KarpelesLab / tmpsecfile / 24973288334

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Hold sparse-detection false-positive at 2^-128 even for sub-block files

ReadAt previously isZero-checked only the bytes of the last AES block
that fell within the logical length. For a 1-byte file that left a
single byte of evidence — 1-in-256 odds of misreading a real
ciphertext byte as a sparse hole.

The fix:
- ReadAt always inspects the full 16-byte block on disk.
- Truncate keeps the on-disk size aesBlockSize-aligned, and on a
  shrink past a partial boundary it re-encrypts the remaining bytes
  of that block to zero so a later re-extension cannot surface old
  plaintext.

Adds an exhaustive 1-byte round-trip test (every value 0..255) and a
truncate-shrink-then-extend leak test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

29 of 33 new or added lines in 1 file covered. (87.88%)

187 of 241 relevant lines covered (77.59%)

459.76 hits per line