Merge pull request #24 from ebfull/sapling_proof_api

Sapling proof API

Cargo.lock

@@ -121,7 +121,7 @@ dependencies = [
  "libc 0.2.40 (registry+https://github.com/rust-lang/crates.io-index)",
  "pairing 0.14.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "sapling-crypto 0.0.1 (git+https://github.com/zcash-hackworks/sapling-crypto?rev=6abfcca25ae233922ecc18a4d2d0b5cb7aab7c8c)",
+ "sapling-crypto 0.0.1 (git+https://github.com/zcash-hackworks/sapling-crypto?rev=c2862a43829632d64f0654e8a9cde9680ba48076)",
 ]
 
 [[package]]
@@ -159,7 +159,7 @@ dependencies = [
 [[package]]
 name = "sapling-crypto"
 version = "0.0.1"
-source = "git+https://github.com/zcash-hackworks/sapling-crypto?rev=6abfcca25ae233922ecc18a4d2d0b5cb7aab7c8c#6abfcca25ae233922ecc18a4d2d0b5cb7aab7c8c"
+source = "git+https://github.com/zcash-hackworks/sapling-crypto?rev=c2862a43829632d64f0654e8a9cde9680ba48076#c2862a43829632d64f0654e8a9cde9680ba48076"
 dependencies = [
  "bellman 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "blake2-rfc 0.2.18 (git+https://github.com/gtank/blake2-rfc?rev=7a5b5fc99ae483a0043db7547fb79a6fa44b88a9)",
@@ -214,7 +214,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum num_cpus 1.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "c51a3322e4bca9d212ad9a158a02abc6934d005490c054a2778df73a70aa0a30"
 "checksum pairing 0.14.2 (registry+https://github.com/rust-lang/crates.io-index)" = "ceda21136251c6d5a422d3d798d8ac22515a6e8d3521bb60c59a8349d36d0d57"
 "checksum rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)" = "eba5f8cb59cc50ed56be8880a5c7b496bfd9bd26394e176bc67884094145c2c5"
-"checksum sapling-crypto 0.0.1 (git+https://github.com/zcash-hackworks/sapling-crypto?rev=6abfcca25ae233922ecc18a4d2d0b5cb7aab7c8c)" = "<none>"
+"checksum sapling-crypto 0.0.1 (git+https://github.com/zcash-hackworks/sapling-crypto?rev=c2862a43829632d64f0654e8a9cde9680ba48076)" = "<none>"
 "checksum typenum 1.10.0 (registry+https://github.com/rust-lang/crates.io-index)" = "612d636f949607bdf9b123b4a6f6d966dedf3ff669f7f045890d3a4a73948169"
 "checksum winapi 0.3.4 (registry+https://github.com/rust-lang/crates.io-index)" = "04e3bd221fcbe8a271359c04f21a76db7d0c6028862d1bb5512d85e1e2eb5bb3"
 "checksum winapi-i686-pc-windows-gnu 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"

Cargo.toml

@@ -27,7 +27,7 @@ rev = "7a5b5fc99ae483a0043db7547fb79a6fa44b88a9"
 
 [dependencies.sapling-crypto]
 git = "https://github.com/zcash-hackworks/sapling-crypto"
-rev = "6abfcca25ae233922ecc18a4d2d0b5cb7aab7c8c"
+rev = "c2862a43829632d64f0654e8a9cde9680ba48076"
 
 [profile.release]
 lto = true

include/librustzcash.h

@@ -60,6 +60,68 @@ extern "C" {
         unsigned char *result
     );
 
+    /// Computes the signature for each Spend description, given the key
+    /// `ask`, the re-randomization `ar`, the 32-byte sighash `sighash`,
+    /// and an output `result` buffer of 64-bytes for the signature.
+    ///
+    /// This function will fail if the provided `ask` or `ar` are invalid.
+    bool librustzcash_sapling_spend_sig(
+        const unsigned char *ask,
+        const unsigned char *ar,
+        const unsigned char *sighash,
+        unsigned char *result
+    );
+
+    /// Creates a Sapling proving context. Please free this when you're done.
+    void * librustzcash_sapling_proving_ctx_init();
+
+    /// This function (using the proving context) constructs a Spend proof
+    /// given the necessary witness information. It outputs `cv` (the value
+    /// commitment) and `rk` (so that you don't have to compute it) along
+    /// with the proof.
+    bool librustzcash_sapling_spend_proof(
+        void *ctx,
+        const unsigned char *ak,
+        const unsigned char *nsk,
+        const unsigned char *diversifier,
+        const unsigned char *rcm,
+        const unsigned char *ar,
+        const uint64_t value,
+        const unsigned char *anchor,
+        const unsigned char *witness,
+        unsigned char *cv,
+        unsigned char *rk,
+        unsigned char *zkproof
+    );
+
+    /// This function (using the proving context) constructs an Output
+    /// proof given the necessary witness information. It outputs `cv`
+    /// and the `zkproof`.
+    bool librustzcash_sapling_output_proof(
+        void *ctx,
+        const unsigned char *esk,
+        const unsigned char *diversifier,
+        const unsigned char *pk_d,
+        const unsigned char *rcm,
+        const uint64_t value,
+        unsigned char *cv,
+        unsigned char *zkproof
+    );
+
+    /// This function (using the proving context) constructs a binding
+    /// signature. You must provide the intended valueBalance so that
+    /// we can internally check consistency.
+    bool librustzcash_sapling_binding_sig(
+        const void *ctx,
+        int64_t valueBalance,
+        const unsigned char *sighash,
+        unsigned char *result
+    );
+
+    /// Frees a Sapling proving context returned from
+    /// `librustzcash_sapling_proving_ctx_init`.
+    void librustzcash_sapling_proving_ctx_free(void *);
+
     /// Creates a Sapling verification context. Please free this
     /// when you're done.
     void * librustzcash_sapling_verification_ctx_init();

src/rustzcash.rs

@@ -23,6 +23,8 @@ use sapling_crypto::{
 };
 
 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,
@@ -33,7 +35,7 @@ use blake2_rfc::blake2s::Blake2s;
 
 use byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
 
-use rand::{OsRng, Rng};
+use rand::{OsRng, Rand, Rng};
 use std::io::BufReader;
 
 use libc::{c_char, c_uchar, int64_t, size_t, uint32_t, uint64_t};
@@ -41,7 +43,7 @@ use std::ffi::CStr;
 use std::fs::File;
 use std::slice;
 
-use sapling_crypto::primitives::ViewingKey;
+use sapling_crypto::primitives::{ProofGenerationKey, ValueCommitment, ViewingKey};
 
 pub mod equihash;
 
@@ -693,7 +695,7 @@ pub extern "system" fn librustzcash_sapling_check_output(
     // Accumulate the value commitment in the context
     {
         let mut tmp = cv.clone();
-        tmp.negate(); // Outputs subtract from the total.
+        tmp = tmp.negate(); // Outputs subtract from the total.
         tmp = tmp.add(&unsafe { &*ctx }.bvk, &JUBJUB);
 
         // Update the context
@@ -1019,3 +1021,493 @@ pub extern "system" fn librustzcash_sprout_verify(
         _ => 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: *const 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) });
+}