Read called XORKeyStream(b, b) — XOR-ing the entire caller buffer even
when the underlying transport returned fewer bytes. AES-CTR's keystream
position is then advanced by len(b), but the peer only consumed n
bytes' worth of keystream. After a single short read the two
keystreams diverge for the lifetime of the connection, every
subsequent MTProto message decrypts to garbage, and the engine fails
with "consume message: decrypt: msg_key is invalid".
The faketls layer makes short reads routine: each Read returns at most
one TLS Application record's payload, regardless of how big the caller
buffer is. So in practice the stream desynced almost immediately on
high-traffic clients (active supergroups, post-relogin catch-up) and
intermittently on quiet ones.
Match the upstream gotd/td fix and only XOR the n bytes that came out
of the transport. Add a regression test (chunkConn delivers ciphertext
in 7-byte chunks; client reads through Obfuscated2.Read with a 128-byte
buffer; plaintext must round-trip).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Igor Artamonov
Adam Van Ymeren