {
        SAPLING_SPEND_PARAMS = Some(spend_params);
        SAPLING_OUTPUT_PARAMS = Some(output_params);
        SPROUT_GROTH16_PARAMS_PATH = Some(sprout_path);

        SAPLING_SPEND_VK = Some(spend_vk);
        SAPLING_OUTPUT_VK = Some(output_vk);
        SPROUT_GROTH16_VK = Some(sprout_vk);
    }
}

#[no_mangle]
pub extern "system" fn librustzcash_tree_uncommitted(result: *mut [c_uchar; 32]) {
    let tmp = sapling_crypto::primitives::Note::<Bls12>::uncommitted().into_repr();

    // Should be okay, caller is responsible for ensuring the pointer
    // is a valid pointer to 32 bytes that can be mutated.
    let result = unsafe { &mut *result };

    write_le(tmp, &mut result[..]);
}

#[no_mangle]
pub extern "system" fn librustzcash_merkle_hash(
    depth: size_t,
    a: *const [c_uchar; 32],
    b: *const [c_uchar; 32],
    result: *mut [c_uchar; 32],
) {
    // Should be okay, because caller is responsible for ensuring
    // the pointer is a valid pointer to 32 bytes, and that is the
    // size of the representation
    let a_repr = read_le(unsafe { &(&*a)[..] });

    // Should be okay, because caller is responsible for ensuring
    // the pointer is a valid pointer to 32 bytes, and that is the
    // size of the representation
    let b_repr = read_le(unsafe { &(&*b)[..] });

    let mut lhs = [false; 256];
    let mut rhs = [false; 256];

    for (a, b) in lhs.iter_mut().rev().zip(BitIterator::new(a_repr)) {
        *a = b;
    }

    for (a, b) in rhs.iter_mut().rev().zip(BitIterator::new(b_repr)) {
        *a = b;
    }

    let tmp = pedersen_hash::<Bls12, _>(
        Personalization::MerkleTree(depth),
        lhs.iter()
            .map(|&x| x)
            .take(Fr::NUM_BITS as usize)
            .chain(rhs.iter().map(|&x| x).take(Fr::NUM_BITS as usize)),
        &JUBJUB,
    ).into_xy()
        .0
        .into_repr();

    // Should be okay, caller is responsible for ensuring the pointer
    // is a valid pointer to 32 bytes that can be mutated.
    let result = unsafe { &mut *result };

    write_le(tmp, &mut result[..]);
}

#[no_mangle] // ToScalar
pub extern "system" fn librustzcash_to_scalar(
    input: *const [c_uchar; 64],
    result: *mut [c_uchar; 32],
) {
    // Should be okay, because caller is responsible for ensuring
    // the pointer is a valid pointer to 32 bytes, and that is the
    // size of the representation
    let scalar = <Bls12 as JubjubEngine>::Fs::to_uniform(unsafe { &(&*input)[..] }).into_repr();

    let result = unsafe { &mut *result };

    scalar
        .write_le(&mut result[..])
        .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");
}

#[no_mangle]
pub extern "system" fn librustzcash_crh_ivk(
    ak: *const [c_uchar; 32],
    nk: *const [c_uchar; 32],
    result: *mut [c_uchar; 32],
) {
    let ak = unsafe { &*ak };
    let nk = unsafe { &*nk };

    let mut h = Blake2s::with_params(32, &[], &[], CRH_IVK_PERSONALIZATION);
    h.update(ak);
    h.update(nk);
    let mut h = h.finalize().as_ref().to_vec();

    // Drop the last five bits, so it can be interpreted as a scalar.
    h[31] &= 0b0000_0111;

    let result = unsafe { &mut *result };

    result.copy_from_slice(&h);
}

#[no_mangle]
pub extern "system" fn librustzcash_check_diversifier(diversifier: *const [c_uchar; 11]) -> bool {
    let diversifier = sapling_crypto::primitives::Diversifier(unsafe { *diversifier });
    diversifier.g_d::<Bls12>(&JUBJUB).is_some()
}

#[no_mangle]
pub extern "system" fn librustzcash_ivk_to_pkd(
    ivk: *const [c_uchar; 32],
    diversifier: *const [c_uchar; 11],
    result: *mut [c_uchar; 32],
) -> bool {
    let ivk = read_fs(unsafe { &*ivk });
    let diversifier = sapling_crypto::primitives::Diversifier(unsafe { *diversifier });
    if let Some(g_d) = diversifier.g_d::<Bls12>(&JUBJUB) {
        let pk_d = g_d.mul(ivk, &JUBJUB);

        let result = unsafe { &mut *result };

        pk_d.write(&mut result[..]).expect("length is 32 bytes");

        true
    } else {
        false
    }
}

/// Test generation of commitment randomness
#[test]
fn test_gen_r() {
    let mut r1 = [0u8; 32];
    let mut r2 = [0u8; 32];

    // Verify different r values are generated
    librustzcash_sapling_generate_r(&mut r1);
    librustzcash_sapling_generate_r(&mut r2);
    assert_ne!(r1, r2);

    // Verify r values are valid in the field
    let mut repr = FsRepr::default();
    repr.read_le(&r1[..]).expect("length is not 32 bytes");
    let _ = Fs::from_repr(repr).unwrap();
    repr.read_le(&r2[..]).expect("length is not 32 bytes");
    let _ = Fs::from_repr(repr).unwrap();
}

/// Return 32 byte random scalar, uniformly.
#[no_mangle]
pub extern "system" fn librustzcash_sapling_generate_r(result: *mut [c_uchar; 32]) {
    // 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();
    }

    // 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");
}

// Private utility function to get Note from C parameters
fn priv_get_note(
    diversifier: *const [c_uchar; 11],
    pk_d: *const [c_uchar; 32],
    value: uint64_t,
    r: *const [c_uchar; 32],
) -> Result<sapling_crypto::primitives::Note<Bls12>, ()> {
    let diversifier = sapling_crypto::primitives::Diversifier(unsafe { *diversifier });
    let g_d = match diversifier.g_d::<Bls12>(&JUBJUB) {
        Some(g_d) => g_d,
        None => return Err(()),
    };

    let pk_d = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*pk_d })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return Err(()),
    };

    let pk_d = match pk_d.as_prime_order(&JUBJUB) {
        Some(pk_d) => pk_d,
        None => return Err(()),
    };

    // Deserialize randomness
    let r = match Fs::from_repr(read_fs(&(unsafe { &*r })[..])) {
        Ok(r) => r,
        Err(_) => return Err(()),
    };

    let note = sapling_crypto::primitives::Note {
        value,
        g_d,
        pk_d,
        r,
    };

    Ok(note)
}

/// Compute Sapling note nullifier.
#[no_mangle]
pub extern "system" fn librustzcash_sapling_compute_nf(
    diversifier: *const [c_uchar; 11],
    pk_d: *const [c_uchar; 32],
    value: uint64_t,
    r: *const [c_uchar; 32],
    ak: *const [c_uchar; 32],
    nk: *const [c_uchar; 32],
    position: uint64_t,
    result: *mut [c_uchar; 32],
) -> bool {
    let note = match priv_get_note(diversifier, pk_d, value, r) {
        Ok(p) => p,
        Err(_) => return false,
    };

    let ak = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*ak })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    let ak = match ak.as_prime_order(&JUBJUB) {
        Some(ak) => ak,
        None => return false,
    };

    let nk = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*nk })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    let nk = match nk.as_prime_order(&JUBJUB) {
        Some(nk) => nk,
        None => return false,
    };

    let vk = ViewingKey { ak, nk };
    let nf = note.nf(&vk, position, &JUBJUB);
    let result = unsafe { &mut *result };
    result.copy_from_slice(&nf);

    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 note = match priv_get_note(diversifier, pk_d, value, r) {
        Ok(p) => p,
        Err(_) => return false,
    };

    let result = unsafe { &mut *result };
    write_le(note.cm(&JUBJUB).into_repr(), &mut result[..]);

    true
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_ka_agree(
    p: *const [c_uchar; 32],
    sk: *const [c_uchar; 32],
    result: *mut [c_uchar; 32],
) -> bool {
    // Deserialize p
    let p = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*p })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // Deserialize sk
    let sk = match Fs::from_repr(read_fs(&(unsafe { &*sk })[..])) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // Multiply by 8
    let p = p.mul_by_cofactor(&JUBJUB);

    // Multiply by sk
    let p = p.mul(sk, &JUBJUB);

    // Produce result
    let result = unsafe { &mut *result };
    p.write(&mut result[..]).expect("length is not 32 bytes");

    true
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_ka_derivepublic(
    diversifier: *const [c_uchar; 11],
    esk: *const [c_uchar; 32],
    result: *mut [c_uchar; 32],
) -> bool {
    let diversifier = sapling_crypto::primitives::Diversifier(unsafe { *diversifier });

    // Compute g_d from the diversifier
    let g_d = match diversifier.g_d::<Bls12>(&JUBJUB) {
        Some(g) => g,
        None => return false,
    };

    // Deserialize esk
    let esk = match Fs::from_repr(read_fs(&(unsafe { &*esk })[..])) {
        Ok(p) => p,
        Err(_) => return false,
    };

    let p = g_d.mul(esk, &JUBJUB);

    let result = unsafe { &mut *result };
    p.write(&mut result[..]).expect("length is not 32 bytes");

    true
}

#[no_mangle]
pub extern "system" fn librustzcash_eh_isvalid(
    n: uint32_t,
    k: uint32_t,
    input: *const c_uchar,
    input_len: size_t,
    nonce: *const c_uchar,
    nonce_len: size_t,
    soln: *const c_uchar,
    soln_len: size_t,
) -> bool {
    if (k >= n) || (n % 8 != 0) || (soln_len != (1 << k) * ((n / (k + 1)) as usize + 1) / 8) {
        return false;
    }
    let rs_input = unsafe { slice::from_raw_parts(input, input_len) };
    let rs_nonce = unsafe { slice::from_raw_parts(nonce, nonce_len) };
    let rs_soln = unsafe { slice::from_raw_parts(soln, soln_len) };
    equihash::is_valid_solution(n, k, rs_input, rs_nonce, rs_soln)
}

pub struct SaplingVerificationContext {
    bvk: edwards::Point<Bls12, Unknown>,
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_verification_ctx_init(
) -> *mut SaplingVerificationContext {
    let ctx = Box::new(SaplingVerificationContext {
        bvk: edwards::Point::zero(),
    });

    Box::into_raw(ctx)
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_verification_ctx_free(
    ctx: *mut SaplingVerificationContext,
) {
    drop(unsafe { Box::from_raw(ctx) });
}

const GROTH_PROOF_SIZE: usize = 48 // π_A
    + 96 // π_B
    + 48; // π_C

#[no_mangle]
pub extern "system" fn librustzcash_sapling_check_spend(
    ctx: *mut SaplingVerificationContext,
    cv: *const [c_uchar; 32],
    anchor: *const [c_uchar; 32],
    nullifier: *const [c_uchar; 32],
    rk: *const [c_uchar; 32],
    zkproof: *const [c_uchar; GROTH_PROOF_SIZE],
    spend_auth_sig: *const [c_uchar; 64],
    sighash_value: *const [c_uchar; 32],
) -> bool {
    // Deserialize the value commitment
    let cv = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*cv })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    if is_small_order(&cv) {
        return false;
    }

    // Accumulate the value commitment in the context
    {
        let mut tmp = cv.clone();
        tmp = tmp.add(&unsafe { &*ctx }.bvk, &JUBJUB);

        // Update the context
        unsafe { &mut *ctx }.bvk = tmp;
    }

    // Deserialize the anchor, which should be an element
    // of Fr.
    let anchor = match Fr::from_repr(read_le(&(unsafe { &*anchor })[..])) {
        Ok(a) => a,
        Err(_) => return false,
    };

    // Grab the nullifier as a sequence of bytes
    let nullifier = &unsafe { &*nullifier }[..];

    // Compute the signature's message for rk/spend_auth_sig
    let mut data_to_be_signed = [0u8; 64];
    (&mut data_to_be_signed[0..32]).copy_from_slice(&(unsafe { &*rk })[..]);
    (&mut data_to_be_signed[32..64]).copy_from_slice(&(unsafe { &*sighash_value })[..]);

    // Deserialize rk
    let rk = match redjubjub::PublicKey::<Bls12>::read(&(unsafe { &*rk })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    if is_small_order(&rk.0) {
        return false;
    }

    // Deserialize the signature
    let spend_auth_sig = match Signature::read(&(unsafe { &*spend_auth_sig })[..]) {
        Ok(sig) => sig,
        Err(_) => return false,
    };

    // Verify the spend_auth_sig
    if !rk.verify(
        &data_to_be_signed,
        &spend_auth_sig,
        FixedGenerators::SpendingKeyGenerator,
        &JUBJUB,
    ) {
        return false;
    }

    // Construct public input for circuit
    let mut public_input = [Fr::zero(); 7];
    {
        let (x, y) = rk.0.into_xy();
        public_input[0] = x;
        public_input[1] = y;
    }
    {
        let (x, y) = cv.into_xy();
        public_input[2] = x;
        public_input[3] = y;
    }
    public_input[4] = anchor;

    // Add the nullifier through multiscalar packing
    {
        let nullifier = multipack::bytes_to_bits_le(nullifier);
        let nullifier = multipack::compute_multipacking::<Bls12>(&nullifier);

        assert_eq!(nullifier.len(), 2);

        public_input[5] = nullifier[0];
        public_input[6] = nullifier[1];
    }

    // Deserialize the proof
    let zkproof = match Proof::<Bls12>::read(&(unsafe { &*zkproof })[..]) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // Verify the proof
    match verify_proof(
        unsafe { SAPLING_SPEND_VK.as_ref() }.unwrap(),
        &zkproof,
        &public_input[..],
    ) {
        // No error, and proof verification successful
        Ok(true) => true,

        // Any other case
        _ => false,
    }
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_check_output(
    ctx: *mut SaplingVerificationContext,
    cv: *const [c_uchar; 32],
    cm: *const [c_uchar; 32],
    epk: *const [c_uchar; 32],
    zkproof: *const [c_uchar; GROTH_PROOF_SIZE],
) -> bool {
    // Deserialize the value commitment
    let cv = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*cv })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    if is_small_order(&cv) {
        return false;
    }

    // Accumulate the value commitment in the context
    {
        let mut tmp = cv.clone();
        tmp = tmp.negate(); // Outputs subtract from the total.
        tmp = tmp.add(&unsafe { &*ctx }.bvk, &JUBJUB);

        // Update the context
        unsafe { &mut *ctx }.bvk = tmp;
    }

    // Deserialize the commitment, which should be an element
    // of Fr.
    let cm = match Fr::from_repr(read_le(&(unsafe { &*cm })[..])) {
        Ok(a) => a,
        Err(_) => return false,
    };

    // Deserialize the ephemeral key
    let epk = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*epk })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    if is_small_order(&epk) {
        return false;
    }

    // Construct public input for circuit
    let mut public_input = [Fr::zero(); 5];
    {
        let (x, y) = cv.into_xy();
        public_input[0] = x;
        public_input[1] = y;
    }
    {
        let (x, y) = epk.into_xy();
        public_input[2] = x;
        public_input[3] = y;
    }
    public_input[4] = cm;

    // Deserialize the proof
    let zkproof = match Proof::<Bls12>::read(&(unsafe { &*zkproof })[..]) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // Verify the proof
    match verify_proof(
        unsafe { SAPLING_OUTPUT_VK.as_ref() }.unwrap(),
        &zkproof,
        &public_input[..],
    ) {
        // No error, and proof verification successful
        Ok(true) => true,

        // Any other case
        _ => false,
    }
}

// This function computes `value` in the exponent of the value commitment base
fn compute_value_balance(value: int64_t) -> Option<edwards::Point<Bls12, Unknown>> {
    // Compute the absolute value (failing if -i64::MAX is
    // the value)
    let abs = match value.checked_abs() {
        Some(a) => a as u64,
        None => return None,
    };

    // Is it negative? We'll have to negate later if so.
    let is_negative = value.is_negative();

    // Compute it in the exponent
    let mut value_balance = JUBJUB
        .generator(FixedGenerators::ValueCommitmentValue)
        .mul(FsRepr::from(abs), &JUBJUB);

    // Negate if necessary
    if is_negative {
        value_balance = value_balance.negate();
    }

    // Convert to unknown order point
    Some(value_balance.into())
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_final_check(
    ctx: *mut SaplingVerificationContext,
    value_balance: int64_t,
    binding_sig: *const [c_uchar; 64],
    sighash_value: *const [c_uchar; 32],
) -> bool {
    // Obtain current bvk from the context
    let mut bvk = redjubjub::PublicKey(unsafe { &*ctx }.bvk.clone());

    // Compute value balance
    let mut value_balance = match compute_value_balance(value_balance) {
        Some(a) => a,
        None => return false,
    };

    // Subtract value_balance from current bvk to get final bvk
    value_balance = value_balance.negate();
    bvk.0 = bvk.0.add(&value_balance, &JUBJUB);

    // Compute the signature's message for bvk/binding_sig
    let mut data_to_be_signed = [0u8; 64];
    bvk.0
        .write(&mut data_to_be_signed[0..32])
        .expect("bvk is 32 bytes");
    (&mut data_to_be_signed[32..64]).copy_from_slice(&(unsafe { &*sighash_value })[..]);

    // Deserialize the signature
    let binding_sig = match Signature::read(&(unsafe { &*binding_sig })[..]) {
        Ok(sig) => sig,
        Err(_) => return false,
    };

    // Verify the binding_sig
    if !bvk.verify(
        &data_to_be_signed,
        &binding_sig,
        FixedGenerators::ValueCommitmentRandomness,
        &JUBJUB,
    ) {
        return false;
    }

    true
}

#[no_mangle]
pub extern "system" fn librustzcash_sprout_prove(
    proof_out: *mut [c_uchar; GROTH_PROOF_SIZE],

    phi: *const [c_uchar; 32],
    rt: *const [c_uchar; 32],
    h_sig: *const [c_uchar; 32],

    // First input
    in_sk1: *const [c_uchar; 32],
    in_value1: uint64_t,
    in_rho1: *const [c_uchar; 32],
    in_r1: *const [c_uchar; 32],
    in_auth1: *const [c_uchar; 1 + 33 * SPROUT_TREE_DEPTH + 8],

    // Second input
    in_sk2: *const [c_uchar; 32],
    in_value2: uint64_t,
    in_rho2: *const [c_uchar; 32],
    in_r2: *const [c_uchar; 32],
    in_auth2: *const [c_uchar; 1 + 33 * SPROUT_TREE_DEPTH + 8],

    // First output
    out_pk1: *const [c_uchar; 32],
    out_value1: uint64_t,
    out_r1: *const [c_uchar; 32],

    // Second output
    out_pk2: *const [c_uchar; 32],
    out_value2: uint64_t,
    out_r2: *const [c_uchar; 32],

    // Public value
    vpub_old: uint64_t,
    vpub_new: uint64_t,
) {
    let phi = unsafe { *phi };
    let rt = unsafe { *rt };
    let h_sig = unsafe { *h_sig };
    let in_sk1 = unsafe { *in_sk1 };
    let in_rho1 = unsafe { *in_rho1 };
    let in_r1 = unsafe { *in_r1 };
    let in_auth1 = unsafe { *in_auth1 };
    let in_sk2 = unsafe { *in_sk2 };
    let in_rho2 = unsafe { *in_rho2 };
    let in_r2 = unsafe { *in_r2 };
    let in_auth2 = unsafe { *in_auth2 };
    let out_pk1 = unsafe { *out_pk1 };
    let out_r1 = unsafe { *out_r1 };
    let out_pk2 = unsafe { *out_pk2 };
    let out_r2 = unsafe { *out_r2 };

    let mut inputs = Vec::with_capacity(2);
    {
        let mut handle_input = |sk, value, rho, r, mut auth: &[u8]| {
            let value = Some(value);
            let rho = Some(sprout::UniqueRandomness(rho));
            let r = Some(sprout::CommitmentRandomness(r));
            let a_sk = Some(sprout::SpendingKey(sk));

            // skip the first byte
            assert_eq!(auth[0], SPROUT_TREE_DEPTH as u8);
            auth = &auth[1..];

            let mut auth_path = [None; SPROUT_TREE_DEPTH];
            for i in (0..SPROUT_TREE_DEPTH).rev() {
                // skip length of inner vector
                assert_eq!(auth[0], 32);
                auth = &auth[1..];

                let mut sibling = [0u8; 32];
                sibling.copy_from_slice(&auth[0..32]);
                auth = &auth[32..];

                auth_path[i] = Some((sibling, false));
            }

            let mut position = auth
                .read_u64::<LittleEndian>()
                .expect("should have had index at the end");

            for i in 0..SPROUT_TREE_DEPTH {
                auth_path[i].as_mut().map(|p| p.1 = (position & 1) == 1);

                position >>= 1;
            }

            inputs.push(sprout::JSInput {
                value: value,
                a_sk: a_sk,
                rho: rho,
                r: r,
                auth_path: auth_path,
            });
        };

        handle_input(in_sk1, in_value1, in_rho1, in_r1, &in_auth1[..]);
        handle_input(in_sk2, in_value2, in_rho2, in_r2, &in_auth2[..]);
    }

    let mut outputs = Vec::with_capacity(2);
    {
        let mut handle_output = |a_pk, value, r| {
            outputs.push(sprout::JSOutput {
                value: Some(value),
                a_pk: Some(sprout::PayingKey(a_pk)),
                r: Some(sprout::CommitmentRandomness(r)),
            });
        };

        handle_output(out_pk1, out_value1, out_r1);
        handle_output(out_pk2, out_value2, out_r2);
    }

    let js = sprout::JoinSplit {
        vpub_old: Some(vpub_old),
        vpub_new: Some(vpub_new),
        h_sig: Some(h_sig),
        phi: Some(phi),
        inputs: inputs,
        outputs: outputs,
        rt: Some(rt),
    };

    // Load parameters from disk
    let sprout_fs = File::open(
        unsafe { &SPROUT_GROTH16_PARAMS_PATH }
            .as_ref()
            .expect("parameters should have been initialized"),
    ).expect("couldn't load Sprout groth16 parameters file");

    let mut sprout_fs = BufReader::with_capacity(1024 * 1024, sprout_fs);

    let params = Parameters::<Bls12>::read(&mut sprout_fs, false)
        .expect("couldn't deserialize Sprout JoinSplit parameters file");

    drop(sprout_fs);

    // Initialize secure RNG
    let mut rng = OsRng::new().expect("should be able to construct RNG");

    let proof = create_random_proof(js, &params, &mut rng).expect("proving should not fail");

    proof
        .write(&mut (unsafe { &mut *proof_out })[..])
        .expect("should be able to serialize a proof");
}

#[no_mangle]
pub extern "system" fn librustzcash_sprout_verify(
    proof: *const [c_uchar; GROTH_PROOF_SIZE],
    rt: *const [c_uchar; 32],
    h_sig: *const [c_uchar; 32],
    mac1: *const [c_uchar; 32],
    mac2: *const [c_uchar; 32],
    nf1: *const [c_uchar; 32],
    nf2: *const [c_uchar; 32],
    cm1: *const [c_uchar; 32],
    cm2: *const [c_uchar; 32],
    vpub_old: uint64_t,
    vpub_new: uint64_t,
) -> bool {
    // Prepare the public input for the verifier
    let mut public_input = Vec::with_capacity((32 * 8) + (8 * 2));
    public_input.extend(unsafe { &(&*rt)[..] });
    public_input.extend(unsafe { &(&*h_sig)[..] });
    public_input.extend(unsafe { &(&*nf1)[..] });
    public_input.extend(unsafe { &(&*mac1)[..] });
    public_input.extend(unsafe { &(&*nf2)[..] });
    public_input.extend(unsafe { &(&*mac2)[..] });
    public_input.extend(unsafe { &(&*cm1)[..] });
    public_input.extend(unsafe { &(&*cm2)[..] });
    public_input.write_u64::<LittleEndian>(vpub_old).unwrap();
    public_input.write_u64::<LittleEndian>(vpub_new).unwrap();

    let public_input = multipack::bytes_to_bits(&public_input);
    let public_input = multipack::compute_multipacking::<Bls12>(&public_input);

    let proof = match Proof::read(unsafe { &(&*proof)[..] }) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // Verify the proof
    match verify_proof(
        unsafe { SPROUT_GROTH16_VK.as_ref() }.expect("parameters should have been initialized"),
        &proof,
        &public_input[..],
    ) {
        // No error, and proof verification successful
        Ok(true) => true,

        // Any other case
        _ => false,
    }
}

pub struct SaplingProvingContext {
    bsk: Fs,
    bvk: edwards::Point<Bls12, Unknown>,
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_output_proof(
    ctx: *mut SaplingProvingContext,
    esk: *const [c_uchar; 32],
    diversifier: *const [c_uchar; 11],
    pk_d: *const [c_uchar; 32],
    rcm: *const [c_uchar; 32],
    value: uint64_t,
    cv: *mut [c_uchar; 32],
    zkproof: *mut [c_uchar; GROTH_PROOF_SIZE],
) -> bool {
    // Grab `esk`, which the caller should have constructed for the DH key exchange.
    let esk = match Fs::from_repr(read_fs(&(unsafe { &*esk })[..])) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // Grab the diversifier from the caller.
    let diversifier = sapling_crypto::primitives::Diversifier(unsafe { *diversifier });

    // Grab pk_d from the caller.
    let pk_d = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*pk_d })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // pk_d should be prime order.
    let pk_d = match pk_d.as_prime_order(&JUBJUB) {
        Some(p) => p,
        None => return false,
    };

    // Construct a payment address
    let payment_address = sapling_crypto::primitives::PaymentAddress {
        pk_d: pk_d,
        diversifier: diversifier,
    };

    // Initialize secure RNG
    let mut rng = OsRng::new().expect("should be able to construct RNG");

    // The caller provides the commitment randomness for the output note
    let rcm = match Fs::from_repr(read_fs(&(unsafe { &*rcm })[..])) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // We construct ephemeral randomness for the value commitment. This
    // randomness is not given back to the caller, but the synthetic
    // blinding factor `bsk` is accumulated in the context.
    let rcv = Fs::rand(&mut rng);

    // Accumulate the value commitment randomness in the context
    {
        let mut tmp = rcv.clone();
        tmp.negate(); // Outputs subtract from the total.
        tmp.add_assign(&unsafe { &*ctx }.bsk);

        // Update the context
        unsafe { &mut *ctx }.bsk = tmp;
    }

    // Construct the value commitment for the proof instance
    let value_commitment = sapling_crypto::primitives::ValueCommitment::<Bls12> {
        value: value,
        randomness: rcv,
    };

    // We now have a full witness for the output proof.
    let instance = sapling_crypto::circuit::sapling::Output {
        params: &*JUBJUB,
        value_commitment: Some(value_commitment.clone()),
        payment_address: Some(payment_address.clone()),
        commitment_randomness: Some(rcm),
        esk: Some(esk.clone()),
    };

    // Create proof
    let proof = create_random_proof(
        instance,
        unsafe { SAPLING_OUTPUT_PARAMS.as_ref() }.unwrap(),
        &mut rng,
    ).expect("proving should not fail");

    // Write the proof out to the caller
    proof
        .write(&mut (unsafe { &mut *zkproof })[..])
        .expect("should be able to serialize a proof");

    // Compute the value commitment
    let value_commitment: edwards::Point<Bls12, Unknown> = value_commitment.cm(&JUBJUB).into();

    // Accumulate the value commitment in the context. We do this to check internal consistency.
    {
        let mut tmp = value_commitment.clone();
        tmp = tmp.negate(); // Outputs subtract from the total.
        tmp = tmp.add(&unsafe { &*ctx }.bvk, &JUBJUB);

        // Update the context
        unsafe { &mut *ctx }.bvk = tmp;
    }

    // Write the value commitment to the caller
    value_commitment
        .write(&mut (unsafe { &mut *cv })[..])
        .expect("should be able to serialize rcv");

    true
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_spend_sig(
    ask: *const [c_uchar; 32],
    ar: *const [c_uchar; 32],
    sighash: *const [c_uchar; 32],
    result: *mut [c_uchar; 64],
) -> bool {
    // The caller provides the re-randomization of `ak`.
    let ar = match Fs::from_repr(read_fs(&(unsafe { &*ar })[..])) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // The caller provides `ask`, the spend authorizing key.
    let ask = match redjubjub::PrivateKey::<Bls12>::read(&(unsafe { &*ask })[..]) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // We compute `rsk`...
    let rsk = ask.randomize(ar);

    // We compute `rk` from there (needed for key prefixing)
    let rk =
        redjubjub::PublicKey::from_private(&rsk, FixedGenerators::SpendingKeyGenerator, &JUBJUB);

    // Compute the signature's message for rk/spend_auth_sig
    let mut data_to_be_signed = [0u8; 64];
    rk.0
        .write(&mut data_to_be_signed[0..32])
        .expect("message buffer should be 32 bytes");
    (&mut data_to_be_signed[32..64]).copy_from_slice(&(unsafe { &*sighash })[..]);

    // Do the signing
    let mut rng = OsRng::new().expect("should be able to construct RNG");
    let sig = rsk.sign(
        &data_to_be_signed,
        &mut rng,
        FixedGenerators::SpendingKeyGenerator,
        &JUBJUB,
    );

    // Write out the signature
    sig.write(&mut (unsafe { &mut *result })[..])
        .expect("result should be 64 bytes");

    true
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_binding_sig(
    ctx: *mut SaplingProvingContext,
    value_balance: int64_t,
    sighash: *const [c_uchar; 32],
    result: *mut [c_uchar; 64],
) -> bool {
    // Grab the current `bsk` from the context
    let bsk = redjubjub::PrivateKey::<Bls12>(unsafe { &*ctx }.bsk);

    // Grab the `bvk` using DerivePublic.
    let bvk = redjubjub::PublicKey::from_private(
        &bsk,
        FixedGenerators::ValueCommitmentRandomness,
        &JUBJUB,
    );

    // In order to check internal consistency, let's use the accumulated value
    // commitments (as the verifier would) and apply valuebalance to compare
    // against our derived bvk.
    {
        // Compute value balance
        let mut value_balance = match compute_value_balance(value_balance) {
            Some(a) => a,
            None => return false,
        };

        // Subtract value_balance from current bvk to get final bvk
        value_balance = value_balance.negate();
        let mut tmp = unsafe { &*ctx }.bvk.clone();
        tmp = tmp.add(&value_balance, &JUBJUB);

        // The result should be the same, unless the provided valueBalance is wrong.
        if bvk.0 != tmp {
            return false;
        }
    }

    // Construct signature message
    let mut data_to_be_signed = [0u8; 64];
    bvk.0
        .write(&mut data_to_be_signed[0..32])
        .expect("message buffer should be 32 bytes");
    (&mut data_to_be_signed[32..64]).copy_from_slice(&(unsafe { &*sighash })[..]);

    // Sign
    let mut rng = OsRng::new().expect("should be able to construct RNG");
    let sig = bsk.sign(
        &data_to_be_signed,
        &mut rng,
        FixedGenerators::ValueCommitmentRandomness,
        &JUBJUB,
    );

    // Write out signature
    sig.write(&mut (unsafe { &mut *result })[..])
        .expect("result should be 64 bytes");

    true
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_spend_proof(
    ctx: *mut SaplingProvingContext,
    ak: *const [c_uchar; 32],
    nsk: *const [c_uchar; 32],
    diversifier: *const [c_uchar; 11],
    rcm: *const [c_uchar; 32],
    ar: *const [c_uchar; 32],
    value: uint64_t,
    anchor: *const [c_uchar; 32],
    witness: *const [c_uchar; 1 + 33 * SAPLING_TREE_DEPTH + 8],
    cv: *mut [c_uchar; 32],
    rk_out: *mut [c_uchar; 32],
    zkproof: *mut [c_uchar; GROTH_PROOF_SIZE],
) -> bool {
    let mut rng = OsRng::new().expect("should be able to construct RNG");

    // We create the randomness of the value commitment
    let rcv = Fs::rand(&mut rng);

    // Accumulate the value commitment randomness in the context
    {
        let mut tmp = rcv.clone();
        tmp.add_assign(&unsafe { &*ctx }.bsk);

        // Update the context
        unsafe { &mut *ctx }.bsk = tmp;
    }

    // Construct the value commitment
    let value_commitment = ValueCommitment::<Bls12> {
        value: value,
        randomness: rcv,
    };

    // Grab `ak` from the caller, which should be a point.
    let ak = match edwards::Point::<Bls12, Unknown>::read(&(unsafe { &*ak })[..], &JUBJUB) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // `ak` should be prime order.
    let ak = match ak.as_prime_order(&JUBJUB) {
        Some(p) => p,
        None => return false,
    };

    // Grab `nsk` from the caller
    let nsk = match Fs::from_repr(read_fs(&(unsafe { &*nsk })[..])) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // Construct the proof generation key
    let proof_generation_key = ProofGenerationKey {
        ak: ak.clone(),
        nsk,
    };

    // Construct the viewing key
    let viewing_key = proof_generation_key.into_viewing_key(&JUBJUB);

    // Grab the diversifier from the caller
    let diversifier = sapling_crypto::primitives::Diversifier(unsafe { *diversifier });

    // Construct the payment address with the viewing key / diversifier
    let payment_address = match viewing_key.into_payment_address(diversifier, &JUBJUB) {
        Some(p) => p,
        None => return false,
    };

    // The caller chooses the note randomness
    let rcm = match Fs::from_repr(read_fs(&(unsafe { &*rcm })[..])) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // The caller also chooses the re-randomization of ak
    let ar = match Fs::from_repr(read_fs(&(unsafe { &*ar })[..])) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // This is the result of the re-randomization, we compute it for the caller
    let rk = redjubjub::PublicKey::<Bls12>(ak.into()).randomize(
        ar,
        FixedGenerators::SpendingKeyGenerator,
        &JUBJUB,
    );

    // Write out `rk` to the caller
    rk.write(&mut unsafe { &mut *rk_out }[..])
        .expect("should be able to write to rk_out");

    // We need to compute the anchor of the Spend.
    let anchor = match Fr::from_repr(read_le(unsafe { &(&*anchor)[..] })) {
        Ok(p) => p,
        Err(_) => return false,
    };

    // The witness contains the incremental tree witness information, in a
    // weird serialized format.
    let mut witness = unsafe { &(&*witness)[..] };

    // Skip the first byte, which should be "32" to signify the length of
    // the following vector of Pedersen hashes.
    assert_eq!(witness[0], SAPLING_TREE_DEPTH as u8);
    witness = &witness[1..];

    // Begin to construct the authentication path
    let mut auth_path = vec![None; SAPLING_TREE_DEPTH];

    // The vector works in reverse
    for i in (0..SAPLING_TREE_DEPTH).rev() {
        // skip length of inner vector
        assert_eq!(witness[0], 32); // the length of a pedersen hash
        witness = &witness[1..];

        // Grab the sibling node at this depth in the tree
        let mut sibling = [0u8; 32];
        sibling.copy_from_slice(&witness[0..32]);
        witness = &witness[32..];

        // Sibling node should be an element of Fr
        let sibling = match Fr::from_repr(read_le(&sibling)) {
            Ok(p) => p,
            Err(_) => return false,
        };

        // Set the value in the auth path; we put false here
        // for now (signifying the position bit) which we'll
        // fill in later.
        auth_path[i] = Some((sibling, false));
    }

    // Read the position from the witness
    let mut position = witness
        .read_u64::<LittleEndian>()
        .expect("should have had index at the end");

    // Let's compute the nullifier while we have the position
    let note = sapling_crypto::primitives::Note {
        value: value,
        g_d: diversifier
            .g_d::<Bls12>(&JUBJUB)
            .expect("was a valid diversifier before"),
        pk_d: payment_address.pk_d.clone(),
        r: rcm,
    };

    let nullifier = note.nf(&viewing_key, position, &JUBJUB);

    // Given the position, let's finish constructing the authentication
    // path
    for i in 0..SAPLING_TREE_DEPTH {
        auth_path[i].as_mut().map(|p| p.1 = (position & 1) == 1);

        position >>= 1;
    }

    // The witness should be empty now; if it wasn't, the caller would
    // have provided more information than they should have, indicating
    // a bug downstream
    assert_eq!(witness.len(), 0);

    // We now have the full witness for our circuit
    let instance = sapling_crypto::circuit::sapling::Spend {
        params: &*JUBJUB,
        value_commitment: Some(value_commitment.clone()),
        proof_generation_key: Some(proof_generation_key),
        payment_address: Some(payment_address),
        commitment_randomness: Some(rcm),
        ar: Some(ar),
        auth_path: auth_path,
        anchor: Some(anchor),
    };

    // Create proof
    let proof = create_random_proof(
        instance,
        unsafe { SAPLING_SPEND_PARAMS.as_ref() }.unwrap(),
        &mut rng,
    ).expect("proving should not fail");

    // Try to verify the proof:
    // Construct public input for circuit
    let mut public_input = [Fr::zero(); 7];
    {
        let (x, y) = rk.0.into_xy();
        public_input[0] = x;
        public_input[1] = y;
    }
    {
        let (x, y) = value_commitment.cm(&JUBJUB).into_xy();
        public_input[2] = x;
        public_input[3] = y;
    }
    public_input[4] = anchor;

    // Add the nullifier through multiscalar packing
    {
        let nullifier = multipack::bytes_to_bits_le(&nullifier);
        let nullifier = multipack::compute_multipacking::<Bls12>(&nullifier);

        assert_eq!(nullifier.len(), 2);

        public_input[5] = nullifier[0];
        public_input[6] = nullifier[1];
    }

    // Verify the proof
    match verify_proof(
        unsafe { SAPLING_SPEND_VK.as_ref() }.unwrap(),
        &proof,
        &public_input[..],
    ) {
        // No error, and proof verification successful
        Ok(true) => {}

        // Any other case
        _ => {
            return false;
        }
    }

    // Compute value commitment
    let value_commitment: edwards::Point<Bls12, Unknown> = value_commitment.cm(&JUBJUB).into();

    // Accumulate the value commitment in the context
    {
        let mut tmp = value_commitment.clone();
        tmp = tmp.add(&unsafe { &*ctx }.bvk, &JUBJUB);

        // Update the context
        unsafe { &mut *ctx }.bvk = tmp;
    }

    // Write value commitment to caller
    value_commitment
        .write(&mut unsafe { &mut *cv }[..])
        .expect("should be able to serialize cv");

    // Write proof out to caller
    proof
        .write(&mut (unsafe { &mut *zkproof })[..])
        .expect("should be able to serialize a proof");

    true
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_proving_ctx_init() -> *mut SaplingProvingContext {
    let ctx = Box::new(SaplingProvingContext {
        bsk: Fs::zero(),
        bvk: edwards::Point::zero(),
    });

    Box::into_raw(ctx)
}

#[no_mangle]
pub extern "system" fn librustzcash_sapling_proving_ctx_free(ctx: *mut SaplingProvingContext) {
    drop(unsafe { Box::from_raw(ctx) });
}