Move Sapling proving and binding signature into zcash_proofs crate

Cargo.lock

@@ -719,7 +719,9 @@ name = "zcash_proofs"
 version = "0.0.0"
 dependencies = [
  "bellman 0.1.0",
+ "byteorder 1.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "pairing 0.14.2",
+ "rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "sapling-crypto 0.0.1",
 ]
 

librustzcash/src/rustzcash.rs

@@ -15,7 +15,7 @@ extern crate lazy_static;
 
 use pairing::{
     bls12_381::{Bls12, Fr, FrRepr},
-    BitIterator, Field, PrimeField, PrimeFieldRepr,
+    BitIterator, PrimeField, PrimeFieldRepr,
 };
 
 use sapling_crypto::{
@@ -30,9 +30,8 @@ use sapling_crypto::{
     redjubjub::{self, Signature},
 };
 
+use sapling_crypto::circuit::sapling::TREE_DEPTH as SAPLING_TREE_DEPTH;
 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,
@@ -43,7 +42,7 @@ use blake2_rfc::blake2s::Blake2s;
 
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
 
-use rand::{OsRng, Rand, Rng};
+use rand::{OsRng, Rng};
 use std::io::{self, BufReader};
 
 use libc::{c_char, c_uchar, int64_t, size_t, uint32_t, uint64_t};
@@ -62,8 +61,10 @@ use std::ffi::OsString;
 #[cfg(target_os = "windows")]
 use std::os::windows::ffi::OsStringExt;
 
-use sapling_crypto::primitives::{ProofGenerationKey, ValueCommitment, ViewingKey};
+use sapling_crypto::primitives::{ProofGenerationKey, ViewingKey};
-use zcash_proofs::sapling::{compute_value_balance, SaplingVerificationContext};
+use zcash_proofs::sapling::{
+    CommitmentTreeWitness, SaplingProvingContext, SaplingVerificationContext,
+};
 
 pub mod equihash;
 
@@ -981,11 +982,6 @@ pub extern "system" fn librustzcash_sprout_verify(
     }
 }
 
-pub struct SaplingProvingContext {
-    bsk: Fs,
-    bvk: edwards::Point<Bls12, Unknown>,
-}
-
 #[no_mangle]
 pub extern "system" fn librustzcash_sapling_output_proof(
     ctx: *mut SaplingProvingContext,
@@ -1030,58 +1026,15 @@ pub extern "system" fn librustzcash_sapling_output_proof(
         Err(_) => return false,
     };
 
-    // Initialize secure RNG
-    let mut rng = OsRng::new().expect("should be able to construct RNG");
-
-    // 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(
+    let (proof, value_commitment) = unsafe { &mut *ctx }.output_proof(
-        instance,
+        esk,
+        payment_address,
+        rcm,
+        value,
         unsafe { SAPLING_OUTPUT_PARAMS.as_ref() }.unwrap(),
-        &mut rng,
+        &JUBJUB,
-    ).expect("proving should not fail");
+    );
-
-    // 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 proof out to the caller
     proof
@@ -1152,54 +1105,11 @@ pub extern "system" fn librustzcash_sapling_binding_sig(
     sighash: *const [c_uchar; 32],
     result: *mut [c_uchar; 64],
 ) -> bool {
-    // Initialize secure RNG
-    let mut rng = OsRng::new().expect("should be able to construct RNG");
-
-    // 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, &JUBJUB) {
-            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 sig = bsk.sign(
+    let sig = match unsafe { &*ctx }.binding_sig(value_balance, unsafe { &*sighash }, &JUBJUB) {
-        &data_to_be_signed,
+        Ok(s) => s,
-        &mut rng,
+        Err(_) => return false,
-        FixedGenerators::ValueCommitmentRandomness,
+    };
-        &JUBJUB,
-    );
 
     // Write out signature
     sig.write(&mut (unsafe { &mut *result })[..])
@@ -1262,43 +1172,6 @@ pub extern "system" fn librustzcash_sapling_spend_proof(
         Err(_) => return false,
     };
 
-    // Initialize secure RNG
-    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,
-    };
-
-    // Construct the viewing key
-    let viewing_key = proof_generation_key.into_viewing_key(&JUBJUB);
-
-    // 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,
-    };
-
-    // 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,
-    );
-
     // We need to compute the anchor of the Spend.
     let anchor = match Fr::from_repr(read_le(unsafe { &(&*anchor)[..] })) {
         Ok(p) => p,
@@ -1307,140 +1180,25 @@ pub extern "system" fn librustzcash_sapling_spend_proof(
 
     // The witness contains the incremental tree witness information, in a
     // weird serialized format.
-    let mut witness = unsafe { &(&*witness)[..] };
+    let witness = match CommitmentTreeWitness::from_slice(unsafe { &(&*witness)[..] }) {
-
+        Ok(w) => w,
-    // Skip the first byte, which should be "32" to signify the length of
+        Err(_) => return false,
-    // 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(
+    let (proof, value_commitment, rk) = unsafe { &mut *ctx }
-        instance,
+        .spend_proof(
-        unsafe { SAPLING_SPEND_PARAMS.as_ref() }.unwrap(),
+            proof_generation_key,
-        &mut rng,
+            diversifier,
-    ).expect("proving should not fail");
+            rcm,
-
+            ar,
-    // Try to verify the proof:
+            value,
-    // Construct public input for circuit
+            anchor,
-    let mut public_input = [Fr::zero(); 7];
+            witness,
-    {
+            unsafe { SAPLING_SPEND_PARAMS.as_ref() }.unwrap(),
-        let (x, y) = rk.0.into_xy();
+            unsafe { SAPLING_SPEND_VK.as_ref() }.unwrap(),
-        public_input[0] = x;
+            &JUBJUB,
-        public_input[1] = y;
+        ).expect("proving should not fail");
-    }
-    {
-        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
@@ -1461,10 +1219,7 @@ pub extern "system" fn librustzcash_sapling_spend_proof(
 
 #[no_mangle]
 pub extern "system" fn librustzcash_sapling_proving_ctx_init() -> *mut SaplingProvingContext {
-    let ctx = Box::new(SaplingProvingContext {
+    let ctx = Box::new(SaplingProvingContext::new());
-        bsk: Fs::zero(),
-        bvk: edwards::Point::zero(),
-    });
 
     Box::into_raw(ctx)
 }

sapling-crypto/src/circuit/sapling/mod.rs

@@ -31,6 +31,8 @@ use super::blake2s;
 use super::num;
 use super::multipack;
 
+pub const TREE_DEPTH: usize = 32;
+
 /// This is an instance of the `Spend` circuit.
 pub struct Spend<'a, E: JubjubEngine> {
     pub params: &'a E::Params,

zcash_proofs/Cargo.toml

@@ -7,5 +7,7 @@ authors = [
 
 [dependencies]
 bellman = { path = "../bellman" }
+byteorder = "1"
 pairing = { path = "../pairing" }
+rand = "0.4"
 sapling-crypto = { path = "../sapling-crypto" }

zcash_proofs/src/lib.rs

@@ -1,5 +1,7 @@
 extern crate bellman;
+extern crate byteorder;
 extern crate pairing;
+extern crate rand;
 extern crate sapling_crypto;
 
 pub mod sapling;

zcash_proofs/src/sapling/mod.rs

@@ -3,12 +3,14 @@ use sapling_crypto::jubjub::{
     edwards, fs::FsRepr, FixedGenerators, JubjubBls12, JubjubParams, Unknown,
 };
 
+mod prover;
 mod verifier;
 
+pub use self::prover::{CommitmentTreeWitness, SaplingProvingContext};
 pub use self::verifier::SaplingVerificationContext;
 
 // This function computes `value` in the exponent of the value commitment base
-pub fn compute_value_balance(
+fn compute_value_balance(
     value: i64,
     params: &JubjubBls12,
 ) -> Option<edwards::Point<Bls12, Unknown>> {

zcash_proofs/src/sapling/prover.rs

@@ -0,0 +1,365 @@
+use bellman::groth16::{
+    create_random_proof, verify_proof, Parameters, PreparedVerifyingKey, Proof,
+};
+use byteorder::{LittleEndian, ReadBytesExt};
+use pairing::{
+    bls12_381::{Bls12, Fr, FrRepr},
+    Field, PrimeField, PrimeFieldRepr,
+};
+use rand::{OsRng, Rand};
+use sapling_crypto::{
+    circuit::{
+        multipack,
+        sapling::{Output, Spend, TREE_DEPTH},
+    },
+    jubjub::{edwards, fs::Fs, FixedGenerators, JubjubBls12, Unknown},
+    primitives::{Diversifier, Note, PaymentAddress, ProofGenerationKey, ValueCommitment},
+    redjubjub::{PrivateKey, PublicKey, Signature},
+};
+
+use super::compute_value_balance;
+
+/// A witness to a path from a postion in a particular Sapling commitment tree
+/// to the root of that tree.
+pub struct CommitmentTreeWitness {
+    auth_path: Vec<Option<(Fr, bool)>>,
+    position: u64,
+}
+
+impl CommitmentTreeWitness {
+    pub fn from_slice(mut witness: &[u8]) -> Result<Self, ()> {
+        // Skip the first byte, which should be "32" to signify the length of
+        // the following vector of Pedersen hashes.
+        assert_eq!(witness[0], TREE_DEPTH as u8);
+        witness = &witness[1..];
+
+        // Begin to construct the authentication path
+        let mut auth_path = vec![None; TREE_DEPTH];
+
+        // The vector works in reverse
+        for i in (0..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 {
+                let mut repr = FrRepr::default();
+                repr.read_le(&sibling[..]).expect("length is 32 bytes");
+                Fr::from_repr(repr)
+            } {
+                Ok(p) => p,
+                Err(_) => return Err(()),
+            };
+
+            // 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 position = witness
+            .read_u64::<LittleEndian>()
+            .expect("should have had index at the end");
+
+        // Given the position, let's finish constructing the authentication
+        // path
+        let mut tmp = position;
+        for i in 0..TREE_DEPTH {
+            auth_path[i].as_mut().map(|p| p.1 = (tmp & 1) == 1);
+
+            tmp >>= 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);
+
+        Ok(CommitmentTreeWitness {
+            auth_path,
+            position,
+        })
+    }
+}
+
+/// A context object for creating the Sapling components of a Zcash transaction.
+pub struct SaplingProvingContext {
+    bsk: Fs,
+    bvk: edwards::Point<Bls12, Unknown>,
+}
+
+impl SaplingProvingContext {
+    /// Construct a new context to be used with a single transaction.
+    pub fn new() -> Self {
+        SaplingProvingContext {
+            bsk: Fs::zero(),
+            bvk: edwards::Point::zero(),
+        }
+    }
+
+    /// Create the value commitment, re-randomized key, and proof for a Sapling
+    /// SpendDescription, while accumulating its value commitment randomness
+    /// inside the context for later use.
+    pub fn spend_proof(
+        &mut self,
+        proof_generation_key: ProofGenerationKey<Bls12>,
+        diversifier: Diversifier,
+        rcm: Fs,
+        ar: Fs,
+        value: u64,
+        anchor: Fr,
+        witness: CommitmentTreeWitness,
+        proving_key: &Parameters<Bls12>,
+        verifying_key: &PreparedVerifyingKey<Bls12>,
+        params: &JubjubBls12,
+    ) -> Result<
+        (
+            Proof<Bls12>,
+            edwards::Point<Bls12, Unknown>,
+            PublicKey<Bls12>,
+        ),
+        (),
+    > {
+        // Initialize secure RNG
+        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(&self.bsk);
+
+            // Update the context
+            self.bsk = tmp;
+        }
+
+        // Construct the value commitment
+        let value_commitment = ValueCommitment::<Bls12> {
+            value: value,
+            randomness: rcv,
+        };
+
+        // Construct the viewing key
+        let viewing_key = proof_generation_key.into_viewing_key(params);
+
+        // Construct the payment address with the viewing key / diversifier
+        let payment_address = match viewing_key.into_payment_address(diversifier, params) {
+            Some(p) => p,
+            None => return Err(()),
+        };
+
+        // This is the result of the re-randomization, we compute it for the caller
+        let rk = PublicKey::<Bls12>(proof_generation_key.ak.clone().into()).randomize(
+            ar,
+            FixedGenerators::SpendingKeyGenerator,
+            params,
+        );
+
+        // Let's compute the nullifier while we have the position
+        let note = Note {
+            value: value,
+            g_d: diversifier
+                .g_d::<Bls12>(params)
+                .expect("was a valid diversifier before"),
+            pk_d: payment_address.pk_d.clone(),
+            r: rcm,
+        };
+
+        let nullifier = note.nf(&viewing_key, witness.position, params);
+
+        // We now have the full witness for our circuit
+        let instance = Spend {
+            params,
+            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: witness.auth_path,
+            anchor: Some(anchor),
+        };
+
+        // Create proof
+        let proof =
+            create_random_proof(instance, proving_key, &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(params).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(verifying_key, &proof, &public_input[..]) {
+            // No error, and proof verification successful
+            Ok(true) => {}
+
+            // Any other case
+            _ => {
+                return Err(());
+            }
+        }
+
+        // Compute value commitment
+        let value_commitment: edwards::Point<Bls12, Unknown> = value_commitment.cm(params).into();
+
+        // Accumulate the value commitment in the context
+        {
+            let mut tmp = value_commitment.clone();
+            tmp = tmp.add(&self.bvk, params);
+
+            // Update the context
+            self.bvk = tmp;
+        }
+
+        Ok((proof, value_commitment, rk))
+    }
+
+    /// Create the value commitment and proof for a Sapling OutputDescription,
+    /// while accumulating its value commitment randomness inside the context
+    /// for later use.
+    pub fn output_proof(
+        &mut self,
+        esk: Fs,
+        payment_address: PaymentAddress<Bls12>,
+        rcm: Fs,
+        value: u64,
+        proving_key: &Parameters<Bls12>,
+        params: &JubjubBls12,
+    ) -> (Proof<Bls12>, edwards::Point<Bls12, Unknown>) {
+        // Initialize secure RNG
+        let mut rng = OsRng::new().expect("should be able to construct RNG");
+
+        // 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(&self.bsk);
+
+            // Update the context
+            self.bsk = tmp;
+        }
+
+        // Construct the value commitment for the proof instance
+        let value_commitment = ValueCommitment::<Bls12> {
+            value: value,
+            randomness: rcv,
+        };
+
+        // We now have a full witness for the output proof.
+        let instance = Output {
+            params,
+            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, proving_key, &mut rng).expect("proving should not fail");
+
+        // Compute the actual value commitment
+        let value_commitment: edwards::Point<Bls12, Unknown> = value_commitment.cm(params).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(&self.bvk, params);
+
+            // Update the context
+            self.bvk = tmp;
+        }
+
+        (proof, value_commitment)
+    }
+
+    /// Create the bindingSig for a Sapling transaction. All calls to spend_proof()
+    /// and output_proof() must be completed before calling this function.
+    pub fn binding_sig(
+        &self,
+        value_balance: i64,
+        sighash: &[u8; 32],
+        params: &JubjubBls12,
+    ) -> Result<Signature, ()> {
+        // Initialize secure RNG
+        let mut rng = OsRng::new().expect("should be able to construct RNG");
+
+        // Grab the current `bsk` from the context
+        let bsk = PrivateKey::<Bls12>(self.bsk);
+
+        // Grab the `bvk` using DerivePublic.
+        let bvk = PublicKey::from_private(&bsk, FixedGenerators::ValueCommitmentRandomness, params);
+
+        // 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, params) {
+                Some(a) => a,
+                None => return Err(()),
+            };
+
+            // Subtract value_balance from current bvk to get final bvk
+            value_balance = value_balance.negate();
+            let mut tmp = self.bvk.clone();
+            tmp = tmp.add(&value_balance, params);
+
+            // The result should be the same, unless the provided valueBalance is wrong.
+            if bvk.0 != tmp {
+                return Err(());
+            }
+        }
+
+        // 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(&sighash[..]);
+
+        // Sign
+        Ok(bsk.sign(
+            &data_to_be_signed,
+            &mut rng,
+            FixedGenerators::ValueCommitmentRandomness,
+            params,
+        ))
+    }
+}