max / synckit

Pin the v3 blob header bound and mid-chunk truncation Feed parse_blob_header bodies one byte short of, exactly, and one byte past BLOB_V3_HEADER_LEN, and hand decrypt_blob_chunked a blob missing the last byte of its final sealed chunk.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Max Johnson <me@maxj.phd> · 2026-08-23 21:58 UTC

Signed with PGP, not checked

Commit: 1e82680f839a5430afbbc728a2ccd761462a6992

Parent: 36e691c

1 file changed, +58 insertions, -0 deletions

M synckit-client/src/crypto.rs +58

			@@ -1883,6 +1883,64 @@
1883	1883		assert!(decrypt_blob_chunked(&buf, &key, "hash").is_err());
1884	1884		}
1885	1885
	1886	+	/// The header-length guard is an exact bound: one byte short of the header
	1887	+	/// must fail, and exactly the header (or more) must parse.
	1888	+	#[test]
	1889	+	fn v3_header_length_bound_is_exact() {
	1890	+	fn body_of(len: usize) -> Vec<u8> {
	1891	+	let mut buf = Vec::from(WIRE_V3_TAG_BYTES);
	1892	+	let mut body = vec![0u8; len];
	1893	+	if !body.is_empty() {
	1894	+	body[0] = 3;
	1895	+	}
	1896	+	// chunk_size must be nonzero for the header to be accepted.
	1897	+	if len >= 5 {
	1898	+	body[1..5].copy_from_slice(&(BLOB_CHUNK_SIZE as u32).to_le_bytes());
	1899	+	}
	1900	+	buf.extend_from_slice(&body);
	1901	+	buf
	1902	+	}
	1903	+
	1904	+	let short = body_of(BLOB_V3_HEADER_LEN - 1);
	1905	+	assert!(
	1906	+	parse_blob_header(&short).is_err(),
	1907	+	"a body one byte short of the header must be rejected"
	1908	+	);
	1909	+
	1910	+	let exact = body_of(BLOB_V3_HEADER_LEN);
	1911	+	let (header, consumed) = parse_blob_header(&exact).expect("exact header parses");
	1912	+	assert_eq!(consumed, WIRE_V3_TAG_BYTES.len() + BLOB_V3_HEADER_LEN);
	1913	+	assert_eq!(header.chunk_size, BLOB_CHUNK_SIZE);
	1914	+	assert_eq!(header.total_len, 0);
	1915	+
	1916	+	let long = body_of(BLOB_V3_HEADER_LEN + 1);
	1917	+	let (header, consumed) =
	1918	+	parse_blob_header(&long).expect("header plus a stream byte parses");
	1919	+	assert_eq!(
	1920	+	consumed,
	1921	+	WIRE_V3_TAG_BYTES.len() + BLOB_V3_HEADER_LEN,
	1922	+	"the trailing byte belongs to the chunk stream, not the header"
	1923	+	);
	1924	+	assert_eq!(header.chunk_size, BLOB_CHUNK_SIZE);
	1925	+	}
	1926	+
	1927	+	/// Losing the last byte of the final sealed chunk is a clean error, not a
	1928	+	/// slice panic.
	1929	+	#[test]
	1930	+	fn v3_blob_truncated_one_byte_into_final_chunk_errs() {
	1931	+	let key = generate_master_key();
	1932	+	let plaintext = vec![7u8; 4096];
	1933	+	let sealed = encrypt_blob_chunked(&plaintext, &key, "hash").unwrap();
	1934	+	let truncated = &sealed[..sealed.len() - 1];
	1935	+	assert!(
	1936	+	matches!(
	1937	+	decrypt_blob_chunked(truncated, &key, "hash"),
	1938	+	Err(SyncKitError::Crypto(_))
	1939	+	),
	1940	+	"a one-byte truncation must report a truncated blob"
	1941	+	);
	1942	+	}
	1943	+
1886	1944		#[test]
1887	1945		fn v2_data_is_tagged_and_roundtrips() {
1888	1946		let key = generate_master_key();