Problem #$2$: Polyalphabetic substitution

$$ Using polyalphabetic substitution, with alphabet a$$z $($no caps$),$

spaces translated to an underscore character $(\_),$ and a period $(.,$ as

the last character of the alphabet$),$ use the private key security to

encrypt and then decrypt the plaintext message: send$\_$money

To help you, use the following formula:

$$ Encryption: ciphertext $=($plaintext $+$ key$)$ mod $28$

$$ Decryption: plaintext $=($ciphertext $-$ key$)$ mod $28$

Number your alphabet so that it starts with zero, e$.$g$.,$ A $=0,$ Z$=25,\_=$

$26,.=27.$ This means that your alphabet will

be abcdefghijklmnopqrstuvwxyz$\_.$

Be sure to show all work. No credit will be given without showing each

step in the process.