Convert python long int to fixed size byte array

Question

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

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Little-endian  reverse the result or the range if you want Big-endian   def int to bytes val  num bytes       return   val  amp   0xff  lt  lt  pos 8    gt  gt  pos 8 for pos in range num bytes     Big-endian   def int to bytes val  num bytes       return   val  amp   0xff  lt  lt  pos 8    gt  gt  pos 8 for pos in reversed range num bytes

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i   0x12345678 s   struct pack   lt I  i  b   struct unpack  BBBB  s

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Python 2 7 does not implement the int to- very slow bytes   method   I tried 3 methods    hex unpack pack   very slow byte shifting 8 bits at a time  significantly faster  using a  C  module and packing into the lower  7 ia64 or 3 i32  bytes  This was about twice as fast as 2    It is the fastest option  but still too slow    All these methods are very inefficient for two reasons     Python 2 7 does not support this useful operation  c does not support extended precision arithmetic using the carry borrow overflow flags available on most platforms

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With Python 3 2 and later  you can use int to bytes and int from bytes  https   docs python org 3 library stdtypes html int to bytes

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long int to the byte array looks like exact purpose of struct pack   For long integers that exceed 4 8  bytes  you can come up with something like the next    gt  gt  gt  limit   256 256 256 256 - 1  gt  gt  gt  i   1234567890987654321  gt  gt  gt  parts       gt  gt  gt  while i          parts append i  amp  limit          i  gt  gt   32   gt  gt  gt  struct pack   gt      L  len parts    parts     xb1l x1c xb1 x11  x10 xf4    gt  gt  gt  struct unpack   gt LL     xb1l x1c xb1 x11  x10 xf4    2976652465L  287445236   gt  gt  gt   287445236L  lt  lt  32    2976652465L 1234567890987654321L

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You can try using struct   import struct struct pack  L  longvalue

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I haven t done any benchmarks  but this recipe  works for me    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      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

User · Answer

Basically what you need to do is convert the int long into its base 256 representation -- i e  a number whose  digits  range from 0-255  Here s a fairly efficient way to do something like that   def base256 encode n  minwidth 0     int long to byte array     if n  gt  0          arr              while n              n  rem   divmod n  256              arr append rem          b   bytearray reversed arr       elif n    0          b   bytearray b  x00       else          raise ValueError      if minwidth  gt  0 and len b   lt  minwidth    zero padding needed          b    minwidth-len b       x00    b     return b   You many not need thereversed  call depending on the endian-ness desired  doing so would require the padding to be done differently as well   Also note that as written it doesn t handle negative numbers   You might also want to take a look at the similar but highly optimized long to bytes   function in thenumber pymodule which is part of the open source Python Cryptography Toolkit  It actually converts the number into a string  not a byte array  but that s a minor issue

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One-liner   bytearray fromhex    0192x   format big int     The 192 is 768   4  because OP wanted 768-bit numbers and there are 4 bits in a hex digit  If you need a bigger bytearray use a format string with a higher number  Example    gt  gt  gt  big int   911085911092802609795174074963333909087482261102921406113936886764014693975052768158290106460018649707059449553895568111944093294751504971131180816868149233377773327312327573120920667381269572962606994373889233844814776702037586419  gt  gt  gt  bytearray fromhex    0192x   format big int   bytearray b  x96 h  xdbJ x8f3obL x9c xc2 xb0- x9e xa4Sj- xf6i xc1 x9e x97 x94 x85M x1d x93 x10   x81 xc2 x89 xcd xe0a xc0D x81v xdf xed xa9 xc1 x83p xdbU xf1 xd0 xfeR  xce x07 xdepM x88 xcc x7fv   x1c x8di x87N x00 x8d xa8 xbd  lt  xdf xaf x13z H xed xc2  xa4 x1e x0f xa7 x92 xa7 xc6 x16 x86 xf1 xf3    gt  gt  gt  lepi int   0x963b685edb4a8f336f624c9cc2b02d9ea4536a2df669c19e9794854d1d93105c81c289cde061c0448176dfeda9c18370db55f1d0fe5229ce07de704d88cc7f765c1c8d69874e008da8bd5b3cdfaf137a3a48edc229a41e0fa792a7c61686f1f  gt  gt  gt  bytearray fromhex    0192x   format lepi int   bytearray b  tc xb6 x85 xed xb4 xa8 xf36 xf6  xc9 xcc  x02 xd9 xeaE6 xa2 xdff x9c x19 xe9yHT xd1 xd91 x05 xc8 x1c  x9c xde x06 x1c x04H x17m xfe xda x9c x187 r xb5  x1d x0f xe5  x9c xe0  xe7 x04 xd8 x8c xc7 xf7e xc1 xc8 xd6 x98t xe0 x08 xda x8b xd5 xb3 xcd xfa xf17 xa3 xa4 x8e xdc  x9aA xe0 xfay  aho x1f      My answer had used hex   before  I corrected it with format   in order to handle ints with odd-sized byte expressions  This fixes previous complaints about ValueError

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Everyone has overcomplicated this answer   some int    lt 256 bit integer gt  some bytes   some int to bytes 32  sys byteorder  my bytearray   bytearray some bytes    You just need to know the number of bytes that you are trying to convert  In my use cases  normally I only use this large of numbers for crypto  and at that point I have to worry about modulus and what-not  so I don t think this is a big problem to be required to know the max number of bytes to return   Since you are doing it as 768-bit math  then instead of 32 as the argument it would be 96

[python] Convert python long/int to fixed size byte array

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