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packet: never emit 1-byte truncated packet numbers #2449

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packet: never emit 1-byte truncated packet numbers #2449

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29 changes: 17 additions & 12 deletions quiche/src/packet.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -570,8 +570,15 @@ pub fn pkt_num_len(pn: u64, largest_acked: u64) -> usize {
let num_unacked: u64 = pn.saturating_sub(largest_acked) + 1;
// computes ceil of num_unacked.log2() + 1
let min_bits = u64::BITS - num_unacked.leading_zeros() + 1;
// get the num len in bytes
min_bits.div_ceil(8) as usize
// get the num len in bytes; floor at 2 to avoid the 1-byte
// truncation ambiguity that breaks AEAD decryption when the
// receiver sees more than 128 packets reordered (the entire
// valid range of a 1-byte truncation collapses, so the decoder
// cannot recover the full packet number unambiguously). The
// reference Go implementation, quic-go, refuses 1-byte truncation
// for the same reason — see PacketNumberLengthForHeader in
// quic-go/internal/protocol/packet_number.go.
(min_bits.div_ceil(8) as usize).max(2)
}

pub fn decrypt_hdr(
Expand Down Expand Up @@ -1356,8 +1363,11 @@ mod tests {

#[test]
fn pkt_num_encode_decode() {
// pkt_num_len floors at 2: the function never emits a 1-byte
// truncated packet number, so the smallest unacked-distance
// encoding still uses 2 bytes.
let num_len = pkt_num_len(0, 0);
assert_eq!(num_len, 1);
assert_eq!(num_len, 2);
let pn = decode_pkt_num(0xa82f30ea, 0x9b32, 2);
assert_eq!(pn, 0xa82f9b32);
let mut d = [0; 10];
Expand All @@ -1380,19 +1390,14 @@ mod tests {
let hdr_num = u64::from(b.get_u24().unwrap());
let pn = decode_pkt_num(0xace9fa, hdr_num, num_len);
assert_eq!(pn, 0xace9fe);
// roundtrip
// roundtrip — every result is at least 2 bytes
let base = 0xdeadbeef;
for i in 1..255 {
let pn = base + i;
let num_len = pkt_num_len(pn, base);
if num_len == 1 {
let decoded = decode_pkt_num(base, pn & 0xff, num_len);
assert_eq!(decoded, pn);
} else {
assert_eq!(num_len, 2);
let decoded = decode_pkt_num(base, pn & 0xffff, num_len);
assert_eq!(decoded, pn);
}
assert_eq!(num_len, 2);
let decoded = decode_pkt_num(base, pn & 0xffff, num_len);
assert_eq!(decoded, pn);
}
}

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