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Promote AACS unit encryption to real API, and assert decrypt byte-exactly

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encrypt_unit becomes public library API rather than a #[cfg(test)] helper.
Authoring an encrypted disc image is a legitimate use of this crate, and
the capability was already written four times over: a pub(crate) test-only
copy in aacs/content.rs plus three hand-rolled duplicates in decrypt.rs,
sector/decrypting.rs and disc/extract.rs. All four now call one function,
removing ~110 lines of duplicated cipher code that could drift from
decrypt_unit independently.

It mirrors decrypt_unit's purity contract: crypto only, no encrypted-flag
handling, because where that flag lives is container-specific (CPI bits in
byte 0 for BD-TS, elsewhere for HD-DVD-PS). Callers set the flag BEFORE
encrypting — bytes 0..16 are the key seed left in plaintext, so touching a
header byte afterwards changes the key a decryptor derives. That footgun is
documented at the function and at every call site.

Two tests pin it: an exact round trip through both directions, and the one
place the pair is deliberately asymmetric — decrypt_unit restores
all-zero-on-disc packets to zero, and the test proves an all-zero plaintext
packet enciphers to non-zero bytes so it is never mistaken for padding.
That asymmetry was previously only prose.

Two decrypt tests were also weaker than their own names:

  * aacs_clear_trailing_partial_passes_through asserted only is_ok(), so a
    mutant corrupting the clear partial while returning Ok passed. It now
    snapshots the buffer and asserts byte equality, matching the
    none_keys_is_noop pattern already in the file.
  * aacs_decorator_decrypts_encrypted_unit_via_map checked only that 0x47
    reappeared at the 192-byte stride, leaving corruption in the other 6112
    bytes undetected. The plaintext is fully known, so it now asserts
    byte-exact recovery against it.

Both were verified red first by mutating the production path.
This commit is contained in:
Matthew Jackson
2026-07-29 17:50:24 -07:00
parent f76688a0dc
commit d09ed76e07
6 changed files with 147 additions and 112 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/disc/extract.rs
+5 -36
View File
@@ -1052,43 +1052,12 @@ mod tests {
s
}
/// Build a clear 6144-byte AACS unit (TS syncs at the 192-byte BD-TS
/// stride) then encrypt it under `unit_key` so `aacs::content::decrypt_unit`
/// recovers it cleanly (zero decrypt loss). Mirrors the encrypt helper in
/// `sector/decrypting.rs` tests. `tag` distinguishes two units' payloads.
/// Encrypt the clear unit from `clear_aacs_unit(tag)` under `unit_key` so
/// `aacs::content::decrypt_unit` recovers it cleanly (zero decrypt loss).
/// `tag` distinguishes two units' payloads.
fn encrypt_aacs_unit(unit_key: &[u8; 16], tag: u8) -> Vec<u8> {
use aes::Aes128;
use aes::cipher::{BlockEncrypt, KeyInit, generic_array::GenericArray};
let mut unit = vec![0u8; crate::aacs::content::ALIGNED_UNIT_LEN];
let mut off = 4;
while off < unit.len() {
unit[off] = 0x47; // TS sync
if off + 1 < unit.len() {
unit[off + 1] = tag; // payload marker so the two extents differ
}
off += 192;
}
// Flag encrypted via CPI bits (byte 0) before key derivation.
unit[0] |= 0xC0;
let header: [u8; 16] = unit[..16].try_into().unwrap();
let derived = crate::aacs::crypto::aes_ecb_encrypt(unit_key, &header);
let mut k = [0u8; 16];
for i in 0..16 {
k[i] = derived[i] ^ header[i];
}
let cipher = Aes128::new(GenericArray::from_slice(&k));
let mut prev = crate::aacs::crypto::AACS_IV;
let blocks = (crate::aacs::content::ALIGNED_UNIT_LEN - 16) / 16;
for i in 0..blocks {
let o = 16 + i * 16;
for j in 0..16 {
unit[o + j] ^= prev[j];
}
let mut blk = GenericArray::clone_from_slice(&unit[o..o + 16]);
cipher.encrypt_block(&mut blk);
unit[o..o + 16].copy_from_slice(&blk);
prev.copy_from_slice(&unit[o..o + 16]);
}
let mut unit = clear_aacs_unit(tag);
crate::aacs::content::encrypt_unit(&mut unit, unit_key);
unit
}