ccl_simplesnappy.py

"""
Copyright 2020, CCL Forensics

Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
the Software without restriction, including without limitation the rights to
use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
of the Software, and to permit persons to whom the Software is furnished to do
so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
"""

import sys
import struct
import io
import typing
import enum

__version__ = "0.1"
__description__ = "Pure Python reimplementation of Google's Snappy decompression"
__contact__ = "Alex Caithness"


DEBUG = False


def log(msg):
    if DEBUG:
        print(msg)


class ElementType(enum.IntEnum):
    """Run type in the compressed snappy data (literal data or offset to backreferenced data_"""
    Literal = 0
    CopyOneByte = 1
    CopyTwoByte = 2
    CopyFourByte = 3


def _read_le_varint(stream: typing.BinaryIO) -> typing.Optional[typing.Tuple[int, bytes]]:
    """Read varint from a stream.
    If the read is successful: returns a tuple of the (unsigned) value and the raw bytes making up that varint,
    otherwise returns None"""
    # this only outputs unsigned
    i = 0
    result = 0
    underlying_bytes = []
    while i < 10:  # 64 bit max possible?
        raw = stream.read(1)
        if len(raw) < 1:
            return None
        tmp, = raw
        underlying_bytes.append(tmp)
        result |= ((tmp & 0x7f) << (i * 7))
        if (tmp & 0x80) == 0:
            break
        i += 1
    return result, bytes(underlying_bytes)


def read_le_varint(stream: typing.BinaryIO) -> typing.Optional[int]:
    """Convenience version of _read_le_varint that only returns the value or None"""
    x = _read_le_varint(stream)
    if x is None:
        return None
    else:
        return x[0]


def read_uint16(stream: typing.BinaryIO) -> int:
    """Reads a Uint16 from stream"""
    return struct.unpack("<H", stream.read(2))[0]


def read_uint24(stream: typing.BinaryIO) -> int:
    """Reads a Uint24 from stream"""
    return struct.unpack("<I", stream.read(3) + b"\x00")[0]


def read_uint32(stream: typing.BinaryIO) -> int:
    """Reads a Uint32 from stream"""
    return struct.unpack("<I", stream.read(4))[0]


def read_byte(stream: typing.BinaryIO) -> typing.Optional[int]:
    """Reads a single byte from stream (or returns None if EOD is met)"""
    x = stream.read(1)
    if x:
        return x[0]

    return None


def decompress(data: typing.BinaryIO) -> bytes:
    """Decompresses the snappy compressed data stream"""
    uncompressed_length = read_le_varint(data)
    log(f"Uncompressed length: {uncompressed_length}")

    out = io.BytesIO()

    while True:
        start_offset = data.tell()
        log(f"Reading tag at offset {start_offset}")
        type_byte = read_byte(data)
        if type_byte is None:
            break

        log(f"Type Byte is {type_byte:02x}")

        tag = type_byte & 0x03

        log(f"Element Type is: {ElementType(tag)}")

        if tag == ElementType.Literal:
            if ((type_byte & 0xFC) >> 2) < 60:  # embedded in tag
                length = 1 + ((type_byte & 0xFC) >> 2)
                log(f"Literal length is embedded in type byte and is {length}")
            elif ((type_byte & 0xFC) >> 2) == 60:  # 8 bit
                length = 1 + read_byte(data)
                log(f"Literal length is 8bit and is {length}")
            elif ((type_byte & 0xFC) >> 2) == 61:  # 16 bit
                length = 1 + read_uint16(data)
                log(f"Literal length is 16bit and is {length}")
            elif ((type_byte & 0xFC) >> 2) == 62:  # 16 bit
                length = 1 + read_uint24(data)
                log(f"Literal length is 24bit and is {length}")
            elif ((type_byte & 0xFC) >> 2) == 63:  # 16 bit
                length = 1 + read_uint32(data)
                log(f"Literal length is 32bit and is {length}")
            else:
                raise ValueError()  # cannot ever happen

            literal_data = data.read(length)
            if len(literal_data) < length:
                raise ValueError("Couldn't read enough literal data")

            out.write(literal_data)

        else:
            if tag == ElementType.CopyOneByte:
                length = ((type_byte & 0x1C) >> 2) + 4
                offset = ((type_byte & 0xE0) << 3) | read_byte(data)
            elif tag == ElementType.CopyTwoByte:
                length = 1 + ((type_byte & 0xFC) >> 2)
                offset = read_uint16(data)
            elif tag == ElementType.CopyFourByte:
                length = 1 + ((type_byte & 0xFC) >> 2)
                offset = read_uint32(data)
            else:
                raise ValueError()  # cannot ever happen

            if offset == 0:
                raise ValueError("Offset cannot be 0")

            actual_offset = out.tell() - offset
            log(f"Current Outstream Length: {out.tell()}")
            log(f"Backreference length: {length}")
            log(f"Backreference relative offset: {offset}")
            log(f"Backreference absolute offset: {actual_offset}")

            # have to read incrementally because you might have to read data that you've just written
            # this is probably a really slow way of doing this.
            for i in range(length):
                out.write(out.getbuffer()[actual_offset + i: actual_offset + i + 1].tobytes())

    result = out.getvalue()
    if uncompressed_length != len(result):
        raise ValueError("Wrong data length in uncompressed data")
        # TODO: allow a partial / potentially bad result via a flag in the function call?

    return result


def main(path):
    import pathlib
    import hashlib
    f = pathlib.Path(path).open("rb")
    decompressed = decompress(f)
    print(decompressed)
    sha1 = hashlib.sha1()
    sha1.update(decompressed)
    print(sha1.hexdigest())


if __name__ == "__main__":
    main(sys.argv[1])