For all the remaining questions, you will work with the following toy encryption scheme. It encrypts plaintexts that consist of only uppercase letters and the underscore character (27 characters total): ABCDEFGHIJKLMNOPQRSTUVWXYZ Note that the underscore character is part of the alphabet and will also be encrypted. A key in this scheme consists of 4 digits, each from 0 to 9. Each digit tells how much to shift one character, that is, change it to the character that many positions later in the alphabet. If the shift goes past'_', then we wrap around to the beginning. The key is applied separately to each group of 4 letters in the message. For example, if the message HI_SALLY is encrypted with the key 2407, the H would be shifted by 2, the I by 4, the underscore by 0, and the S by 7. Then we would start over with the first digit of the key, shifting A by 2, L by 4, and so on. So the ciphertext would be: JM_ZCPLE. 6.(2 pts) Say you obtain a ciphertext of an English sentence encrypted with an unknown key using this scheme. For a brute-force attack, what is the maximum number of keys would you have to test? 7. (5 pts) Say that your best frenemy is using this encryption scheme. You accidentally find out that the encryption of the message MEET_TUESDAY is TFGWGUWHZECA. a. (2 pts) Give the key that was used for this encryption. b. (1 point) Wasn't finding the key much easier than with a brute-force attack? c. (2 pts) Which of the four attack models discussed in class does the scenario in this question correspond to