Chapter 4, Problem 47SE

The encrypted version of message is LJMKG MG-MXF QEXMW. IF IT WAS ENCRYPTED

USING THE AFFINE CHIPHER $f\left(p\right)=\text{7p+1}0)\text{ mod}\text{26}$ , what was the original message?

Autokey ciphers are ciphers where the nth letter of the plaintext is shifted by the numerical equivalent of the nth letter of a keystream. The keystream begins with a seed letter; its subsequent letters are constructed using either the plaintext or the ciphertext. When the plaintext is used, each character of the keystream, after the first, is the pervious letter of the plaintext. When the ciphertext is used, each subsequent character of the keystream, after first, is the pervious letter of the ciphertext computed so far. In both case, plaintext letters are encrypted by shifting each character by the numerical equivalent of the corresponding keystream letter.