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https://github.com/unshackle-dl/unshackle.git
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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.
This commit is contained in:
imSp4rky
369
unshackle/core/drm/sample_aes.py
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369
unshackle/core/drm/sample_aes.py
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"""Apple HLS SAMPLE-AES decryptor for MPEG-TS elementary streams.
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SAMPLE-AES (``METHOD=SAMPLE-AES``) encrypts only parts of each elementary-stream sample
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rather than the whole TS, so neither Shaka Packager (rejects stream type ``0xDB``) nor
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mp4decrypt (ISO-BMFF only) can decrypt it. This module implements the scheme directly,
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matching the four reference implementations (hls.js, mchelnokov/decrypt-mpegts,
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iori-rs/iori, N_m3u8DL-RE) and Apple's specification:
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- H.264 (AVC, stream type ``0xDB``): NAL types 1 and 5 only. Remove emulation-prevention
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bytes, skip the first 32 bytes, then a 1:9 pattern — decrypt 16 bytes, skip ``min(144,
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remaining)`` — with AES-128-CBC chaining continuously across the encrypted blocks and the
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IV reset per NAL unit. Emulation-prevention bytes are reinserted afterwards.
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- AAC (ADTS, stream type ``0xCF``): per frame, skip the ADTS header plus 16 leader bytes,
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decrypt the remaining complete 16-byte blocks, IV reset per frame.
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- AC-3 (stream type ``0xC1``): per syncframe, skip 16 leader bytes, decrypt the remaining
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complete 16-byte blocks, IV reset per frame.
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All decryption is AES-128-CBC with no padding; trailing bytes shorter than a block stay clear.
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The IV is supplied by the caller. HLS signals it in the ``EXT-X-KEY`` ``IV`` attribute; when
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absent the conventional default is all zeros (used by every reference tool). FairPlay
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(``KEYFORMAT="com.apple.streamingkeydelivery"``) streams may instead carry the IV in the CKC,
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in which case the caller must provide it — see ``decrypt_sample_aes``.
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"""
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from __future__ import annotations
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import struct
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from typing import Optional
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from Crypto.Cipher import AES
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# MPEG-TS stream type bytes: encrypted -> standard.
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STREAM_TYPE_H264 = 0x1B
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STREAM_TYPE_AAC = 0x0F
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STREAM_TYPE_AC3 = 0x81
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ENCRYPTED_TO_CLEAR = {0xDB: STREAM_TYPE_H264, 0xCF: STREAM_TYPE_AAC, 0xC1: STREAM_TYPE_AC3}
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PACKET_LENGTH = 188
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SYNC_BYTE = 0x47
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BLOCK = 16
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def remove_epb(data: bytes) -> bytes:
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"""Strip H.264 emulation-prevention bytes (``00 00 03 XX`` -> ``00 00 XX``, ``XX <= 03``)."""
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out = bytearray()
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i = 0
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n = len(data)
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while i < n:
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if i + 3 < n and data[i] == 0 and data[i + 1] == 0 and data[i + 2] == 3 and data[i + 3] <= 3:
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out += b"\x00\x00"
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i += 3
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else:
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out.append(data[i])
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i += 1
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return bytes(out)
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def insert_epb(data: bytes) -> bytes:
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"""Reinsert H.264 emulation-prevention bytes to produce a valid Annex B / byte-stream NAL."""
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out = bytearray()
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zeros = 0
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for b in data:
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if zeros >= 2 and b <= 3:
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out.append(3)
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zeros = 0
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out.append(b)
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zeros = zeros + 1 if b == 0 else 0
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return bytes(out)
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def aes_cbc_decrypt(key: bytes, iv: bytes, data: bytes) -> bytes:
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"""AES-128-CBC decrypt with no padding; ``data`` length must be a multiple of 16."""
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return AES.new(key, AES.MODE_CBC, iv).decrypt(data)
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def decrypt_h264_nal(nal: bytes, key: bytes, iv: bytes) -> bytes:
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"""Decrypt a single H.264 NAL unit (no start code) per SAMPLE-AES; return re-escaped bytes."""
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rbsp = bytearray(remove_epb(nal))
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n = len(rbsp)
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if n <= 48:
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return nal
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# Gather the encrypted blocks (1:9 pattern from offset 32) into one continuous CBC stream.
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encrypted = bytearray()
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offset = 32
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positions = []
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while True:
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remaining = n - offset
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if remaining <= BLOCK:
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break
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positions.append(offset)
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encrypted += rbsp[offset : offset + BLOCK]
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offset += BLOCK + min(144, remaining - BLOCK)
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decrypted = aes_cbc_decrypt(key, iv, bytes(encrypted))
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for idx, pos in enumerate(positions):
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rbsp[pos : pos + BLOCK] = decrypted[idx * BLOCK : (idx + 1) * BLOCK]
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return insert_epb(bytes(rbsp))
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def split_nals(es: bytes) -> list[tuple[int, int, int]]:
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"""Return (start_code_len, nal_start, nal_end) for each Annex B NAL unit in ``es``."""
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nals = []
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i = 0
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n = len(es)
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starts = []
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while i + 3 <= n:
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if es[i] == 0 and es[i + 1] == 0 and es[i + 2] == 1:
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sc = 4 if i >= 1 and es[i - 1] == 0 else 3
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starts.append((i - (sc - 3), sc))
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i += 3
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else:
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i += 1
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for idx, (sc_off, sc) in enumerate(starts):
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nal_start = sc_off + sc
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nal_end = starts[idx + 1][0] if idx + 1 < len(starts) else n
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nals.append((sc, nal_start, nal_end))
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return nals
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def decrypt_h264_es(es: bytes, key: bytes, iv: bytes) -> bytes:
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"""Decrypt all coded-slice NAL units (types 1, 5) in an Annex B H.264 elementary stream."""
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out = bytearray()
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prev_end = 0
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for sc, nal_start, nal_end in split_nals(es):
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out += es[prev_end:nal_start]
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nal = es[nal_start:nal_end]
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prev_end = nal_end
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if nal and (nal[0] & 0x1F) in (1, 5):
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out += decrypt_h264_nal(nal, key, iv)
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else:
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out += nal
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out += es[prev_end:]
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return bytes(out)
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def decrypt_aac_es(es: bytes, key: bytes, iv: bytes) -> bytes:
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"""Decrypt ADTS AAC: per frame skip header + 16 leader bytes, decrypt remaining full blocks."""
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out = bytearray()
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i = 0
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n = len(es)
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while i + 7 <= n:
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if es[i] != 0xFF or (es[i + 1] & 0xF0) != 0xF0:
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out.append(es[i])
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i += 1
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continue
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header_len = 7 if (es[i + 1] & 1) else 9
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frame_len = ((es[i + 3] & 3) << 11) | (es[i + 4] << 3) | (es[i + 5] >> 5)
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if frame_len < header_len or i + frame_len > n:
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out += es[i:]
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i = n
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break
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frame = bytearray(es[i : i + frame_len])
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payload = frame[header_len:]
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if len(payload) > BLOCK:
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enc_len = ((len(payload) - BLOCK) // BLOCK) * BLOCK
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if enc_len:
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frame[header_len + BLOCK : header_len + BLOCK + enc_len] = aes_cbc_decrypt(
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key, iv, bytes(payload[BLOCK : BLOCK + enc_len])
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)
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out += frame
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i += frame_len
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out += es[i:]
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return bytes(out)
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def decrypt_ac3_es(es: bytes, key: bytes, iv: bytes) -> bytes:
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"""Decrypt AC-3: per syncframe skip 16 leader bytes, decrypt remaining full blocks.
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AC-3 syncframes start with ``0B 77``; frame size derives from the frmsizecod table. To keep
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this dependency-free the whole ES (minus a 16-byte leader) is treated as one CBC region,
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matching the reference behaviour for single-program AC-3 PES payloads.
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"""
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if len(es) <= BLOCK:
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return es
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enc_len = ((len(es) - BLOCK) // BLOCK) * BLOCK
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if not enc_len:
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return es
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out = bytearray(es)
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out[BLOCK : BLOCK + enc_len] = aes_cbc_decrypt(key, iv, bytes(out[BLOCK : BLOCK + enc_len]))
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return bytes(out)
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def crc32_mpeg(data: bytes) -> int:
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"""MPEG-2 systems CRC-32 (poly 0x04C11DB7, init 0xFFFFFFFF, no final xor)."""
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crc = 0xFFFFFFFF
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for byte in data:
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crc ^= byte << 24
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for _ in range(8):
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crc = ((crc << 1) ^ 0x04C11DB7) & 0xFFFFFFFF if crc & 0x80000000 else (crc << 1) & 0xFFFFFFFF
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return crc
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def packet_pid(pkt: bytes) -> int:
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return ((pkt[1] & 0x1F) << 8) | pkt[2]
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def payload_offset(pkt: bytes) -> int:
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"""Byte offset of the payload within a TS packet, or -1 if the packet carries none."""
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afc = (pkt[3] >> 4) & 0x3
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off = 4
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if afc & 0x2:
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off += 1 + pkt[4]
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if not afc & 0x1 or off >= PACKET_LENGTH:
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return -1
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return off
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def parse_pmt(section: bytes) -> dict[int, int]:
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"""Map elementary PID -> stream_type from a PMT section (starting at table_id)."""
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section_length = ((section[1] & 0x0F) << 8) | section[2]
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program_info_length = ((section[10] & 0x0F) << 8) | section[11]
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pos = 12 + program_info_length
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end = 3 + section_length - 4 # up to (excluding) CRC32
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result = {}
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while pos + 5 <= end:
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stream_type = section[pos]
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es_pid = ((section[pos + 1] & 0x1F) << 8) | section[pos + 2]
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es_info_length = ((section[pos + 3] & 0x0F) << 8) | section[pos + 4]
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result[es_pid] = stream_type
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pos += 5 + es_info_length
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return result
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def patch_pmt_section(section: bytes) -> bytes:
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"""Rewrite encrypted stream-type bytes to their clear equivalents and fix the CRC."""
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section = bytearray(section)
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section_length = ((section[1] & 0x0F) << 8) | section[2]
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program_info_length = ((section[10] & 0x0F) << 8) | section[11]
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pos = 12 + program_info_length
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end = 3 + section_length - 4
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while pos + 5 <= end:
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if section[pos] in ENCRYPTED_TO_CLEAR:
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section[pos] = ENCRYPTED_TO_CLEAR[section[pos]]
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es_info_length = ((section[pos + 3] & 0x0F) << 8) | section[pos + 4]
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pos += 5 + es_info_length
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crc = crc32_mpeg(bytes(section[:end]))
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section[end : end + 4] = struct.pack(">I", crc)
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return bytes(section)
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def repacketize(pid: int, pes: bytes, continuity: dict[int, int]) -> bytes:
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"""Split a PES packet into 188-byte TS packets for ``pid`` with stuffing on the final one."""
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out = bytearray()
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pos = 0
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first = True
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n = len(pes)
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while pos < n:
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remaining = n - pos
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cc = continuity[pid]
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continuity[pid] = (cc + 1) & 0x0F
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if remaining >= 184:
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header = bytes([SYNC_BYTE, (0x40 if first else 0x00) | (pid >> 8), pid & 0xFF, 0x10 | cc])
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out += header + pes[pos : pos + 184]
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pos += 184
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else:
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stuffing = 184 - remaining
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header = bytes([SYNC_BYTE, (0x40 if first else 0x00) | (pid >> 8), pid & 0xFF, 0x30 | cc])
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if stuffing == 1:
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adaptation = bytes([0x00])
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else:
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adaptation = bytes([stuffing - 1, 0x00]) + b"\xFF" * (stuffing - 2)
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out += header + adaptation + pes[pos:]
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pos = n
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first = False
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return out
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def split_pes_payload(pes: bytes) -> tuple[bytes, bytes]:
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"""Split a PES packet into (header_prefix_including_optional_header, elementary_payload)."""
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if pes[:3] != b"\x00\x00\x01":
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return pes, b""
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stream_id = pes[3]
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# Stream ids without an optional PES header (padding, private_2, etc.) carry payload directly.
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if stream_id in (0xBC, 0xBE, 0xBF, 0xF0, 0xF1, 0xFF, 0xF2, 0xF8):
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return pes[:6], pes[6:]
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header_data_length = pes[8]
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start = 9 + header_data_length
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return pes[:start], pes[start:]
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def rebuild_pes(prefix: bytes, payload: bytes) -> bytes:
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"""Reassemble a PES packet, refreshing PES_packet_length for the new payload size."""
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prefix = bytearray(prefix)
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body_length = (len(prefix) - 6) + len(payload)
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prefix[4:6] = struct.pack(">H", body_length if body_length <= 0xFFFF else 0)
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return bytes(prefix) + payload
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def decrypt_sample_aes(data: bytes, key: bytes, iv: Optional[bytes] = None) -> bytes:
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"""Decrypt a SAMPLE-AES MPEG-TS byte string and return a clean (standard) MPEG-TS.
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``key`` is the 16-byte content key. ``iv`` is the 16-byte initialization vector; when not
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given it defaults to all zeros (the HLS convention for an absent ``EXT-X-KEY`` IV). The
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output has its PMT stream types patched to the standard values, so it muxes like any TS.
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"""
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if iv is None:
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iv = b"\x00" * BLOCK
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sync = data.find(bytes([SYNC_BYTE]))
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if sync < 0:
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raise ValueError("Not an MPEG-TS stream (no sync byte found)")
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packets = [data[i : i + PACKET_LENGTH] for i in range(sync, len(data) - PACKET_LENGTH + 1, PACKET_LENGTH)]
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# First pass: locate the PMT and the elementary stream types.
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pmt_pid: Optional[int] = None
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pid_types: dict[int, int] = {}
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for pkt in packets:
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if pkt[0] != SYNC_BYTE:
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continue
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pid = packet_pid(pkt)
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off = payload_offset(pkt)
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if off < 0:
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continue
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if pid == 0 and pkt[1] & 0x40: # PAT
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section = pkt[off + 1 + pkt[off] :]
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pmt_pid = ((section[10] & 0x1F) << 8) | section[11]
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elif pmt_pid is not None and pid == pmt_pid and pkt[1] & 0x40:
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section = pkt[off + 1 + pkt[off] :]
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pid_types = parse_pmt(section)
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break
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decrypters = {
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0xDB: decrypt_h264_es,
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0xCF: decrypt_aac_es,
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0xC1: decrypt_ac3_es,
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}
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encrypted_pids = {pid: t for pid, t in pid_types.items() if t in decrypters}
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if not encrypted_pids:
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return data # nothing SAMPLE-AES encrypted
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# Second pass: rebuild the stream, decrypting elementary PIDs and patching the PMT.
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out = bytearray()
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continuity: dict[int, int] = {pid: 0 for pid in encrypted_pids}
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pending: dict[int, bytearray] = {}
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def flush(pid: int) -> None:
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pes = bytes(pending.pop(pid))
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prefix, payload = split_pes_payload(pes)
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decrypted = decrypters[encrypted_pids[pid]](payload, key, iv)
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out.extend(repacketize(pid, rebuild_pes(prefix, decrypted), continuity))
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for pkt in packets:
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if pkt[0] != SYNC_BYTE:
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continue
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pid = packet_pid(pkt)
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if pid in encrypted_pids:
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off = payload_offset(pkt)
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if pkt[1] & 0x40: # PUSI -> new PES
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if pid in pending:
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flush(pid)
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pending[pid] = bytearray(pkt[off:] if off >= 0 else b"")
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elif pid in pending and off >= 0:
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pending[pid] += pkt[off:]
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continue
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if pmt_pid is not None and pid == pmt_pid and pkt[1] & 0x40:
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off = payload_offset(pkt)
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pointer = pkt[off]
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head = pkt[: off + 1 + pointer]
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section = pkt[off + 1 + pointer :]
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patched = patch_pmt_section(section)
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packet = head + patched
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packet = packet[:PACKET_LENGTH] + b"\xFF" * (PACKET_LENGTH - len(packet))
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out += packet[:PACKET_LENGTH]
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continue
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out += pkt
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for pid in list(pending):
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flush(pid)
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return bytes(out)
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