I am creating a Python function to perform counter mode encryption using the PyCrypto module. I am aware of the builtin, but want to implement it myself.
I'm trying Test Vector #1 from RFC 3686, and have the correct Counter Block and the correct Key in ASCII form. But when I encrypt the Counter Block using the Key, I don't get the expected Key Stream.
The relevant parts of my code:
cipher = AES.new(key)
ctr_block = iv + nonce + ctr
key_stream = base64.b64decode(cipher.encrypt(ctr_block))
I can provide more code if needed, but I'm not sure how because ctr_block and key have many question mark characters when I print them.
Why am I not getting the expected answer? It seems like everything should go right. Perhaps I made some mistake with the encoding of the string.
Edit
Self-contained code:
from Crypto.Cipher import AES
import base64
def hex_to_str(hex_str):
return str(bytearray([int(n, 16) for n in hex_str.split()]))
key = hex_to_str("AE 68 52 F8 12 10 67 CC 4B F7 A5 76 55 77 F3 9E")
iv = hex_to_str("00 00 00 00 00 00 00 00")
nonce = hex_to_str("00 00 00 30")
ctr = hex_to_str("00 00 00 01")
cipher = AES.new(key)
ctr_block = iv + nonce + ctr
key_stream = base64.b64decode(cipher.encrypt(ctr_block))
print "".join([hex(ord(char)) for char in key_stream])
# 0xd90xda0x72
First, the correct CTR block order is nonce + iv + ctr. Second, that base64.b64decode call is wrong: cipher.encrypt produces a decoded string. After these two fixes your code prints 0xb70x600x330x280xdb0xc20x930x1b0x410xe0x160xc80x60x7e0x620xdf which seems to be a correct key stream.