cipher = AESnewkey AESMODE_EAX

This line of code initializes an instance of the AES cipher algorithm in EAX mode with the given key. The AES cipher is a symmetric-key block cipher that encrypts and decrypts data in blocks of 128 bits. EAX mode is a block cipher mode of operation that provides authenticated encryption with additional data (AEAD), which means that it not only encrypts the data but also provides integrity and authenticity protection. The key is a secret key used to encrypt and decrypt the data.