Topic: Keyed Hashing and Data Authentication

Figure 1: A sample script using hmac

Figure 2: Alice verifies Bob’s identity with a shared key and an HMAC function

Figure 3: A Python script that simulates the above scenario

Figure 4: Examples of using json.dumps( ) and json.loads()

Figure 1: A sample script using hmac

import hashlib

import hmac

# reference: https://docs.python.org/3/library/hmac.html

msg1 = 'message'

print("\nhmac using sha256 ...........")

hmac_sha256 = hmac.new(key=b'key1', msg=msg1.encode(), digestmod=hashlib.sha256)

#A string must be encoded first before hashing.

print("original mesg: ", msg1)

print("mac produced by digest(): ", hmac_sha256.digest())

print("mac produced by hexdigest(): ", hmac_sha256.hexdigest())

print("size: ", hmac_sha256.digest_size)

print("\nhmac using md5 ...........")

hmac_md5 = hmac.new(key=b'key1', msg=msg1.encode(), digestmod=hashlib.md5)

print("original mesg: ", msg1)

print("mac produced by digest(): ", hmac_md5.digest())

print("mac produced by hexdigest(): ", hmac_md5.hexdigest())

print("size: ", hmac_md5.digest_size)

digest1 = hmac_md5.hexdigest()

#change the message to msg2

msg2 = "message2"

hmac_sha256.update(msg2.encode())

print("\nhmac using sha256 ...........")

print("new mesg: ", msg2)

print("mac produced by hexdigest(): ", hmac_sha256.hexdigest())

print("size: ", hmac_sha256.digest_size)

hmac_md5.update(msg2.encode())

print("\nhmac using md5 ...........")

print("new mesg: ", msg2)

print("mac produced by hexdigest(): ", hmac_md5.hexdigest())

print("size: ", hmac_md5.digest_size)

digest2 = hmac_md5.hexdigest()

digest2b = hmac_md5.hexdigest()

#DO NOT compare two hashes using ==; it is subject to timing attack.

#Use compare_digest() instead

if ( hmac.compare_digest(digest1, digest2)):

print(digest1, "is equal to ", digest2)

else:

print(digest1, "is NOT equal to ", digest2)

if ( hmac.compare_digest(digest2, digest2b)):

print(digest2, "is equal to ", digest2b)

else:

print(digest2, "is NOT equal to ", digest2b)

Data Authentication using HMAC:

Figure 2: Alice verifies Bob’s identity with a shared key and an HMAC function (Figure 3.3 of the textbook)

Figure 3: A Python script that simulates the above scenario

#This script simulates that Bob, the sender, sends a message and

# the message's hash to Alice, the receiver.

# Alice reads both the message and the accompanied hash and then

# verifies the integrity of the received message using hmac by comparing

# the accompanied hash with a freshly generated hash.

import hashlib

import hmac

import json

def Bob():

hmac_sha256 = hmac.new(b'shared_key', digestmod=hashlib.sha256)

message = b'from Bob to Alice'

hmac_sha256.update(message)

hash_value = hmac_sha256.hexdigest()

authenticated_msg = {

'message': list(message),

'hash_value': hash_value, }

print("message: ", authenticated_msg['message'])

print("hash: ", authenticated_msg['hash_value'])

print("authenticated_msg: ", authenticated_msg)

outbound_msg_to_alice = json.dumps(authenticated_msg)

print("outbound_msg_to_alice: ", outbound_msg_to_alice)

return outbound_msg_to_alice

def Alice (inbound_msg_from_bob):

authenticated_msg = json.loads(inbound_msg_from_bob)

message = bytes(authenticated_msg['message'])

received_hash = authenticated_msg['hash_value']

print("\nTrying to verify the message: ", message,

"and hash: ", received_hash)

hmac_sha256 = hmac.new(b'shared_key', digestmod=hashlib.sha256)

hmac_sha256.update(message)

hash_value = hmac_sha256.hexdigest()

#if hash_value == authenticated_msg['hash_value']:

if hmac.compare_digest(hash_value, received_hash):

print('trust message')

else:

print('cannot trust message')

message = Bob()

Alice(message)

Figure 4: Examples of using json.dumps( ) and json.loads()

# Examples of using json.dumps() and json.loads() in Python

# source: Google Search Labs|AI Overview (keywords: example python json dumps loads)

import json

# Example data (Python dictionary)

data = {

"name": "John Doe",

"age": 30,

"city": "New York",

"is_student": False,

"courses": ["Math", "Science"]

}

# Using json.dumps() to convert Python dictionary to JSON string

json_string = json.dumps(data, indent=4) # indent for pretty printing

print("JSON string:")

print(json_string)

# Using json.loads() to convert JSON string back to Python dictionary

parsed_data = json.loads(json_string)

print("\nParsed Python dictionary:")

print(parsed_data)

# Accessing data from the parsed dictionary

print("\nName:", parsed_data["name"])

print("Age:", parsed_data["age"])

print("Is student:", parsed_data["is_student"])

print("First course:", parsed_data["courses"][0])