input: Write a function to decrypt an encrypted string using base64 decoding and XOR operation.
  assert decrypt_string("U29tZUVuY3J5cHRlZFN0cmluZw==", "U29tZUtleQ==") == "SomeDecryptedString"
  assert decrypt_string("Rk1TU1ZP", "SEtFSVM=") == "HELLO"
  assert decrypt_string("Q09ERQ==", "V0VSUw==") == "TEST"
output: def decrypt_string(encrypted_str, key_str):
      import base64
  
      # Decode the encrypted string and the key from base64
      encrypted_bytes = base64.b64decode(encrypted_str)
      key_bytes = base64.b64decode(key_str)
  
      # XOR decryption
      decrypted_bytes = bytes([e ^ k for e, k in zip(encrypted_bytes, key_bytes)])
  
      # Convert bytes back to string
      return decrypted_bytes.decode('utf-8')