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fix: use subtle for constant-time comparison #784

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fix: use subtle for constant-time comparison #784

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21 changes: 10 additions & 11 deletions crates/daphne/src/messages/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@ use std::{
fmt,
io::{Cursor, Read},
};
use subtle::ConstantTimeEq;

// Batch modes
const BATCH_MODE_TIME_INTERVAL: u8 = 0x01;
Expand Down Expand Up @@ -1485,18 +1486,8 @@ impl ParameterizedDecode<DapVersion> for PlaintextInputShare {
}
}

// NOTE ring provides a similar function, but as of version 0.16.20, it doesn't compile to
// wasm32-unknown-unknown.
pub fn constant_time_eq(left: &[u8], right: &[u8]) -> bool {
if left.len() != right.len() {
return false;
}

let mut r = 0;
for (x, y) in left.iter().zip(right) {
r |= x ^ y;
}
r == 0
bool::from(left.ct_eq(right))
}

pub(crate) fn encode_u16_bytes(bytes: &mut Vec<u8>, input: &[u8]) -> Result<(), CodecError> {
Expand Down Expand Up @@ -1705,6 +1696,14 @@ mod test {
));
}

#[test]
fn constant_time_eq_matches_slice_equality() {
assert!(constant_time_eq(b"", b""));
assert!(constant_time_eq(b"same bytes", b"same bytes"));
assert!(!constant_time_eq(b"same bytes", b"same bytez"));
assert!(!constant_time_eq(b"short", b"shorter"));
}

fn partial_batch_selector_encode_decode(version: DapVersion) {
const TEST_DATA_DRAFT09: &[u8] = &[1];
const TEST_DATA_LATEST: &[u8] = &[1, 0, 0];
Expand Down
16 changes: 15 additions & 1 deletion crates/daphne/src/vdaf/prio3.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@ use prio::{
},
};
use std::io::Cursor;
use subtle::ConstantTimeEq;

impl Prio3Config {
pub(crate) fn shard(
Expand All @@ -35,10 +36,11 @@ impl Prio3Config {
(DapVersion::Latest, Prio3Config::Count, DapMeasurement::U64(measurement))
if measurement < 2 =>
{
let measurement = bool::from(measurement.ct_eq(&1));
let vdaf = Prio3::new_count(2).map_err(|e| {
VdafError::Dap(fatal_error!(err = ?e, "initializing {self:?} failed"))
})?;
shard_then_encode(&vdaf, task_id, &(measurement != 0), nonce)
shard_then_encode(&vdaf, task_id, &measurement, nonce)
}
(
DapVersion::Latest,
Expand Down Expand Up @@ -612,6 +614,18 @@ mod test {
assert_eq!(got, DapAggregateResult::U64(3));
}

#[test]
fn count_rejects_non_binary_measurement() {
let got = Prio3Config::Count.shard(
DapVersion::Latest,
DapMeasurement::U64(2),
&[0; 16],
crate::messages::TaskId([0; 32]),
);

assert!(got.is_err());
}

#[test]
fn roundtrip_sum() {
let mut t = AggregationJobTest::new(
Expand Down