Encrypt the message "this is an exercise" using one of the following ciphers. Ignore the space between words. Decrypt the message to get the original plaintext.
a. Additive cipher with key =20

Character B Q M A V L Z I T W P
Frequency 18 15 14 12 11 10 10 9 9 9 7
Character E G N C O U X J S K D
Frequency 6 6 5 5 4 4 3 2 1 1 1