mirror of
https://github.com/freemkv/libfreemkv.git
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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.
77 lines
2.9 KiB
Markdown
77 lines
2.9 KiB
Markdown
# CSS spec table tests — rationale and mutation notes
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Background for the property tests in `src/css/tables.rs`'s `mod tests`. The
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comments in that file point here for detail beyond the internal-comment cap.
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## tab1_is_a_permutation
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TAB1 is a bijection on 0..256. CSS uses it as an invertible output
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permutation in `css_DecryptKey`'s chained-XOR rounds; if two inputs
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collided, the key mangling would not be invertible.
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Mutation: duplicate any value (e.g. set `TAB1[1] = TAB1[0]`) -> the "maps
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two inputs" assert fires.
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## tab1_known_spec_anchors
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TAB1's fixed structural anchors from the CSS spec table: `TAB1[0x00] ==
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0x33` and the inverse `TAB1[0x33] == 0x00`. These two entries are the
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canonical first-row / inverse-lookup landmarks of the published CSS TAB1
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and pin the table's orientation.
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Grounding: CSS specification TAB1, row 0 col 0 = 0x33; index 0x33 (row 3
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col 3) = 0x00.
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Mutation: change the first literal `0x33` in TAB1 -> first assert fails.
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## tab2_is_a_permutation
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TAB2 is a permutation of 0..256 (it is the LFSR1 high-byte feedback
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substitution). A non-bijective TAB2 would bias the LFSR1 keystream.
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Mutation: set `TAB2[8] = 0x00` (collides with `TAB2[0]`) -> assert fires.
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## tab3_matches_lfsr1_generating_formula
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TAB3 is the CSS LFSR1 low-word table: the 8-value feedback block
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`BASE = [0x00,0x24,0x49,0x6d,0x92,0xb6,0xdb,0xff]` repeated 64 times —
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`TAB3[i] == BASE[i & 7]`. The high bits of the 9-bit index do not affect
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the output (the LFSR1 step indexes with the full 9-bit low register but
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only `& 7` matters). This pins all 512 entries to the published cipher's
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table.
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Mutation: flip any single byte in the TAB3 literal -> the formula check
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fails at that index.
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## tab4_bit_reversal
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TAB4 is the exact bit-reversal of each byte (CSS uses it to permute LFSR0
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bytes on seed and output). `TAB4[b]` reverses b's 8 bits MSB<->LSB.
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Therefore it is also an involution: `TAB4[TAB4[b]] == b`.
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Grounding: `TAB4[0x01]=0x80`, `TAB4[0x80]=0x01`, `TAB4[0x00]=0x00`,
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`TAB4[0xFF]=0xFF`.
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Mutation: set `TAB4[1] = 0x40` (not the reversal `0x80`) -> bit-reversal
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check fails at index 1.
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## tab4_is_a_permutation
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TAB4 is a permutation (bit-reversal is bijective). Distinct from the
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reversal test: a table that is "reversal except two swapped entries" would
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still be a permutation, and a table that is "reversal except one
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duplicated entry" would fail this but might pass a sampled reversal check
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— the two tests pin different failure modes.
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Mutation: set `TAB4[2] = TAB4[1]` -> permutation assert fires.
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## tab5_permutation_anchors
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TAB5 is also a permutation (complement of a bijection is a bijection) and
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its own self-consistency landmark: `TAB5[0x00] == 0xFF` (`TAB4[0]^0xFF`)
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and `TAB5[0xFF] == 0x00` (`TAB4[0xFF]^0xFF`). Pins orientation independent
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of the complement-loop test.
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Mutation: change the first TAB5 literal `0xff -> 0xfe` -> the landmark and
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permutation checks both catch it.
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