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Matthew Jackson
6e14fc0c0c
Security - Remove the unsafe Vec::set_len over recycled sector buffers — UB reachable from safe code (GHSA-j8ww-f5fg-9pmh, low). Uses Vec::resize; pooling unchanged. CI / tooling - clippy on stable with the MSRV single-sourced from Cargo.toml; cargo-deny dependency-audit gate with first-party git sources allowed (#15); audience-based comment-guard (#17). Docs - README (keydb ownership, BD-J parser count), drive-access transport/reset notes, mux timeline module-doc links.
40 lines
1.8 KiB
Markdown
40 lines
1.8 KiB
Markdown
# AACS 2.1 FMTS forensic segment map (`src/aacs/segment.rs`)
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Background moved out of the module's rustdoc to keep the doc comments within
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the comment-guard's prose caps.
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## Terminology
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The **index** carried by each segment record (1..32) is NOT the AACS 2.1
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*Media Key Variant* — that is the 65536-value device selector in the MKB
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that decides *which set* of index keys a device receives, a layer this
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module does not deal with. All the index keys belong to one variant, whose
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number is unknown and irrelevant to the segment map. Decrypting a segment
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with the ordinary CPS Unit Key yields garbage — broken HEVC reference
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frames (empirically: `Could not find ref with POC …` on a plain unit-key
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rip).
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## Measurements from a retail AACS 2.1 disc
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`index` is the 1..32 forensic index tag, NOT a sequential segment id:
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measured on a retail 2.1 disc it cycles 1,2,…,32,1,2,… across records in
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file order — 24 full cycles of 32 plus a final partial cycle of 24 = 792
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records. Source-packet numbers are the 192-byte BDAV packet index: byte
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offset = `spn * 192`. Each segment is ~2560 packets (~480 KB) = 80 aligned
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units, spread across the entire 54 GB feature (one roughly every 67 MB).
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Inside a segment the 80 units interleave in two stride-2 halves: applying
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the segment's index key decrypts ~40 of them to clean TS and garbles the
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other ~40 (a second interleaved half, unidentified), which the demux then
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drops — leaving one coherent stream. Confirmed by decoding a retail disc
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with a full set of 32 index keys.
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## `index_to_key_idx`
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The `fmts_key_ranges` callback maps a segment's `index` to a position in the
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key pool, e.g. `|i| i as usize` when the pool is laid out as
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`[base, idx1, idx2, …]`.
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See also `docs/segment-key.md` for the on-disc key store these indices
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select from.
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