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