How to convert a sha256 object to integer and pack it to bytearray in python?

Question

I want to convert a hash256 object to a 32-byte integer first, and then pack it into a bytearray.

>>> import hashlib
>>> hashobj = hashlib.sha256('something')
>>> val_hex = hashobj.hexdigest()
>>> print val_hex
3fc9b689459d738f8c88a3a48aa9e33542016b7a4052e001aaa536fca74813cb
>>> print len(val_hex)
64

The hex string is 64-byte instead of 32-byte, which is not what I want.

>>> val = hashobj.digest()
>>> print val
?ɶ?E?s????????5Bkz@R???6??H?
>>> print len(val)
32

This is a 32-byte string and I want to convert it to a 32-byte integer.

It gave me an error message when I try:

>>> val_int = int(val, 10)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '?\xc9\xb6\x89E\x9ds\x8f\x8c\x88\xa3\xa4\x8a\xa9\xe35B\x01kz@R\xe0\x01\xaa\xa56\xfc\xa7H\x13\xcb'

What should I do to get my int_val?

And how can I use struct to pack it (32-byte) to a bytearray? I found the longest format in python struct document is 'Q' which is only 8-byte.

Thank you very much.

Answer 1

The simplest way in Python 2 to get the integer value of the SHA-256 digest is via the hexdigest. Alternatively, you can loop over the bytearray constructed from the binary digest. Both methods are illustrated below.

import hashlib

hashobj = hashlib.sha256('something')
val_hex = hashobj.hexdigest()
print val_hex

# Build bytearray from binary digest
val_bytes = bytearray(hashobj.digest())
print ''.join(['%02x' % byte for byte in val_bytes])

# Get integer value of digest from the hexdigest
val_int = int(val_hex, 16)
print '%064x' % val_int

# Get integer value of digest from the bytearray
n = 0
for byte in val_bytes:
    n = n<<8 | byte
print '%064x' % n

output

3fc9b689459d738f8c88a3a48aa9e33542016b7a4052e001aaa536fca74813cb
3fc9b689459d738f8c88a3a48aa9e33542016b7a4052e001aaa536fca74813cb
3fc9b689459d738f8c88a3a48aa9e33542016b7a4052e001aaa536fca74813cb
3fc9b689459d738f8c88a3a48aa9e33542016b7a4052e001aaa536fca74813cb

---

In Python 3, we can't pass a plain text string to the hashlib hash function, we must pass a bytes string or a bytearray, eg

b'something' 

or

'something'.encode('utf-8')

or

bytearray('something', 'utf-8')

We can simplify the second version to

'something'.encode()

since UTF-8 is the default encoding for str.encode (and bytes.decode()).

To perform the conversion to int, any of the above techniques can be used, but we also have an additional option: the int.from_bytes method. To get the correct integer we need to tell it to interpret the bytes as a big-endian number:

import hashlib

hashobj = hashlib.sha256(b'something')
val = int.from_bytes(hashobj.digest(), 'big')
print('%064x' % val)

output

3fc9b689459d738f8c88a3a48aa9e33542016b7a4052e001aaa536fca74813cb