Test nullifiers in constant time

Checking for spent notes in a block is still not completely constant
time, due to filtering out negative results of the constant-time
comparison.

Part of #84.

Cargo.lock

@@ -493,6 +493,11 @@ dependencies = [
  "opaque-debug 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "subtle"
+version = "2.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "syn"
 version = "0.14.9"
@@ -545,6 +550,7 @@ dependencies = [
  "rand_core 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "rand_os 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "rand_xorshift 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "subtle 2.1.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "zcash_primitives 0.0.0",
 ]
 
@@ -642,6 +648,7 @@ dependencies = [
 "checksum rand_os 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "ddb525a78d3a0b0e05b6fe0f7df14d7a4dc957944c7b403911ba5a0f1c694967"
 "checksum rand_xorshift 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "77d416b86801d23dde1aa643023b775c3a462efc0ed96443add11546cdf1dca8"
 "checksum sha2 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "7b4d8bfd0e469f417657573d8451fb33d16cfe0989359b93baf3a1ffc639543d"
+"checksum subtle 2.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "01f40907d9ffc762709e4ff3eb4a6f6b41b650375a3f09ac92b641942b7fb082"
 "checksum syn 0.14.9 (registry+https://github.com/rust-lang/crates.io-index)" = "261ae9ecaa397c42b960649561949d69311f08eeaea86a65696e6e46517cf741"
 "checksum typenum 1.10.0 (registry+https://github.com/rust-lang/crates.io-index)" = "612d636f949607bdf9b123b4a6f6d966dedf3ff669f7f045890d3a4a73948169"
 "checksum unicode-xid 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "fc72304796d0818e357ead4e000d19c9c174ab23dc11093ac919054d20a6a7fc"

zcash_client_backend/Cargo.toml

@@ -12,6 +12,7 @@ ff = { path = "../ff" }
 hex = "0.3"
 pairing = { path = "../pairing" }
 protobuf = "2"
+subtle = "2"
 zcash_primitives = { path = "../zcash_primitives" }
 
 [build-dependencies]

zcash_client_backend/src/welding_rig.rs

@@ -3,6 +3,7 @@
 use ff::{PrimeField, PrimeFieldRepr};
 use pairing::bls12_381::{Bls12, Fr, FrRepr};
 use std::collections::HashSet;
+use subtle::{ConditionallySelectable, ConstantTimeEq, CtOption};
 use zcash_primitives::{
     jubjub::{edwards, fs::Fs},
     merkle_tree::{CommitmentTree, IncrementalWitness},
@@ -116,28 +117,29 @@ pub fn scan_block(
         let num_outputs = tx.outputs.len();
 
         // Check for spent notes
-        let shielded_spends: Vec<_> =
+        // The only step that is not constant-time is the filter() at the end.
-            tx.spends
+        let shielded_spends: Vec<_> = tx
-                .into_iter()
+            .spends
-                .enumerate()
+            .into_iter()
-                .filter_map(|(index, spend)| {
+            .enumerate()
-                    if let Some(account) = nullifiers.iter().find_map(|&(nf, acc)| {
+            .map(|(index, spend)| {
-                        if nf == &spend.nf[..] {
+                // Find the first tracked nullifier that matches this spend, and produce
-                            Some(acc)
+                // a WalletShieldedSpend if there is a match, in constant time.
-                        } else {
+                nullifiers
-                            None
+                    .iter()
-                        }
+                    .map(|&(nf, account)| CtOption::new(account as u64, nf.ct_eq(&spend.nf[..])))
-                    }) {
+                    .fold(CtOption::new(0, 0.into()), |first, next| {
-                        Some(WalletShieldedSpend {
+                        CtOption::conditional_select(&next, &first, first.is_some())
-                            index,
+                    })
-                            nf: spend.nf,
+                    .map(|account| WalletShieldedSpend {
-                            account,
+                        index,
-                        })
+                        nf: spend.nf,
-                    } else {
+                        account: account as usize,
-                        None
+                    })
-                    }
+            })
-                })
+            .filter(|spend| spend.is_some().into())
-                .collect();
+            .map(|spend| spend.unwrap())
+            .collect();
 
         // Collect the set of accounts that were spent from in this transaction
         let spent_from_accounts: HashSet<_> =