Decrypting secret hex output from AES CTR example in 3rd party application (python)

[Prantae]

Here is the main.c file for AES CTR - /examples/crypto/nrf_crypto/aes/aes_ctr example project

'NORDIC SEMICONDUCTORAES&MAC TEST' is the key

Encrypted hex output: "12 DD C3 EE DF 6E 7E 04 C7 3B DD 0D A3 F5 8E 7E CF 6D A0 A6 76 60 8A 63 01 87 A5 F6 F6 26 B6 86 3A 07 B3 C0 FE D3 E5 3E 94 95 D5 05 EA 62 E7 23 4A F8 9A 20 4C 68 B2 2C D6 E5"

In plain text it is - "Example string to demonstrate basic usage of AES CTR mode."

Here is my code for decrypting the message in python using the above key. https://gist.github.com/neelratanguria/f5e86bc4594882760013a2d19acf7e9e

from Crypto.Cipher import AES

# Hex prints from debug console from segger
nrf_plain_hex = "45 78 61 6D 70 6C 65 20 73 74 72 69 6E 67 20 74 6F 20 64 65 6D 6F 6E 73 74 72 61 74 65 20 62 61 73 69 63 20 75 73 61 67 65 20 6F 66 20 41 45 53 20 43 54 52 20 6D 6F 64 65 2E"
nrf_cipher = "12 DD C3 EE DF 6E 7E 04 C7 3B DD 0D A3 F5 8E 7E CF 6D A0 A6 76 60 8A 63 01 87 A5 F6 F6 26 B6 86 3A 07 B3 C0 FE D3 E5 3E 94 95 D5 05 EA 62 E7 23 4A F8 9A 20 4C 68 B2 2C D6 E5"
# Initiat vector for aes_ctr project
iv = "00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00"

# Method for converton of of hex string from NRF to bytes in python
def nrf_hex_str_to_bytes(m_hex_str):
    split_nrf_plain_hex = x = m_hex_str.split(' ')
    split_nrf_plain_hex = "".join(split_nrf_plain_hex)
    return bytes.fromhex(split_nrf_plain_hex)

# Convert plain text to bytes
def str_to_byte(plain_key):
    return str.encode(plain_key)

# Convert Hex string from segger's debug console to python bytes, verifying decoding in python
pl_text = nrf_hex_str_to_bytes(nrf_plain_hex)
# Print Hex string from segger's debug console to python bytes, verifying decoding in python
print(pl_text)
# Convert cihper hex string from segger's debug console to python bytes object
encrypted_byte = nrf_hex_str_to_bytes(nrf_cipher)
# Convert 16 bytes IV to python bytes object
iv_byte = nrf_hex_str_to_bytes(iv)
# Secret used for encryption in nRF AES CTR project
plain_key = 'NORDIC SEMICONDUCTORAES&MAC TEST'
# Convert secret string to python bytes object
key_byte = str_to_byte(plain_key)

# Create instance of Mode of operation
crypto = AES.new(key_byte, AES.MODE_CTR, counter=lambda: iv_byte)
# Decrypte cipher text
plaintext = crypto.decrypt(encrypted_byte)
# Print cipher text
print(plaintext)

I figured out that IV is 16 bytes of 0. i.e. "00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00". This is what I am using a counter in python code base.

The problem is the application works fine for any encrypted generated from an AES python app, but it is unable for to successfully decrypt the above encrypted message from nRF output.

I realized that in nRF project the message is being encrypted in two part and then are joined. First 10 bytes is encrypted first, then rest of the message is encrypted.

I tried decrypting with the same logic by decrypting 10 bytes of the message first separately. But still the message is not being decrypted properly.

[Charlie]

Hi Neel,

I am checking the details with our development team and will feedback to you later.

Best regards,

Charlie

[Prantae]

Thanks

[Prantae]

Does the encryption process have PKCS padding?

[Charlie]

Hi Neel,

I test on AES Encryption: Encrypt and decrypt online — Cryptii seems everything works fine.

Here AES-128 use on 128bit key, so only "4E 4F 52 44 49 43 20 53 45 4D 49 43 4F 4E 44 55"(NORDIC SEMICONDU) is used.

Best regards,

Charlie

[Prantae]

Thanks Charlie I managed to resolve the problem with the key you hinted.

for any this is my code.

import binascii
import os
from Crypto.Cipher import AES
from Crypto.Util import Counter
def int_of_string(s):
    return int(binascii.hexlify(s), 16)
def encrypt_message(key, plaintext):
    iv = os.urandom(16)
    ctr = Counter.new(128, initial_value=int_of_string(iv))
    aes = AES.new(key, AES.MODE_CTR, counter=ctr)
    return iv + aes.encrypt(plaintext)
def decrypt_message(key, ciphertext):
    iv = ciphertext[:16]
    print(iv)
    ctr = Counter.new(128, initial_value=int_of_string(iv))
    aes = AES.new(key, AES.MODE_CTR, counter=ctr)
    return aes.decrypt(ciphertext[16:])

def nrf_hex_str_to_bytes(m_hex_str):
    split_nrf_plain_hex = x = m_hex_str.split(' ')
    split_nrf_plain_hex = "".join(split_nrf_plain_hex)
    return bytes.fromhex(split_nrf_plain_hex)



if __name__ == '__main__':
    #plain_key = 'NORDIC SEMICONDUCTORAES&MAC TEST'
    plain_key = 'NORDIC SEMICONDU'
    key = str.encode(plain_key)
    nrf_cipher = "12 DD C3 EE DF 6E 7E 04 C7 3B DD 0D A3 F5 8E 7E CF 6D A0 A6 76 60 8A 63 01 87 A5 F6 F6 26 B6 86 3A 07 B3 C0 FE D3 E5 3E 94 95 D5 05 EA 62 E7 23 4A F8 9A 20 4C 68 B2 2C D6 E5"
    nrf_cipher_iv = "00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 12 DD C3 EE DF 6E 7E 04 C7 3B DD 0D A3 F5 8E 7E CF 6D A0 A6 76 60 8A 63 01 87 A5 F6 F6 26 B6 86 3A 07 B3 C0 FE D3 E5 3E 94 95 D5 05 EA 62 E7 23 4A F8 9A 20 4C 68 B2 2C D6 E5"
    
    encrypted_byte = nrf_hex_str_to_bytes(nrf_cipher_iv)
    print(decrypt_message(key, encrypted_byte))