将 python long/int 转换为固定大小的字节数组 [英] Convert python long/int to fixed size byte array

问题描述

我正在尝试在 python 中实现 RC4 和 DH 密钥交换.问题是我不知道如何将 python long/int 从密钥交换转换为 RC4 实现所需的字节数组.有没有一种简单的方法可以将 long 转换为所需长度的字节数组?

I'm trying to implement RC4 and DH key exchange in python. Problem is that I have no idea about how to convert the python long/int from the key exchange to the byte array I need for the RC4 implementation. Is there a simple way to convert a long to the required length byte array?

更新:忘记提及我正在处理的数字是 768 位无符号整数.

Update: forgot to mention that the numbers I'm dealing with are 768 bit unsigned integers.

推荐答案

我没有做过任何基准测试，但这个配方对我有用".

I haven't done any benchmarks, but this recipe "works for me".

简短版本:使用 '%x' % val，然后 unhexlify 结果.然而，问题在于细节，因为 unhexlify 需要偶数个十六进制数字，而 %x 不能保证.有关详细信息，请参阅文档字符串和自由内联注释.

The short version: use '%x' % val, then unhexlify the result. The devil is in the details, though, as unhexlify requires an even number of hex digits, which %x doesn't guarantee. See the docstring, and the liberal inline comments for details.

from binascii import unhexlify

def long_to_bytes (val, endianness='big'):
    """
    Use :ref:`string formatting` and :func:`~binascii.unhexlify` to
    convert ``val``, a :func:`long`, to a byte :func:`str`.

    :param long val: The value to pack

    :param str endianness: The endianness of the result. ``'big'`` for
      big-endian, ``'little'`` for little-endian.

    If you want byte- and word-ordering to differ, you're on your own.

    Using :ref:`string formatting` lets us use Python's C innards.
    """

    # one (1) hex digit per four (4) bits
    width = val.bit_length()

    # unhexlify wants an even multiple of eight (8) bits, but we don't
    # want more digits than we need (hence the ternary-ish 'or')
    width += 8 - ((width % 8) or 8)

    # format width specifier: four (4) bits per hex digit
    fmt = '%%0%dx' % (width // 4)

    # prepend zero (0) to the width, to zero-pad the output
    s = unhexlify(fmt % val)

    if endianness == 'little':
        # see http://stackoverflow.com/a/931095/309233
        s = s[::-1]

    return s

...以及我的 nosetest 单元测试 ;-)

...and my nosetest unit tests ;-)

class TestHelpers (object):
    def test_long_to_bytes_big_endian_small_even (self):
        s = long_to_bytes(0x42)
        assert s == '\x42'

        s = long_to_bytes(0xFF)
        assert s == '\xff'

    def test_long_to_bytes_big_endian_small_odd (self):
        s = long_to_bytes(0x1FF)
        assert s == '\x01\xff'

        s = long_to_bytes(0x201FF)
        assert s == '\x02\x01\xff'

    def test_long_to_bytes_big_endian_large_even (self):
        s = long_to_bytes(0xab23456c8901234567)
        assert s == '\xab\x23\x45\x6c\x89\x01\x23\x45\x67'

    def test_long_to_bytes_big_endian_large_odd (self):
        s = long_to_bytes(0x12345678901234567)
        assert s == '\x01\x23\x45\x67\x89\x01\x23\x45\x67'

    def test_long_to_bytes_little_endian_small_even (self):
        s = long_to_bytes(0x42, 'little')
        assert s == '\x42'

        s = long_to_bytes(0xFF, 'little')
        assert s == '\xff'

    def test_long_to_bytes_little_endian_small_odd (self):
        s = long_to_bytes(0x1FF, 'little')
        assert s == '\xff\x01'

        s = long_to_bytes(0x201FF, 'little')
        assert s == '\xff\x01\x02'

    def test_long_to_bytes_little_endian_large_even (self):
        s = long_to_bytes(0xab23456c8901234567, 'little')
        assert s == '\x67\x45\x23\x01\x89\x6c\x45\x23\xab'

    def test_long_to_bytes_little_endian_large_odd (self):
        s = long_to_bytes(0x12345678901234567, 'little')
        assert s == '\x67\x45\x23\x01\x89\x67\x45\x23\x01'

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