Encrypt with arduino and decrypt with python using AES CBC mode - python-3.x

Question

I'm very new to programming and I've been trying to encrypt a message using Arduino and decrypting with Python.
On the Arduino, I managed to encrypt and decrypt correctly, but when I try to decrypt with Python it doesn't show an error but the result isn't right.
I've used the library AESlib with the latest version (2.2.1) on Arduino with MEGA2560.
On the Arduino part I encrypted and decrypted the message correctly, I used the simple example that the AESlib offer but changed a bit to be able to do what I need it, encrypting with AES and encoding with base64, and then decoding with base64 to be able to decrypt with AES again. When that worked I printed the base64 encoded message and then copied it into a function on the python program and tried to decrypt it without working.
On the Python part, I've used the CBC mode for the decryption. Copied the key, the IV, and the encoded message for then decoded and decrypted.
Here is the message with the key and IV that I've used:
#define INPUT_BUFFER_LIMIT (400 + 1) //Maximum message caracters
unsigned char cleartext[INPUT_BUFFER_LIMIT] = {0}; // THIS IS INPUT BUFFER (FOR TEXT)
unsigned char ciphertext[2*INPUT_BUFFER_LIMIT] = {0}; // THIS IS OUTPUT BUFFER (FOR BASE64-ENCODED ENCRYPTED DATA)
unsigned char decryptedtext[INPUT_BUFFER_LIMIT] = {0}; // THIS IS OUTPUT BUFFER (FOR DECRYPTED TEXT)
unsigned char readBuffer[399] = "0013;0013;0013;15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3;NULL";//THIS IS THE VARIABLE THAT CONTAINS THE MESSAGE TO ENCRYPT
byte aes_key[N_BLOCK] = "06a9214036b8a15b512e03d534120006"; // THIS IS THE VARIABLE THAT CONTAINS THE KEY
byte aes_iv[N_BLOCK] = "6543210987654"; // THIS IS THE VARIABLE THAT CONTAINS THE IV
Arduino code:
#include "AESLib.h"
#define BAUD 9600
AESLib aesLib;
#define INPUT_BUFFER_LIMIT (400 + 1)
unsigned char cleartext[INPUT_BUFFER_LIMIT] = {0}; // THIS IS INPUT BUFFER (FOR TEXT)
unsigned char ciphertext[2*INPUT_BUFFER_LIMIT] = {0}; // THIS IS OUTPUT BUFFER (FOR BASE64-ENCODED ENCRYPTED DATA)
unsigned char decryptedtext[INPUT_BUFFER_LIMIT] = {0}; // THIS IS OUTPUT BUFFER (FOR DECRYPTED TEXT)
unsigned char readBuffer[399] = "0013;0013;0013;15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3;NULL";//THIS IS THE VARIABLE THAT CONTAINS THE MESSAGE TO ENCRYPT
byte aes_key[N_BLOCK] = "06a9214036b8a15b512e03d534120006"; // THIS IS THE VARIABLE THAT CONTAINS THE KEY
byte aes_iv[N_BLOCK] = "6543210987654"; // THIS IS THE VARIABLE THAT CONTAINS THE IV
// Generate IV
void aes_init() {
aesLib.gen_iv(aes_iv);
aesLib.set_paddingmode((paddingMode)0);
}
uint16_t encrypt_to_ciphertext(char * msg, uint16_t msgLen, byte iv[]) {
int i = 0;
Serial.println("Calling encrypt (string)...");
int cipherlength = aesLib.encrypt((byte*)msg, msgLen, (char*)ciphertext, aes_key, sizeof(aes_key), iv);
// uint16_t encrypt(byte input[], uint16_t input_length, char * output, byte key[],int bits, byte my_iv[]);
return cipherlength;
}
uint16_t decrypt_to_cleartext(byte msg[], uint16_t msgLen, byte iv[]) {
int i = 0;
Serial.print("Calling decrypt...; ");
uint16_t dec_bytes = aesLib.decrypt(msg, msgLen, (char*)decryptedtext, aes_key, sizeof(aes_key), iv);
Serial.print("Decrypted bytes: "); Serial.println(dec_bytes);
return dec_bytes;
}
void setup() {
Serial.begin(BAUD);
Serial.setTimeout(60000);
delay(2000);
aes_init(); // generate random IV, should be called only once? causes crash if repeated...
Serial.println(readBuffer[2]);
}
/* non-blocking wait function */
void wait(unsigned long milliseconds) {
unsigned long timeout = millis() + milliseconds;
while (millis() < timeout) {
yield();
}
}
byte enc_iv_to[N_BLOCK] = "6543210987654"; //A COPY OF THE IV TO DECRYPT WITH THE SAME IV
void loop() {
int i = 0;
Serial.print("readBuffer length: "); Serial.println(sizeof(readBuffer));
// must not exceed INPUT_BUFFER_LIMIT bytes; may contain a newline
sprintf((char*)cleartext, "%s", readBuffer);
// Encrypt
// iv_block gets written to, provide own fresh copy... so each iteration of encryption will be the same.
uint16_t msgLen = sizeof(readBuffer);
memcpy(aes_iv, enc_iv_to, sizeof(enc_iv_to));
uint16_t encLen = encrypt_to_ciphertext((char*)cleartext, msgLen, aes_iv); //CALL THE FUNCTION TO ENCRYPT THE MESSAGE
unsigned char base64encoded[2*INPUT_BUFFER_LIMIT] = {0};
base64_encode((char*)base64encoded, (char*)ciphertext, sizeof(ciphertext)); //CALL THE FUNCTION TO ENCODE THE ENCRYPTED MESSAGE
Serial.println("ciphertext_base64_encoded");
Serial.println((char*)base64encoded);
delay(5000);
Serial.print("Encrypted length = "); Serial.println(encLen);
Serial.print("Encrypted base64 length = "); Serial.println(sizeof(base64encoded));
Serial.println("Encrypted. Decrypting..."); Serial.println(sizeof(base64encoded)); Serial.flush();
unsigned char base64decoded[2*INPUT_BUFFER_LIMIT] = {0};
base64_decode((char*)base64decoded, (char*)base64encoded, sizeof(base64encoded));
Serial.println((char*)base64decoded);
delay(3000);
memcpy(aes_iv, enc_iv_to, sizeof(enc_iv_to));
uint16_t decLen = decrypt_to_cleartext((char*)base64decoded, encLen, aes_iv);
Serial.print("Decrypted cleartext of length: "); Serial.println(decLen);
Serial.print("Decrypted cleartext:\n"); Serial.println((char*)decryptedtext);
if (strcmp((char*)readBuffer, (char*)decryptedtext) == 0) {
Serial.println("Decrypted correctly.");
} else {
Serial.println("Decryption test failed.");
}
delay(3000);
Serial.println("---");
exit(0);
}
Python code:
from Crypto.Cipher import AES
from Crypto.Util.Padding import unpad, pad
from Crypto.Random import get_random_bytes
from Crypto.Util.strxor import strxor
import random, base64, hashlib, getpass, argparse, re
def decrypt_CBC_base64(key, ciphertext_base64, iv):
ciphertext_base64 += "=" * ((4 - len(ciphertext_base64) % 4) % 4)
ciphertext = base64.b64decode(ciphertext_base64)
ciphertext = pad (ciphertext, 16)
py_bytes = decrypt_CBC(key, ciphertext, iv)
return py_bytes
def decrypt_CBC(key, ciphertext, iv):
cipher = AES.new(key, AES.MODE_CBC, iv)
pt_bytes = cipher.decrypt(ciphertext)
return pt_bytes
try:
key = b'06a9214036b8a15b512e03d534120006'
iv = b'6543210987654321'
plaintext = b'0013;0013;0013;15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3,15.3;NULL'
ciphertext_base64 = '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'
print(ciphertext_base64)
decrypted = decrypt_CBC_base64(key, ciphertext_base64, iv)
print(decrypted)
except Exception as err:
print("error: {0}".format(err))
except KeyboardInterrupt:
print("\n\n[*] (Ctrl-C) Detected, shutting down...")
exit()
Any solutions? Hope there's enough information.

Answer 1

The AESLib iterates over data in 16 byte chunks, encrypting one chunk of 16 bytes at a time. To decrypt the data in Python using the pycryptodome library, you would need to decrypt 16 bytes at a time and then concatenate all the decrypted data together.