CODEWITHSUNDEEP
pythonpython-2.7int
Slava
Slava

Reputation: 44268

How to pack big negative long to binary string in Python

I need to calculate md5 hash of random 8 bytes long number, so I am trying to pack it:

import struct

num = 123L
bin = struct.pack( 'q', num )

which works fine, but, for big negative values:

num = -14710095416404972671L
bin = struct.pack( 'q', num )

I get this error:

struct.error: long too large to convert to int

from python 2.6

Which way I can convert it to use as input for md5 hash?

Upvotes: 3

Views: 407

Answers (2)

Michael F
Michael F

Reputation: 40839

You have a variety of choices, as soon as you realise MD5 operates on any byte stream. It is important to note that sign does not matter for MD5; it does not interpret integers, it manipulates a stream. Therefore you don't need to care about the sign of your input or converting to signed integers. You need a byte array, which you can obtain in multiple ways, my preference is: 

In [30]: 0xEB95EC9D994ED78D.to_bytes(8, 'big')
Out[30]: b'\xeb\x95\xec\x9d\x99N\xd7\x8d'

which will handle arbitrary integers within the limits Python's integers allow.

Upvotes: 1

Dimitris Fasarakis Hilliard
Dimitris Fasarakis Hilliard

Reputation: 160557

You can't, that number just doesn't fit in an 8 byte long long (signed). The maximum value you can supply with 'q' is -2 ** 63, no less:

num = -2 ** 63 
bin = struct.pack('q', num )

while:

num = -2 ** 63 - 1
bin = struct.pack('q', num )

leads to error: argument out of range. That's the ceiling, with 'Q' you can achieve larger positive (unsigned) values but with a limit there of 2 ** 64.

Upvotes: 1

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