//! AACS content decryption — AES primitives, unit decryption, bus encryption.

use aes::Aes128;
use aes::cipher::{BlockDecrypt, BlockEncrypt, KeyInit, generic_array::GenericArray};

// ── AACS constants ──────────────────────────────────────────────────────────

/// Fixed IV used by AACS for all AES-CBC operations.
pub(crate) const AACS_IV: [u8; 16] = [
    0x0B, 0xA0, 0xF8, 0xDD, 0xFE, 0xA6, 0x1F, 0xB3, 0xD8, 0xDF, 0x9F, 0x56, 0x6A, 0x05, 0x0F, 0x78,
];

/// Size of an AACS aligned unit (3 × 2048-byte sectors).
pub const ALIGNED_UNIT_LEN: usize = 6144;

/// Size of one sector.
const SECTOR_LEN: usize = 2048;

/// Transport stream packet spacing in Blu-ray m2ts (192 bytes = 4 TP_extra + 188 TS).
const TS_PACKET_LEN: usize = 192;

/// TS sync byte.
const TS_SYNC: u8 = 0x47;

// ── AES primitives ──────────────────────────────────────────────────────────

/// AES-128-ECB encrypt a single 16-byte block.
pub(crate) fn aes_ecb_encrypt(key: &[u8; 16], data: &[u8; 16]) -> [u8; 16] {
    let cipher = Aes128::new(GenericArray::from_slice(key));
    let mut block = GenericArray::clone_from_slice(data);
    cipher.encrypt_block(&mut block);
    let mut out = [0u8; 16];
    out.copy_from_slice(&block);
    out
}

/// AES-128-ECB decrypt a single 16-byte block.
pub(crate) fn aes_ecb_decrypt(key: &[u8; 16], data: &[u8; 16]) -> [u8; 16] {
    let cipher = Aes128::new(GenericArray::from_slice(key));
    let mut block = GenericArray::clone_from_slice(data);
    cipher.decrypt_block(&mut block);
    let mut out = [0u8; 16];
    out.copy_from_slice(&block);
    out
}

/// AES-128-CBC decrypt in-place with the fixed AACS IV.
///
/// Precondition: `data.len()` is a multiple of 16. Any trailing partial
/// block is silently ignored; all callers pass aligned regions (6128 and
/// 2032 bytes), and the assert documents/enforces that contract.
pub(crate) fn aes_cbc_decrypt(key: &[u8; 16], data: &mut [u8]) {
    debug_assert!(
        data.len() % 16 == 0,
        "aes_cbc_decrypt requires a block-aligned slice"
    );
    let cipher = Aes128::new(GenericArray::from_slice(key));
    let num_blocks = data.len() / 16;
    // Process blocks in reverse to avoid clobbering ciphertext needed for XOR
    for i in (0..num_blocks).rev() {
        let offset = i * 16;
        let prev = if i == 0 {
            AACS_IV
        } else {
            let mut p = [0u8; 16];
            p.copy_from_slice(&data[(i - 1) * 16..i * 16]);
            p
        };
        let mut block = GenericArray::clone_from_slice(&data[offset..offset + 16]);
        cipher.decrypt_block(&mut block);
        for j in 0..16 {
            data[offset + j] = block[j] ^ prev[j];
        }
    }
}

// ── Content decryption ──────────────────────────────────────────────────────

/// True if a 6144-byte aligned unit is AACS-scrambled on disc.
///
/// AACS encrypts the unit body, which destroys the MPEG-TS sync bytes (`0x47`)
/// a clear unit carries at offsets 4, 196, 388, … (one per 192-byte source
/// packet). So "scrambled" = "the TS syncs are NOT intact". This is
/// flag-independent: it does NOT read the TP_extra copy-control bits (byte 0)
/// or the TS scrambling-control bits (byte 7) — AACS sets neither reliably
/// across discs/players.
///
/// This is the single shared definition of "encrypted" for the whole ecosystem
/// — libfreemkv's decrypt gate, autorip's sample selection, and the online key
/// service's validation gate all call THIS, so they always agree on what is
/// encrypted. A correctly-decrypted (or natively-clear) unit reports `false`,
/// so the decrypt path never double-decrypts and there is no flag to clear.
pub fn is_aacs_scrambled(unit: &[u8]) -> bool {
    unit.len() >= ALIGNED_UNIT_LEN && !ts_syncs_intact(unit)
}

/// Count the MPEG-TS sync bytes (`0x47`) present at the BD-TS packet stride
/// (offset 4 and every 192 bytes after — 4-byte TP_extra_header + 188-byte
/// TS packet). A clear or correctly-decrypted m2ts unit shows ~one per
/// packet; an encrypted unit, or a non-content unit decrypted under a key
/// that doesn't apply, shows ~none.
pub fn ts_sync_count(unit: &[u8]) -> usize {
    let mut count = 0;
    let mut offset = 4;
    while offset < unit.len() {
        if unit[offset] == TS_SYNC {
            count += 1;
        }
        offset += TS_PACKET_LEN;
    }
    count
}

/// Number of BD-TS packets in the unit — the maximum possible sync count.
pub fn ts_packet_total(unit: &[u8]) -> usize {
    // One sync byte per 192-byte BD-TS packet (at offset 4 of each). The old
    // `(len - 4) / TS_PACKET_LEN + 1` over-counted by one for lengths of the
    // form `4 + k·192`.
    unit.len() / TS_PACKET_LEN
}

fn ts_syncs_intact(unit: &[u8]) -> bool {
    ts_sync_count(unit) > ts_packet_total(unit) / 2
}

/// Verify a decrypted unit looks like clear MPEG-TS (sync bytes intact).
fn verify_ts(unit: &[u8]) -> bool {
    ts_syncs_intact(unit)
}

/// Decrypt one AACS aligned unit (6144 bytes) in-place.
/// Returns true if decryption succeeded (verified by TS sync bytes).
///
/// Algorithm:
/// 1. AES-128-ECB encrypt first 16 bytes with unit_key → derived
/// 2. XOR derived with original 16 bytes → unit_decrypt_key
/// 3. AES-128-CBC decrypt bytes 16..6143 with unit_decrypt_key and AACS IV
///
/// Decryption restores the TS sync bytes, so the unit reads as clear afterward;
/// there is no flag to clear.
pub fn decrypt_unit(unit: &mut [u8], unit_key: &[u8; 16]) -> bool {
    if unit.len() < ALIGNED_UNIT_LEN {
        return false;
    }
    if !is_aacs_scrambled(unit) {
        return true; // not encrypted
    }

    // Save original first 16 bytes (they're plaintext TP_extra_header)
    let mut header = [0u8; 16];
    header.copy_from_slice(&unit[..16]);

    // Step 1: Encrypt header with unit key to derive per-unit key
    let derived = aes_ecb_encrypt(unit_key, &header);

    // Step 2: XOR to get the actual decryption key
    let mut decrypt_key = [0u8; 16];
    for i in 0..16 {
        decrypt_key[i] = derived[i] ^ header[i];
    }

    // Step 3: Decrypt bytes 16..6143 with AES-CBC
    aes_cbc_decrypt(&decrypt_key, &mut unit[16..ALIGNED_UNIT_LEN]);

    // Decryption restored the TS syncs; verify the unit now looks like clear TS.
    verify_ts(unit)
}

/// Fast, NON-MUTATING unit-key validation for the brute-force key search.
///
/// `decrypt_unit` pays a full 6128-byte (383-block) CBC decrypt before
/// `verify_ts` can reject a wrong key — but in a brute scan ~every candidate is
/// wrong. In CBC the plaintext of block *i* is `AES_dec(C_i) XOR C_{i-1}`, so
/// the FIRST restored TS sync byte (payload offset 196, which lands in CBC
/// block 11 of the `unit[16..]` region) can be recovered with a SINGLE block
/// decrypt instead of 383. A wrong key fails this 1-byte gate ~255/256 of the
/// time for the cost of one AES block; the rare survivor is then confirmed with
/// the full [`decrypt_unit`], so the set of accepted keys is bit-for-bit
/// identical to the slow path.
///
/// The caller MUST pass an aligned, already-[`is_aacs_scrambled`] unit
/// (`unit.len() >= ALIGNED_UNIT_LEN`). The brute pre-filters its units, so the
/// per-candidate scramble re-scan is intentionally skipped here.
///
/// NOTE: this is a search accelerator — it never writes the input and never
/// participates in the content decrypt path. Aggregate correctness (does a key
/// validate against *any* of the disc's units) is preserved because a true key
/// restores offset-196 on every standard BD-TS unit.
pub fn unit_key_validates(unit: &[u8], unit_key: &[u8; 16]) -> bool {
    if unit.len() < ALIGNED_UNIT_LEN {
        return false;
    }
    // Per-unit decrypt key: AES-ECB-encrypt the 16-byte plaintext header with
    // the unit key, XOR with the header (same derivation as `decrypt_unit`).
    let mut header = [0u8; 16];
    header.copy_from_slice(&unit[..16]);
    let derived = aes_ecb_encrypt(unit_key, &header);
    let mut decrypt_key = [0u8; 16];
    for i in 0..16 {
        decrypt_key[i] = derived[i] ^ header[i];
    }

    // Cheap gate: recover ONLY payload byte 196 (the 2nd BD-TS packet's sync).
    // The CBC region is `unit[16..]`; payload offset 196 → region offset 180 =
    // block 11, byte 4. P[11] = AES_dec(C[11]) XOR C[10]; C[10] is raw
    // ciphertext (no decrypt needed). Constant offsets for the fixed 6144 unit.
    const SYNC_PAYLOAD_OFF: usize = 196;
    let region_off = SYNC_PAYLOAD_OFF - 16; // 180
    let blk = region_off / 16; // 11
    let byte = region_off % 16; // 4
    let c11 = 16 + blk * 16; // absolute offset of C[11] in `unit` (=192)
    let cipher = Aes128::new(GenericArray::from_slice(&decrypt_key));
    let mut b = GenericArray::clone_from_slice(&unit[c11..c11 + 16]);
    cipher.decrypt_block(&mut b);
    let prev = unit[c11 - 16 + byte]; // C[10] byte (region block 10)
    if b[byte] ^ prev != TS_SYNC {
        return false;
    }

    // Survivor (~1/256 of candidates): confirm with the authoritative full
    // decrypt + verify, so the verdict matches `decrypt_unit` exactly.
    let mut full = [0u8; ALIGNED_UNIT_LEN];
    full.copy_from_slice(&unit[..ALIGNED_UNIT_LEN]);
    decrypt_unit(&mut full, unit_key)
}

/// Outcome of [`decrypt_unit_try_keys`].
///
/// Distinguishes "the unit was already clear, no key was consumed" from "key
/// at index `i` decrypted it" — the bare `Option<usize>` form conflated the two
/// (a clear unit reported `Some(0)`, indistinguishable from key index 0, and
/// possibly out of range when `unit_keys` is empty).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum UnitKeyResult {
    /// The unit was not scrambled; it was left untouched and no key was used.
    AlreadyClear,
    /// The unit was decrypted in place by `unit_keys[index]`.
    DecryptedWith(usize),
}

/// Decrypt one aligned unit trying multiple unit keys.
///
/// Returns [`UnitKeyResult::AlreadyClear`] if the unit was not scrambled (no key
/// consumed), [`UnitKeyResult::DecryptedWith(i)`] if key `i` decrypted it, or
/// `None` if no key worked (the unit is restored to its original bytes).
pub fn decrypt_unit_try_keys(unit: &mut [u8], unit_keys: &[[u8; 16]]) -> Option<UnitKeyResult> {
    if !is_aacs_scrambled(unit) {
        return Some(UnitKeyResult::AlreadyClear);
    }

    // Save original for retry. Stack-backed buffer — no heap allocation, and the
    // restore-on-failure contract holds uniformly regardless of key count.
    let mut original = [0u8; ALIGNED_UNIT_LEN];
    original.copy_from_slice(&unit[..ALIGNED_UNIT_LEN]);

    for (i, key) in unit_keys.iter().enumerate() {
        unit[..ALIGNED_UNIT_LEN].copy_from_slice(&original);
        if decrypt_unit(unit, key) {
            return Some(UnitKeyResult::DecryptedWith(i));
        }
    }

    // Restore original on failure
    unit[..ALIGNED_UNIT_LEN].copy_from_slice(&original);
    None
}

/// Remove bus encryption from an aligned unit (AACS 2.0 / UHD).
/// Bus encryption uses read_data_key, decrypting bytes 16..2047 of each 2048-byte sector.
pub fn decrypt_bus(unit: &mut [u8], read_data_key: &[u8; 16]) {
    for sector_start in (0..ALIGNED_UNIT_LEN).step_by(SECTOR_LEN) {
        if sector_start + SECTOR_LEN > unit.len() {
            break;
        }
        // First 16 bytes of each sector are plaintext
        aes_cbc_decrypt(
            read_data_key,
            &mut unit[sector_start + 16..sector_start + SECTOR_LEN],
        );
    }
}

/// Full decrypt of an aligned unit: bus decrypt (if needed) then AACS decrypt.
pub fn decrypt_unit_full(
    unit: &mut [u8],
    unit_key: &[u8; 16],
    read_data_key: Option<&[u8; 16]>,
) -> bool {
    if !is_aacs_scrambled(unit) {
        return true;
    }
    if let Some(rdk) = read_data_key {
        decrypt_bus(unit, rdk);
    }
    decrypt_unit(unit, unit_key)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_aes_ecb_roundtrip() {
        let key = [
            0x15u8, 0x66, 0x5F, 0x98, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A,
            0x0B, 0x0C,
        ];
        let plain = [0x41u8; 16];
        let enc = aes_ecb_encrypt(&key, &plain);
        let dec = aes_ecb_decrypt(&key, &enc);
        assert_eq!(dec, plain);
    }

    #[test]
    fn test_decrypt_unit_unencrypted() {
        // A clear unit (TS syncs intact) is not scrambled → passes through.
        let mut unit = vec![0u8; ALIGNED_UNIT_LEN];
        let mut off = 4;
        while off < ALIGNED_UNIT_LEN {
            unit[off] = TS_SYNC;
            off += TS_PACKET_LEN;
        }
        let key = [0u8; 16];
        assert!(!is_aacs_scrambled(&unit));
        assert!(decrypt_unit(&mut unit, &key));
    }

    #[test]
    fn ts_packet_total_no_off_by_one() {
        // The maximum sync count is exactly the number of stride
        // positions the counting loop visits (offset 4, 196, ...), i.e.
        // len / 192, NOT (len - 4) / 192 + 1. For the 6144-byte aligned unit
        // the loop checks offsets 4..=5956 → 32 positions.
        let unit = vec![0u8; ALIGNED_UNIT_LEN];
        assert_eq!(ts_packet_total(&unit), 32);
        // Confirm the loop visits exactly that many stride positions.
        let visited = (4..ALIGNED_UNIT_LEN).step_by(TS_PACKET_LEN).count();
        assert_eq!(visited, ts_packet_total(&unit));
    }

    #[test]
    fn scramble_detection_at_16_32_boundary() {
        // With 32 stride positions the majority threshold is
        // total/2 = 16. A unit with EXACTLY half its syncs intact (16) must
        // NOT be over-counted into the "scrambled" bucket by an inflated
        // total: 16 > 16 is false → not-intact → scrambled. 17 intact → clear.
        // The fix is that `total` is 32 (not 33), so the boundary sits cleanly
        // at the real midpoint.
        let set_syncs = |n: usize| {
            let mut unit = vec![0u8; ALIGNED_UNIT_LEN];
            let mut off = 4;
            let mut placed = 0;
            while off < ALIGNED_UNIT_LEN && placed < n {
                unit[off] = TS_SYNC;
                off += TS_PACKET_LEN;
                placed += 1;
            }
            unit
        };

        assert_eq!(ts_sync_count(&set_syncs(16)), 16);
        assert_eq!(ts_sync_count(&set_syncs(17)), 17);

        // Exactly half intact → classified scrambled (16 > 16 is false).
        assert!(is_aacs_scrambled(&set_syncs(16)));
        // One past half → classified clear.
        assert!(!is_aacs_scrambled(&set_syncs(17)));
    }

    #[test]
    fn scramble_detection_extremes() {
        // Detection semantics for the clear-cut cases must be preserved:
        // a fully-clear unit (all 32 syncs) is NOT scrambled; a unit with no
        // syncs (fully scrambled body) IS scrambled.
        let mut clear = vec![0u8; ALIGNED_UNIT_LEN];
        let mut off = 4;
        while off < ALIGNED_UNIT_LEN {
            clear[off] = TS_SYNC;
            off += TS_PACKET_LEN;
        }
        assert_eq!(ts_sync_count(&clear), 32);
        assert!(
            !is_aacs_scrambled(&clear),
            "fully-clear unit → not scrambled"
        );

        let scrambled = vec![0u8; ALIGNED_UNIT_LEN];
        assert_eq!(ts_sync_count(&scrambled), 0);
        assert!(is_aacs_scrambled(&scrambled), "no syncs → scrambled");
    }

    #[test]
    fn test_aes_cbc_roundtrip() {
        let key = [
            0x11u8, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xAA, 0xBB, 0xCC, 0xDD, 0xEE,
            0xFF, 0x00,
        ];
        let original = vec![0x42u8; 128]; // 8 blocks
        let mut data = original.clone();

        // Encrypt with CBC manually (forward direction)
        fn aes_cbc_encrypt(key: &[u8; 16], data: &mut [u8]) {
            let cipher = Aes128::new(GenericArray::from_slice(key));
            let mut prev = super::AACS_IV;
            let num_blocks = data.len() / 16;
            for i in 0..num_blocks {
                let offset = i * 16;
                for j in 0..16 {
                    data[offset + j] ^= prev[j];
                }
                let mut block = GenericArray::clone_from_slice(&data[offset..offset + 16]);
                cipher.encrypt_block(&mut block);
                data[offset..offset + 16].copy_from_slice(&block);
                prev.copy_from_slice(&data[offset..offset + 16]);
            }
        }

        aes_cbc_encrypt(&key, &mut data);
        assert_ne!(data, original); // should be different after encrypt

        super::aes_cbc_decrypt(&key, &mut data);
        assert_eq!(data, original); // should match after roundtrip
    }

    #[test]
    fn test_decrypt_unit_synthetic() {
        // Build a fake 6144-byte aligned unit with known TS sync pattern,
        // encrypt it with the AACS algorithm, then decrypt and verify.
        let unit_key = [0xAAu8; 16];

        // Build plaintext unit with TS sync bytes every 192 bytes starting at offset 4
        let mut plain = vec![0u8; ALIGNED_UNIT_LEN];
        let mut offset = 4;
        while offset < ALIGNED_UNIT_LEN {
            plain[offset] = TS_SYNC;
            offset += TS_PACKET_LEN;
        }
        // No flag set: CBC-encrypting the body below scrambles packets 1..31's
        // TS syncs, which is exactly what `is_aacs_scrambled` (raw-sync) detects.

        // Now encrypt bytes 16..6143 using the AACS algorithm (reverse of decrypt)
        let header: [u8; 16] = plain[..16].try_into().unwrap();
        let derived = aes_ecb_encrypt(&unit_key, &header);
        let mut encrypt_key = [0u8; 16];
        for i in 0..16 {
            encrypt_key[i] = derived[i] ^ header[i];
        }

        // CBC encrypt bytes 16..6143
        let cipher = Aes128::new(GenericArray::from_slice(&encrypt_key));
        let mut prev = AACS_IV;
        let num_blocks = (ALIGNED_UNIT_LEN - 16) / 16;
        for i in 0..num_blocks {
            let off = 16 + i * 16;
            for j in 0..16 {
                plain[off + j] ^= prev[j];
            }
            let mut block = GenericArray::clone_from_slice(&plain[off..off + 16]);
            cipher.encrypt_block(&mut block);
            plain[off..off + 16].copy_from_slice(&block);
            prev.copy_from_slice(&plain[off..off + 16]);
        }

        // Now plain contains encrypted data. Decrypt it.
        let mut unit = plain;
        assert!(is_aacs_scrambled(&unit));
        assert!(decrypt_unit(&mut unit, &unit_key));
        assert!(!is_aacs_scrambled(&unit)); // decrypted: TS syncs restored

        // Verify TS sync bytes
        let mut count = 0;
        let mut off = 4;
        while off < ALIGNED_UNIT_LEN {
            if unit[off] == TS_SYNC {
                count += 1;
            }
            off += TS_PACKET_LEN;
        }
        // Assert against the single canonical packet count, not the old
        // `(len - 4) / 192 + 1` form that `ts_packet_total` corrected away from.
        assert_eq!(count, ts_packet_total(&unit));
    }
}