Python - Read text file to String

Question

I have the following python sript, which double hashes a hex value:

import hashlib
linestring = open('block_header.txt', 'r').read()
header_hex = linestring.encode("hex") // Problem!!!
print header_hex
header_bin = header_hex.decode('hex')
hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest()

hash.encode('hex_codec')
print hash[::-1].encode('hex_codec')

My text file "block_header.txt" (hex) looks like this:

0100000081cd02ab7e569e8bcd9317e2fe99f2de44d49ab2b8851ba4a308000000000000e320b6c2fffc8d750423db8b1eb942ae710e951ed797f7affc8892b0f1fc122bc7f5d74df2b9441a42a14695

Unfortunately, the result from printing the variable header_hex looks like this (not like the txt file):

303130303030303038316364303261623765353639653862636439333137653266653939663264653434643439616232623838353162613461333038303030303030303030303030653332306236633266666663386437353034323364623862316562393432616537313065393531656437393766376166666338383932623066316663313232626337663564373464663262393434316134326131343639350a

I think the problem is in this line:

header_hex = linestring.encode("hex")

If I remove the ".encode("hex")"-part, then I get the error

unhandled TypeError "Odd-length string"

Can anyone give me a hint what might be wrong? Thank you a lot :)

Answer 1

You're doing too much encoding/decoding.

Like others mentioned, if your input data is hex, then it's a good idea to strip leading / trailing whitespace with strip().

Then, you can use decode('hex') to turn the hex ASCII into binary. After performing whatever hashing you want, you'll have the binary digest.

If you want to be able to "see" that digest, you can turn it back into hex with encode('hex').

The following code works on your input file with any kinds of whitespace added at the beginning or end.

import hashlib

def multi_sha256(data, iterations):
    for i in xrange(iterations):
        data = hashlib.sha256(data).digest()
    return data

with open('block_header.txt', 'r') as f:
    hdr = f.read().strip().decode('hex')
    _hash = multi_sha256(hdr, 2)

    # Print the hash (in hex)
    print 'Hash (hex):', _hash.encode('hex')

    # Save the hash to a hex file
    open('block_header_hash.hex', 'w').write(_hash.encode('hex'))

    # Save the hash to a binary file
    open('block_header_hash.bin', 'wb').write(_hash)