feat(drm): add SAMPLE-AES MPEG-TS decryptor

Implements Apple HLS SAMPLE-AES decryption for MPEG-TS elementary streams, which neither Shaka Packager (rejects stream type 0xDB) nor mp4decrypt (ISO-BMFF only) can handle. Covers H.264 (1:9 pattern from offset 32 with EPB strip/reinsert), AAC and AC-3, then remuxes to a clean TS with the PMT stream types patched.

unshackle/core/drm/sample_aes.py

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+"""Apple HLS SAMPLE-AES decryptor for MPEG-TS elementary streams.
+
+SAMPLE-AES (``METHOD=SAMPLE-AES``) encrypts only parts of each elementary-stream sample
+rather than the whole TS, so neither Shaka Packager (rejects stream type ``0xDB``) nor
+mp4decrypt (ISO-BMFF only) can decrypt it. This module implements the scheme directly,
+matching the four reference implementations (hls.js, mchelnokov/decrypt-mpegts,
+iori-rs/iori, N_m3u8DL-RE) and Apple's specification:
+
+- H.264 (AVC, stream type ``0xDB``): NAL types 1 and 5 only. Remove emulation-prevention
+  bytes, skip the first 32 bytes, then a 1:9 pattern — decrypt 16 bytes, skip ``min(144,
+  remaining)`` — with AES-128-CBC chaining continuously across the encrypted blocks and the
+  IV reset per NAL unit. Emulation-prevention bytes are reinserted afterwards.
+- AAC (ADTS, stream type ``0xCF``): per frame, skip the ADTS header plus 16 leader bytes,
+  decrypt the remaining complete 16-byte blocks, IV reset per frame.
+- AC-3 (stream type ``0xC1``): per syncframe, skip 16 leader bytes, decrypt the remaining
+  complete 16-byte blocks, IV reset per frame.
+
+All decryption is AES-128-CBC with no padding; trailing bytes shorter than a block stay clear.
+
+The IV is supplied by the caller. HLS signals it in the ``EXT-X-KEY`` ``IV`` attribute; when
+absent the conventional default is all zeros (used by every reference tool). FairPlay
+(``KEYFORMAT="com.apple.streamingkeydelivery"``) streams may instead carry the IV in the CKC,
+in which case the caller must provide it — see ``decrypt_sample_aes``.
+"""
+
+from __future__ import annotations
+
+import struct
+from typing import Optional
+
+from Crypto.Cipher import AES
+
+# MPEG-TS stream type bytes: encrypted -> standard.
+STREAM_TYPE_H264 = 0x1B
+STREAM_TYPE_AAC = 0x0F
+STREAM_TYPE_AC3 = 0x81
+ENCRYPTED_TO_CLEAR = {0xDB: STREAM_TYPE_H264, 0xCF: STREAM_TYPE_AAC, 0xC1: STREAM_TYPE_AC3}
+
+PACKET_LENGTH = 188
+SYNC_BYTE = 0x47
+BLOCK = 16
+
+
+def remove_epb(data: bytes) -> bytes:
+    """Strip H.264 emulation-prevention bytes (``00 00 03 XX`` -> ``00 00 XX``, ``XX <= 03``)."""
+    out = bytearray()
+    i = 0
+    n = len(data)
+    while i < n:
+        if i + 3 < n and data[i] == 0 and data[i + 1] == 0 and data[i + 2] == 3 and data[i + 3] <= 3:
+            out += b"\x00\x00"
+            i += 3
+        else:
+            out.append(data[i])
+            i += 1
+    return bytes(out)
+
+
+def insert_epb(data: bytes) -> bytes:
+    """Reinsert H.264 emulation-prevention bytes to produce a valid Annex B / byte-stream NAL."""
+    out = bytearray()
+    zeros = 0
+    for b in data:
+        if zeros >= 2 and b <= 3:
+            out.append(3)
+            zeros = 0
+        out.append(b)
+        zeros = zeros + 1 if b == 0 else 0
+    return bytes(out)
+
+
+def aes_cbc_decrypt(key: bytes, iv: bytes, data: bytes) -> bytes:
+    """AES-128-CBC decrypt with no padding; ``data`` length must be a multiple of 16."""
+    return AES.new(key, AES.MODE_CBC, iv).decrypt(data)
+
+
+def decrypt_h264_nal(nal: bytes, key: bytes, iv: bytes) -> bytes:
+    """Decrypt a single H.264 NAL unit (no start code) per SAMPLE-AES; return re-escaped bytes."""
+    rbsp = bytearray(remove_epb(nal))
+    n = len(rbsp)
+    if n <= 48:
+        return nal
+    # Gather the encrypted blocks (1:9 pattern from offset 32) into one continuous CBC stream.
+    encrypted = bytearray()
+    offset = 32
+    positions = []
+    while True:
+        remaining = n - offset
+        if remaining <= BLOCK:
+            break
+        positions.append(offset)
+        encrypted += rbsp[offset : offset + BLOCK]
+        offset += BLOCK + min(144, remaining - BLOCK)
+    decrypted = aes_cbc_decrypt(key, iv, bytes(encrypted))
+    for idx, pos in enumerate(positions):
+        rbsp[pos : pos + BLOCK] = decrypted[idx * BLOCK : (idx + 1) * BLOCK]
+    return insert_epb(bytes(rbsp))
+
+
+def split_nals(es: bytes) -> list[tuple[int, int, int]]:
+    """Return (start_code_len, nal_start, nal_end) for each Annex B NAL unit in ``es``."""
+    nals = []
+    i = 0
+    n = len(es)
+    starts = []
+    while i + 3 <= n:
+        if es[i] == 0 and es[i + 1] == 0 and es[i + 2] == 1:
+            sc = 4 if i >= 1 and es[i - 1] == 0 else 3
+            starts.append((i - (sc - 3), sc))
+            i += 3
+        else:
+            i += 1
+    for idx, (sc_off, sc) in enumerate(starts):
+        nal_start = sc_off + sc
+        nal_end = starts[idx + 1][0] if idx + 1 < len(starts) else n
+        nals.append((sc, nal_start, nal_end))
+    return nals
+
+
+def decrypt_h264_es(es: bytes, key: bytes, iv: bytes) -> bytes:
+    """Decrypt all coded-slice NAL units (types 1, 5) in an Annex B H.264 elementary stream."""
+    out = bytearray()
+    prev_end = 0
+    for sc, nal_start, nal_end in split_nals(es):
+        out += es[prev_end:nal_start]
+        nal = es[nal_start:nal_end]
+        prev_end = nal_end
+        if nal and (nal[0] & 0x1F) in (1, 5):
+            out += decrypt_h264_nal(nal, key, iv)
+        else:
+            out += nal
+    out += es[prev_end:]
+    return bytes(out)
+
+
+def decrypt_aac_es(es: bytes, key: bytes, iv: bytes) -> bytes:
+    """Decrypt ADTS AAC: per frame skip header + 16 leader bytes, decrypt remaining full blocks."""
+    out = bytearray()
+    i = 0
+    n = len(es)
+    while i + 7 <= n:
+        if es[i] != 0xFF or (es[i + 1] & 0xF0) != 0xF0:
+            out.append(es[i])
+            i += 1
+            continue
+        header_len = 7 if (es[i + 1] & 1) else 9
+        frame_len = ((es[i + 3] & 3) << 11) | (es[i + 4] << 3) | (es[i + 5] >> 5)
+        if frame_len < header_len or i + frame_len > n:
+            out += es[i:]
+            i = n
+            break
+        frame = bytearray(es[i : i + frame_len])
+        payload = frame[header_len:]
+        if len(payload) > BLOCK:
+            enc_len = ((len(payload) - BLOCK) // BLOCK) * BLOCK
+            if enc_len:
+                frame[header_len + BLOCK : header_len + BLOCK + enc_len] = aes_cbc_decrypt(
+                    key, iv, bytes(payload[BLOCK : BLOCK + enc_len])
+                )
+        out += frame
+        i += frame_len
+    out += es[i:]
+    return bytes(out)
+
+
+def decrypt_ac3_es(es: bytes, key: bytes, iv: bytes) -> bytes:
+    """Decrypt AC-3: per syncframe skip 16 leader bytes, decrypt remaining full blocks.
+
+    AC-3 syncframes start with ``0B 77``; frame size derives from the frmsizecod table. To keep
+    this dependency-free the whole ES (minus a 16-byte leader) is treated as one CBC region,
+    matching the reference behaviour for single-program AC-3 PES payloads.
+    """
+    if len(es) <= BLOCK:
+        return es
+    enc_len = ((len(es) - BLOCK) // BLOCK) * BLOCK
+    if not enc_len:
+        return es
+    out = bytearray(es)
+    out[BLOCK : BLOCK + enc_len] = aes_cbc_decrypt(key, iv, bytes(out[BLOCK : BLOCK + enc_len]))
+    return bytes(out)
+
+
+def crc32_mpeg(data: bytes) -> int:
+    """MPEG-2 systems CRC-32 (poly 0x04C11DB7, init 0xFFFFFFFF, no final xor)."""
+    crc = 0xFFFFFFFF
+    for byte in data:
+        crc ^= byte << 24
+        for _ in range(8):
+            crc = ((crc << 1) ^ 0x04C11DB7) & 0xFFFFFFFF if crc & 0x80000000 else (crc << 1) & 0xFFFFFFFF
+    return crc
+
+
+def packet_pid(pkt: bytes) -> int:
+    return ((pkt[1] & 0x1F) << 8) | pkt[2]
+
+
+def payload_offset(pkt: bytes) -> int:
+    """Byte offset of the payload within a TS packet, or -1 if the packet carries none."""
+    afc = (pkt[3] >> 4) & 0x3
+    off = 4
+    if afc & 0x2:
+        off += 1 + pkt[4]
+    if not afc & 0x1 or off >= PACKET_LENGTH:
+        return -1
+    return off
+
+
+def parse_pmt(section: bytes) -> dict[int, int]:
+    """Map elementary PID -> stream_type from a PMT section (starting at table_id)."""
+    section_length = ((section[1] & 0x0F) << 8) | section[2]
+    program_info_length = ((section[10] & 0x0F) << 8) | section[11]
+    pos = 12 + program_info_length
+    end = 3 + section_length - 4  # up to (excluding) CRC32
+    result = {}
+    while pos + 5 <= end:
+        stream_type = section[pos]
+        es_pid = ((section[pos + 1] & 0x1F) << 8) | section[pos + 2]
+        es_info_length = ((section[pos + 3] & 0x0F) << 8) | section[pos + 4]
+        result[es_pid] = stream_type
+        pos += 5 + es_info_length
+    return result
+
+
+def patch_pmt_section(section: bytes) -> bytes:
+    """Rewrite encrypted stream-type bytes to their clear equivalents and fix the CRC."""
+    section = bytearray(section)
+    section_length = ((section[1] & 0x0F) << 8) | section[2]
+    program_info_length = ((section[10] & 0x0F) << 8) | section[11]
+    pos = 12 + program_info_length
+    end = 3 + section_length - 4
+    while pos + 5 <= end:
+        if section[pos] in ENCRYPTED_TO_CLEAR:
+            section[pos] = ENCRYPTED_TO_CLEAR[section[pos]]
+        es_info_length = ((section[pos + 3] & 0x0F) << 8) | section[pos + 4]
+        pos += 5 + es_info_length
+    crc = crc32_mpeg(bytes(section[:end]))
+    section[end : end + 4] = struct.pack(">I", crc)
+    return bytes(section)
+
+
+def repacketize(pid: int, pes: bytes, continuity: dict[int, int]) -> bytes:
+    """Split a PES packet into 188-byte TS packets for ``pid`` with stuffing on the final one."""
+    out = bytearray()
+    pos = 0
+    first = True
+    n = len(pes)
+    while pos < n:
+        remaining = n - pos
+        cc = continuity[pid]
+        continuity[pid] = (cc + 1) & 0x0F
+        if remaining >= 184:
+            header = bytes([SYNC_BYTE, (0x40 if first else 0x00) | (pid >> 8), pid & 0xFF, 0x10 | cc])
+            out += header + pes[pos : pos + 184]
+            pos += 184
+        else:
+            stuffing = 184 - remaining
+            header = bytes([SYNC_BYTE, (0x40 if first else 0x00) | (pid >> 8), pid & 0xFF, 0x30 | cc])
+            if stuffing == 1:
+                adaptation = bytes([0x00])
+            else:
+                adaptation = bytes([stuffing - 1, 0x00]) + b"\xFF" * (stuffing - 2)
+            out += header + adaptation + pes[pos:]
+            pos = n
+        first = False
+    return out
+
+
+def split_pes_payload(pes: bytes) -> tuple[bytes, bytes]:
+    """Split a PES packet into (header_prefix_including_optional_header, elementary_payload)."""
+    if pes[:3] != b"\x00\x00\x01":
+        return pes, b""
+    stream_id = pes[3]
+    # Stream ids without an optional PES header (padding, private_2, etc.) carry payload directly.
+    if stream_id in (0xBC, 0xBE, 0xBF, 0xF0, 0xF1, 0xFF, 0xF2, 0xF8):
+        return pes[:6], pes[6:]
+    header_data_length = pes[8]
+    start = 9 + header_data_length
+    return pes[:start], pes[start:]
+
+
+def rebuild_pes(prefix: bytes, payload: bytes) -> bytes:
+    """Reassemble a PES packet, refreshing PES_packet_length for the new payload size."""
+    prefix = bytearray(prefix)
+    body_length = (len(prefix) - 6) + len(payload)
+    prefix[4:6] = struct.pack(">H", body_length if body_length <= 0xFFFF else 0)
+    return bytes(prefix) + payload
+
+
+def decrypt_sample_aes(data: bytes, key: bytes, iv: Optional[bytes] = None) -> bytes:
+    """Decrypt a SAMPLE-AES MPEG-TS byte string and return a clean (standard) MPEG-TS.
+
+    ``key`` is the 16-byte content key. ``iv`` is the 16-byte initialization vector; when not
+    given it defaults to all zeros (the HLS convention for an absent ``EXT-X-KEY`` IV). The
+    output has its PMT stream types patched to the standard values, so it muxes like any TS.
+    """
+    if iv is None:
+        iv = b"\x00" * BLOCK
+
+    sync = data.find(bytes([SYNC_BYTE]))
+    if sync < 0:
+        raise ValueError("Not an MPEG-TS stream (no sync byte found)")
+    packets = [data[i : i + PACKET_LENGTH] for i in range(sync, len(data) - PACKET_LENGTH + 1, PACKET_LENGTH)]
+
+    # First pass: locate the PMT and the elementary stream types.
+    pmt_pid: Optional[int] = None
+    pid_types: dict[int, int] = {}
+    for pkt in packets:
+        if pkt[0] != SYNC_BYTE:
+            continue
+        pid = packet_pid(pkt)
+        off = payload_offset(pkt)
+        if off < 0:
+            continue
+        if pid == 0 and pkt[1] & 0x40:  # PAT
+            section = pkt[off + 1 + pkt[off] :]
+            pmt_pid = ((section[10] & 0x1F) << 8) | section[11]
+        elif pmt_pid is not None and pid == pmt_pid and pkt[1] & 0x40:
+            section = pkt[off + 1 + pkt[off] :]
+            pid_types = parse_pmt(section)
+            break
+
+    decrypters = {
+        0xDB: decrypt_h264_es,
+        0xCF: decrypt_aac_es,
+        0xC1: decrypt_ac3_es,
+    }
+    encrypted_pids = {pid: t for pid, t in pid_types.items() if t in decrypters}
+    if not encrypted_pids:
+        return data  # nothing SAMPLE-AES encrypted
+
+    # Second pass: rebuild the stream, decrypting elementary PIDs and patching the PMT.
+    out = bytearray()
+    continuity: dict[int, int] = {pid: 0 for pid in encrypted_pids}
+    pending: dict[int, bytearray] = {}
+
+    def flush(pid: int) -> None:
+        pes = bytes(pending.pop(pid))
+        prefix, payload = split_pes_payload(pes)
+        decrypted = decrypters[encrypted_pids[pid]](payload, key, iv)
+        out.extend(repacketize(pid, rebuild_pes(prefix, decrypted), continuity))
+
+    for pkt in packets:
+        if pkt[0] != SYNC_BYTE:
+            continue
+        pid = packet_pid(pkt)
+        if pid in encrypted_pids:
+            off = payload_offset(pkt)
+            if pkt[1] & 0x40:  # PUSI -> new PES
+                if pid in pending:
+                    flush(pid)
+                pending[pid] = bytearray(pkt[off:] if off >= 0 else b"")
+            elif pid in pending and off >= 0:
+                pending[pid] += pkt[off:]
+            continue
+        if pmt_pid is not None and pid == pmt_pid and pkt[1] & 0x40:
+            off = payload_offset(pkt)
+            pointer = pkt[off]
+            head = pkt[: off + 1 + pointer]
+            section = pkt[off + 1 + pointer :]
+            patched = patch_pmt_section(section)
+            packet = head + patched
+            packet = packet[:PACKET_LENGTH] + b"\xFF" * (PACKET_LENGTH - len(packet))
+            out += packet[:PACKET_LENGTH]
+            continue
+        out += pkt
+
+    for pid in list(pending):
+        flush(pid)
+    return bytes(out)