Some reverse engineering of the Carmin Android app backend

carmin_crypto.py

# pip3 install pycryptodome

# Encryption for the "http://carmin-backend.appspot.com/rs" URL paths

import base64
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad, unpad

class CarminURLEncryptor:
    def __init__(self):
        # The SHA256 digest (from the Android app signature)
        hex_digest = "27561785fbc8213f459a091e0c24c6d1e1eea5264500e660382b8eb5cf6141bd"
        digest_bytes = bytes.fromhex(hex_digest)

        # Base64 encode the digest, then use the first 16 characters as the key/IV
        base64_digest = base64.b64encode(digest_bytes).decode()
        key_iv = base64_digest[:16].encode()

        self.key = key_iv
        self.iv = key_iv

    def encrypt(self, plaintext: str) -> str:
        if not plaintext:
            raise Exception("Empty string")

        cipher = AES.new(self.key, AES.MODE_CBC, self.iv)
        padded = pad(plaintext.encode(), AES.block_size)
        encrypted = cipher.encrypt(padded)

        # URL-safe Base64
        encoded = base64.b64encode(encrypted).decode()
        return encoded.replace('+', '-').replace('/', '_')
    
    def decrypt(self, encoded_str: str) -> str:
        if not encoded_str:
            raise Exception("Empty string")

        # URL-safe Base64
        safe_str = encoded_str.replace('-', '+').replace('_', '/')
        encrypted_bytes = base64.b64decode(safe_str)

        cipher = AES.new(self.key, AES.MODE_CBC, self.iv)
        decrypted = unpad(cipher.decrypt(encrypted_bytes), AES.block_size)

        return decrypted.decode()


if __name__ == "__main__":
    encryptor = CarminURLEncryptor()
    ciphertext = encryptor.encrypt("4843060768800768")      # uYuTMMCAFMCWF3v_Pa1pg-JIZC1gmN8trwkDiY97rQs=
    print(ciphertext)
    print(encryptor.decrypt("0mAo3kpeErHLTHAhiUH8Og=="))    # com.min.car