probe_folder_encryption sampled AACS units starting from the largest extent
anywhere in the disc. AACS units are 3 sectors and a unit boundary is only
guaranteed at the START of a clip, so that anchor is only correct when the
largest extent happens to be a clip first extent.
For any clip over ~2 GiB it is not. The planner caps an allocation descriptor
at MAX_AD_BYTES = 524287 sectors, so every full piece of a split file ties on
sector_count and max_by_key returns the LAST tie — an extent beginning
(k-1)*524287 sectors into the file. 524287 % 3 == 1, so that start misses the
unit boundary for two file sizes in three.
The 6144-byte sample windows then begin mid-source-packet and the byte read as
the CPI flag is content. Both verdicts fail in a costly direction: a decrypted
folder is rejected as encrypted (DirImageEncrypted on something perfectly
rippable — the exact case scan_dir was added to rescue), or real ciphertext
reads as clear and the mux writes it out as video at exit 0. is_unit_aligned
cannot catch it: it measures against the same wrong base.
Now the largest TITLE first extent, which is unit-aligned by construction and
is also the more meaningful sample — the main feature rather than whichever
fragment happened to be biggest.
The existing tests could not reach this: their fixture m2ts is 786,432 bytes,
a single extent, which is always its own first.
Matthew Jackson
84e0ba9fa8