Move Sapling verification checks into zcash_proofs crate

zcash_proofs/Cargo.toml

@@ -6,3 +6,6 @@ authors = [
 ]
 
 [dependencies]
+bellman = { path = "../bellman" }
+pairing = { path = "../pairing" }
+sapling-crypto = { path = "../sapling-crypto" }

zcash_proofs/src/lib.rs

@@ -1,7 +1,5 @@
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn it_works() {
-        assert_eq!(2 + 2, 4);
-    }
-}
+extern crate bellman;
+extern crate pairing;
+extern crate sapling_crypto;
+
+pub mod sapling;

zcash_proofs/src/sapling/mod.rs

@@ -0,0 +1,37 @@
+use pairing::bls12_381::Bls12;
+use sapling_crypto::jubjub::{
+    edwards, fs::FsRepr, FixedGenerators, JubjubBls12, JubjubParams, Unknown,
+};
+
+mod verifier;
+
+pub use self::verifier::SaplingVerificationContext;
+
+// This function computes `value` in the exponent of the value commitment base
+pub fn compute_value_balance(
+    value: i64,
+    params: &JubjubBls12,
+) -> 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 = params
+        .generator(FixedGenerators::ValueCommitmentValue)
+        .mul(FsRepr::from(abs), params);
+
+    // Negate if necessary
+    if is_negative {
+        value_balance = value_balance.negate();
+    }
+
+    // Convert to unknown order point
+    Some(value_balance.into())
+}

zcash_proofs/src/sapling/verifier.rs

@@ -0,0 +1,207 @@
+use bellman::groth16::{verify_proof, PreparedVerifyingKey, Proof};
+use pairing::{
+    bls12_381::{Bls12, Fr},
+    Field,
+};
+use sapling_crypto::{
+    circuit::multipack,
+    jubjub::{edwards, FixedGenerators, JubjubBls12, Unknown},
+    redjubjub::{PublicKey, Signature},
+};
+
+use super::compute_value_balance;
+
+fn is_small_order<Order>(p: &edwards::Point<Bls12, Order>, params: &JubjubBls12) -> bool {
+    p.double(params).double(params).double(params) == edwards::Point::zero()
+}
+
+/// A context object for verifying the Sapling components of a Zcash transaction.
+pub struct SaplingVerificationContext {
+    bvk: edwards::Point<Bls12, Unknown>,
+}
+
+impl SaplingVerificationContext {
+    /// Construct a new context to be used with a single transaction.
+    pub fn new() -> Self {
+        SaplingVerificationContext {
+            bvk: edwards::Point::zero(),
+        }
+    }
+
+    /// Perform consensus checks on a Sapling SpendDescription, while
+    /// accumulating its value commitment inside the context for later use.
+    pub fn check_spend(
+        &mut self,
+        cv: edwards::Point<Bls12, Unknown>,
+        anchor: Fr,
+        nullifier: &[u8; 32],
+        rk: PublicKey<Bls12>,
+        sighash_value: &[u8; 32],
+        spend_auth_sig: Signature,
+        zkproof: Proof<Bls12>,
+        verifying_key: &PreparedVerifyingKey<Bls12>,
+        params: &JubjubBls12,
+    ) -> bool {
+        if is_small_order(&cv, params) {
+            return false;
+        }
+
+        if is_small_order(&rk.0, params) {
+            return false;
+        }
+
+        // Accumulate the value commitment in the context
+        {
+            let mut tmp = cv.clone();
+            tmp = tmp.add(&self.bvk, params);
+
+            // Update the context
+            self.bvk = tmp;
+        }
+
+        // Grab the nullifier as a sequence of bytes
+        let nullifier = &nullifier[..];
+
+        // 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(&sighash_value[..]);
+
+        // Verify the spend_auth_sig
+        if !rk.verify(
+            &data_to_be_signed,
+            &spend_auth_sig,
+            FixedGenerators::SpendingKeyGenerator,
+            params,
+        ) {
+            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];
+        }
+
+        // Verify the proof
+        match verify_proof(verifying_key, &zkproof, &public_input[..]) {
+            // No error, and proof verification successful
+            Ok(true) => true,
+
+            // Any other case
+            _ => false,
+        }
+    }
+
+    /// Perform consensus checks on a Sapling OutputDescription, while
+    /// accumulating its value commitment inside the context for later use.
+    pub fn check_output(
+        &mut self,
+        cv: edwards::Point<Bls12, Unknown>,
+        cm: Fr,
+        epk: edwards::Point<Bls12, Unknown>,
+        zkproof: Proof<Bls12>,
+        verifying_key: &PreparedVerifyingKey<Bls12>,
+        params: &JubjubBls12,
+    ) -> bool {
+        if is_small_order(&cv, params) {
+            return false;
+        }
+
+        if is_small_order(&epk, params) {
+            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(&self.bvk, params);
+
+            // Update the context
+            self.bvk = tmp;
+        }
+
+        // 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;
+
+        // Verify the proof
+        match verify_proof(verifying_key, &zkproof, &public_input[..]) {
+            // No error, and proof verification successful
+            Ok(true) => true,
+
+            // Any other case
+            _ => false,
+        }
+    }
+
+    /// Perform consensus checks on the valueBalance and bindingSig parts of a
+    /// Sapling transaction. All SpendDescriptions and OutputDescriptions must
+    /// have been checked before calling this function.
+    pub fn final_check(
+        &self,
+        value_balance: i64,
+        sighash_value: &[u8; 32],
+        binding_sig: Signature,
+        params: &JubjubBls12,
+    ) -> bool {
+        // Obtain current bvk from the context
+        let mut bvk = PublicKey(self.bvk.clone());
+
+        // Compute value balance
+        let mut value_balance = match compute_value_balance(value_balance, params) {
+            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, params);
+
+        // 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(&sighash_value[..]);
+
+        // Verify the binding_sig
+        bvk.verify(
+            &data_to_be_signed,
+            &binding_sig,
+            FixedGenerators::ValueCommitmentRandomness,
+            params,
+        )
+    }
+}