Convert 16 bytes of random data to integer in Python

Question

I have generated a random 16 byte string. It looks like this:

b'\xb68 \xe9L\xbd\x97\xe0\xd6Q\x91c\t\xc3z\\'

I want to convert this to a (positive) integer. What's the best way to do this in Python?

I appreciate the help.

Possibly related: http://stackoverflow.com/questions/444591/convert-a-string-of-bytes-into-an-int-python — Logan, Oct 23 '16 at 21:04
@Logan I saw that post too, but I don't understand why to use struct? That can't be the solution. — awaelchli, Oct 23 '16 at 21:06

Logan · Accepted Answer · 2016-10-23T21:15:36.877

5

In Python 3.2+, you can use int.from_bytes():

>>> int.from_bytes(b'\xb68 \xe9L\xbd\x97\xe0\xd6Q\x91c\t\xc3z\\', byteorder='little')
122926391642694380673571917327050487990

You can also use 'big' byteorder:

>>> int.from_bytes(b'\xb68 \xe9L\xbd\x97\xe0\xd6Q\x91c\t\xc3z\\', byteorder='big')
242210931377951886843917078789492013660

You can also specify if you want to use two's complement representation. For more info: https://docs.python.org/3/library/stdtypes.html

edited Oct 23 '16 at 21:15

answered Oct 23 '16 at 21:11

Logan

Oh I think that's exactly what I need. If my data is random, I think it does not matter if I use little or big endian, it will just shuffle the number, right? – awaelchli Oct 23 '16 at 21:14
I guess it would depend on what exactly you're planning on doing with the integer, but yes, the number will be different if you change the byteorder. – Logan Oct 23 '16 at 21:16
What is the command to go back to the original string from that number? – awaelchli Oct 23 '16 at 21:18
Looks like you could use int.to_bytes(): https://docs.python.org/3/library/stdtypes.html#int.to_bytes – Logan Oct 23 '16 at 21:21

score 2 · Answer 2 · answered Oct 23 '16 at 21:15

A solution compatible both with Python 2 and Python 3 is to use struct.unpack:

import struct
n = b'\xb68 \xe9L\xbd\x97\xe0\xd6Q\x91c\t\xc3z\\'
m = struct.unpack("<QQ", n)
res = (m[0] << 64) | m[1]
print(res)

Result: 298534947350364316483256053893818307030L

2 Answers2