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fix(quic): bound decrypt header associated data #11

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2c8eee9
fix(quic): bound decrypt header associated data
alanhoff Mar 17, 2026
51538e1
fix(packet): validate decrypt payload bounds
alanhoff Mar 17, 2026
2f13b45
fix(packet): bound decrypt associated data rebuild
alanhoff Mar 17, 2026
bb3bde2
fix(packet): narrow helper error set
alanhoff Mar 17, 2026
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145 changes: 133 additions & 12 deletions src/quic/packet.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -308,6 +308,24 @@ pub fn parseIncoming(bytes: []const u8) void {
_ = bytes;
}

fn buildHeaderBytes(header_bytes_buf: []u8, first_byte: u8, packet_buf: []const u8, packet_start: usize, header_end: usize) error{InvalidPacket}![]const u8 {
if (header_end <= packet_start or header_end > packet_buf.len) {
return error.InvalidPacket;
}

const header_len = header_end - packet_start;
if (header_len > header_bytes_buf.len) {
return error.InvalidPacket;
}

header_bytes_buf[0] = first_byte;
if (header_len > 1) {
@memcpy(header_bytes_buf[1..header_len], packet_buf[(packet_start + 1)..header_end]);
}

return header_bytes_buf[0..header_len];
}

pub fn decrypt(header: *Header, fbs: anytype, space: PacketNumSpace) ![]u8 {
// We need at least 4 bytes for packet number + 16 for sample
if (fbs.pos + 4 + crypto.SAMPLE_LEN > fbs.buffer.len) {
Expand Down Expand Up @@ -375,19 +393,20 @@ pub fn decrypt(header: *Header, fbs: anytype, space: PacketNumSpace) ![]u8 {
// Skip the packet number bytes in the buffer
try fbs.seekBy(@intCast(header.packet_number_len));

if (header.remainder_len < header.packet_number_len) {
return error.InvalidPacket;
}
const payload_len = header.remainder_len - header.packet_number_len;
if (payload_len > (fbs.buffer.len - fbs.pos)) {
return error.InvalidPacket;
}

// RFC 9001 Section 5.2: AD includes the unprotected first byte and everything up to and including packet number
// For coalesced packets, use packet_start to get the correct offset within the buffer
const pkt_start = header.packet_start;
const header_len = fbs.pos - pkt_start; // total header length including packet number
var header_bytes_buf: [512]u8 = undefined;
// Copy first byte as unprotected
header_bytes_buf[0] = first_byte;
// Copy the rest of the header and packet number (from byte after first to current position)
@memcpy(header_bytes_buf[1..][0..(header_len - 1)], fbs.buffer[(pkt_start + 1)..fbs.pos]);
const header_bytes = header_bytes_buf[0..header_len];
const encrypted_payload = fbs.buffer[(fbs.pos)..(fbs.pos + payload_len)];
const header_bytes = try buildHeaderBytes(&header_bytes_buf, first_byte, fbs.buffer, pkt_start, fbs.pos);
const encrypted_payload = fbs.buffer[fbs.pos..(fbs.pos + payload_len)];

// Decode packet number
header.packet_number = decodePacketNumber(space.next_packet_number, truncated_packet_number, header.packet_number_len * 8);
Expand Down Expand Up @@ -457,16 +476,19 @@ pub fn decryptWithKeyUpdate(header: *Header, fbs: anytype, space: *PacketNumSpac

try fbs.seekBy(@intCast(header.packet_number_len));

if (header.remainder_len < header.packet_number_len) {
return error.InvalidPacket;
}
const payload_len = header.remainder_len - header.packet_number_len;
if (payload_len > (fbs.buffer.len - fbs.pos)) {
return error.InvalidPacket;
}

// Build associated data
const pkt_start = header.packet_start;
const header_len = fbs.pos - pkt_start;
var header_bytes_buf: [512]u8 = undefined;
header_bytes_buf[0] = first_byte;
@memcpy(header_bytes_buf[1..][0..(header_len - 1)], fbs.buffer[(pkt_start + 1)..fbs.pos]);
const header_bytes = header_bytes_buf[0..header_len];
const encrypted_payload = fbs.buffer[(fbs.pos)..(fbs.pos + payload_len)];
const header_bytes = try buildHeaderBytes(&header_bytes_buf, first_byte, fbs.buffer, pkt_start, fbs.pos);
const encrypted_payload = fbs.buffer[fbs.pos..(fbs.pos + payload_len)];

// Decode packet number
header.packet_number = decodePacketNumber(space.next_packet_number, truncated_packet_number, header.packet_number_len * 8);
Expand Down Expand Up @@ -1061,6 +1083,105 @@ fn decodePacketNumber(expected_pkt_num: u64, truncated_pkt_num: u64, num_bits: u
return candidate;
}

fn testOpen() crypto.Open {
return .{
.key = .{0} ** crypto.max_key_len,
.hp_key = .{0} ** crypto.max_key_len,
.nonce = .{0} ** crypto.nonce_len,
};
}

fn testKeyUpdateManager() crypto.KeyUpdateManager {
return crypto.KeyUpdateManager.init(
.{0} ** 32,
.{0} ** 32,
.{0} ** crypto.max_key_len,
.{0} ** crypto.max_key_len,
);
}

fn initProtectedShortHeaderPacket(buffer: []u8, hp_key: [crypto.max_key_len]u8, unprotected_first_byte: u8, truncated_packet_number: u8) void {
@memset(buffer, 0);

const pn_offset = 1;
const sample: *const [crypto.SAMPLE_LEN]u8 = buffer[(pn_offset + 4)..][0..crypto.SAMPLE_LEN];
const mask = crypto.computeHpMask(sample, hp_key, .aes_128_gcm_sha256);

buffer[0] = unprotected_first_byte ^ (mask[0] & 0x1f);
buffer[pn_offset] = truncated_packet_number ^ mask[1];
}

test "QUIC: buildHeaderBytes rejects oversized associated data headers" {
var header_bytes_buf: [512]u8 = undefined;
var packet_buf: [513]u8 = .{0} ** 513;

try std.testing.expectError(
error.InvalidPacket,
buildHeaderBytes(&header_bytes_buf, FIXED_BIT, &packet_buf, 0, packet_buf.len),
);
}

test "QUIC: buildHeaderBytes keeps the unprotected first byte" {
var header_bytes_buf: [8]u8 = undefined;
const packet_buf = [_]u8{ 0xff, 0xaa, 0xbb, 0xcc };

const header_bytes = try buildHeaderBytes(&header_bytes_buf, FIXED_BIT, &packet_buf, 0, packet_buf.len);

try std.testing.expectEqualSlices(u8, &[_]u8{ FIXED_BIT, 0xaa, 0xbb, 0xcc }, header_bytes);
}

test "QUIC: decrypt rejects malformed payload lengths" {
const open = testOpen();
const space = PacketNumSpace{ .crypto_open = open };
const unprotected_first_byte = FIXED_BIT;

{
var buf: [32]u8 = undefined;
initProtectedShortHeaderPacket(&buf, open.hp_key, unprotected_first_byte, 0);
var fbs = io.fixedBufferStream(&buf);
fbs.pos = 1;
var header = Header{ .packet_start = 0, .remainder_len = 0 };

try std.testing.expectError(error.InvalidPacket, decrypt(&header, &fbs, space));
}

{
var buf: [32]u8 = undefined;
initProtectedShortHeaderPacket(&buf, open.hp_key, unprotected_first_byte, 0);
var fbs = io.fixedBufferStream(&buf);
fbs.pos = 1;
var header = Header{ .packet_start = 0, .remainder_len = buf.len };

try std.testing.expectError(error.InvalidPacket, decrypt(&header, &fbs, space));
}
}

test "QUIC: decryptWithKeyUpdate rejects malformed payload lengths" {
var ku = testKeyUpdateManager();
var space = PacketNumSpace{};
const unprotected_first_byte = FIXED_BIT;

{
var buf: [32]u8 = undefined;
initProtectedShortHeaderPacket(&buf, ku.hp_open, unprotected_first_byte, 0);
var fbs = io.fixedBufferStream(&buf);
fbs.pos = 1;
var header = Header{ .packet_start = 0, .remainder_len = 0 };

try std.testing.expectError(error.InvalidPacket, decryptWithKeyUpdate(&header, &fbs, &space, &ku));
}

{
var buf: [32]u8 = undefined;
initProtectedShortHeaderPacket(&buf, ku.hp_open, unprotected_first_byte, 0);
var fbs = io.fixedBufferStream(&buf);
fbs.pos = 1;
var header = Header{ .packet_start = 0, .remainder_len = buf.len };

try std.testing.expectError(error.InvalidPacket, decryptWithKeyUpdate(&header, &fbs, &space, &ku));
}
}

// Retry token roundtrip test
test "Retry token: generate and validate roundtrip" {
var token_key: [crypto.key_len]u8 = undefined;
Expand Down