Single-part encrypted objects in the legacy format could not serve range
reads without decrypting from byte 0: v1 frames are emitted per upstream
read, so no closed-form plaintext-to-ciphertext mapping exists. The v2
layout fixed the frame length (8218 ciphertext bytes per 8 KiB plaintext
frame), making the mapping closed-form.
Consume it:
- Single-part PUTs that encrypt locally under the v2 write switch stamp
the frame-layout marker, valued with the object's data_dir token -
ciphertext passthrough, data movement and copies mint a new data_dir
or strip the marker, so a re-homed marker disqualifies itself.
- The encrypted read plan seeks marked, uncompressed single-part objects
to frame_index * 8218 and decrypts from that frame: the frame index
rides the plan's sequence-number slot into DecryptReader::new_at_block,
whose nonce and AAD bind absolute indices. New metric path label
frame_seek.
- A lying marker fails closed: v2 authentication rejects bytes at a fake
frame boundary; plaintext is never served from the wrong offset.
Compressed objects and multipart sub-part seeks keep the conservative
paths (follow-up work); reading needs no switch - seekability follows
the marker.
唐小鸭