Unbounded memory allocation in header parsing #572

The helper Header::read and its buffered/async variants (fn read, fn read_async, fn read_buffered, and fn read_async_buffered) call the nom parser in a loop. When the parser returns Incomplete(Needed::Size(n)), the code blindly executes data.resize(m + n, 0) and then read_exact the additional bytes. Because n is attacker-influenced and no upper bound is enforced, a hostile file can force arbitrarily large allocations, leading to memory-exhaustion denial of service.

Recommendation:

Abort header parsing when the accumulated buffer exceeds 64 KiB (the limit used by the reference Go implementation) or a tighter value mandated by project policy. Propagate a DecryptError::InvalidHeaderSize to callers.