Implementing-cryptography-using-python

202 Chapter 7 ■ Message Integrity
It is important to note that hash functions that are not collision resistant can
be vulnerable to the birthday attack; you learn more about this attack shortly.
First, you will explore the HMAC function and learn how to incorporate the
same logic into a standard hash library. Open the Python interpreter and enter
the first two lines of code to verify that you are getting the same tag:
import hmac, hashlib
print(hmac.new(b"secretkey", b"our secret message", hashlib.sha256).
hexdigest())
78c736db86abd16023a23355f4ad3005e77dec6d8c960d06ea3c4a9aba9c449f
SHA256 has a 64-byte block length; to build an HMAC by hand, you need to
build K + , ipad, and opad:
import binascii, hashlib
k = b"secretkey"
msg = b"our secret message"
kplus = k + b"\x00"*(64-len(k))
ipad = b"\x36"*64
opad = b"\x5C"*64
def XOR(raw1, raw2):
return binascii.unhexlify(format(int(binascii.hexlify(raw1), 16) ^
int(binascii.hexlify(raw2), 16), 'x'))
tag = hashlib.sha256(XOR(kplus, opad) + hashlib.sha256(XOR(kplus, ipad)
+ msg).digest()).digest()
print(binascii.hexlify(tag))
Confirm that this manual computation matches the library implementation.
In our next script, we will apply an HMAC digest to a signed message.
Using HMAC to Sign Message
The file that we are creating the message digest for is a simple text file that contains
only Hello. When run, the code reads a data file and computes an HMAC
signature for it:
import hmac
myKey = b'this_is_my_secret'
digest_maker = hmac.new(myKey)
with open('test.txt', 'rb') as f:
while True: