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ashutosh1206
GitHub Repository: ashutosh1206/crypton
 Path: blob/master/Authenticated-Encryption/AE-with-MACs/MAC-then-Encrypt/mac-then-encrypt.py
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"""

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An illustration of MAC then Encrypt technique of Authenticated Encryption with MACs

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MAC algorithm: CBC-MAC

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Encryption: AES in CBC mode

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Note that this is only for illustrative purposes (the script is vulnerable to CBC-MAC forgery and more implementation attacks-

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even the unpad function is vulnerable!) 

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"""

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from Crypto.Cipher import AES

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from os import urandom

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from Crypto.Util.number import *

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key = urandom(16)

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iv = urandom(16)

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mac_key = urandom(16)

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blocksize = 16

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def pad(input_str, blocksize):

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	input_str += chr(blocksize - len(input_str) % blocksize)*(blocksize - len(input_str) % blocksize)

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	assert len(input_str) % blocksize == 0

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	return input_str

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def unpad(input_str):

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	return input_str[:-ord(input_str[-1])]

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def cbc_mac_gen(input_str, iv, mac_key, blocksize):

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	input_str = pad(input_str, blocksize)

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	obj1 = AES.new(mac_key, AES.MODE_CBC, iv)

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	auth_tag = obj1.encrypt(input_str)[-blocksize:]

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	return auth_tag.encode("hex")

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def cbc_mac_auth(input_str, iv, mac_key, blocksize, auth_tag):

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	input_str = pad(input_str, blocksize)

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	obj1 = AES.new(mac_key, AES.MODE_CBC, iv)

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	chk_tag = obj1.encrypt(input_str)[-blocksize:]

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	if chk_tag == auth_tag:

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		print "Verification Successful"

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		return 1

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	else:

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		print "Verification Failed"

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		return 0

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def encrypt(input_str, iv, key, blocksize):

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	input_str = pad(input_str, blocksize)

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	obj1 = AES.new(key, AES.MODE_CBC, iv)

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	ciphertext = obj1.encrypt(input_str)

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	return ciphertext.encode("hex")

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def decrypt(ciphertext, iv, key, blocksize):

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	obj1 = AES.new(key, AES.MODE_CBC, iv)

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	plaintext = obj1.decrypt(ciphertext)

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	return unpad(plaintext)

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def mac_then_encrypt(input_str, iv, key, mac_key, blocksize):

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	tag = cbc_mac_gen(input_str, iv, mac_key, blocksize)

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	tag = tag.decode("hex")

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	plaintext = input_str + ":" + tag

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	ciphertext = encrypt(plaintext, iv, key, blocksize)

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	return iv.encode("hex") + ":" + ciphertext

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def auth_check(cookie, iv, key, mac_key, blocksize):

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	iv, ciphertext = cookie.split(":")

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	iv = iv.decode("hex")

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	ciphertext = ciphertext.decode("hex")

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	plaintext = decrypt(ciphertext, iv, key, blocksize)

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	input_str, auth_tag = plaintext.split(":")

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	if cbc_mac_auth(input_str, iv, mac_key, blocksize, auth_tag):

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		print "Plaintext: ", input_str

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	else:

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		return "Verification failed, so nothing for you"

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str1 = mac_then_encrypt("testplaintext", iv, key, mac_key, blocksize)

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print str1

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print auth_check(str1, iv, key, mac_key, blocksize)

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