For the purpose of hiding the meaning of the messages, it encrypted using Caesar cipher substitution then using another substitution where the key is 567. Once the message is processed through the aforementioned methods, an extra layer is added of security by encrypting the message with One Time Pad that increments by one each time it’s used but remains less or equals to 15 for encryption and decryption.
For one particular message between two people, that key was:
When the receiver received the message, he/she received the following ciphertext:
LC DOMX IZY XVHP XMJQSH AANW FIHABRT
What is the plaintext?
Please note all calculation steps are required.