sandishee
Lis 💚🦒🧡Every time one of them is on a longer mission away from home they would post little ads in the paper for he other to read at the breakfast table.
Now it's Izuku who is on a top secret mission the last few days and Katsuki sits at home having breakfast and hoping he's coming back soon. He sees the funny looking ad in the paper which doesn't seem to make any sense. Katsuki smiles softly (no one's there to see it, so no harm done, shut up!) and grabs pen and paper.
He writes the ad message down carefully and adds the keyword he knows his nerd always uses below it. Counting the letters in the original message he repeats the keyword twice more so he is able to decrypt the whole message.
“SMKU ZUGZ CTYI LOWV BBRT UIYB WRTY SOIW HKFL”
Katsuki consults the alphabet square, which is used to encryt the original message but also works for decrypting it.
After a few minutes he's successfully reading Izuku's intended message still smiling softly and feeling much better.
“Kacchan sugoi, indeed.”
/end
If you want to know what Izuku wrote to Kacchan, you can use this table. Just write the ciphertext down and the keyword below it. With the table you can decrypt the message.
All information you need (keyword etc.) you'll find in the little drabble.
There are online tools for decrypting this message. Just google "Vigenère cipher" and you'll find some. You need to put in the ciphertext and the keyword & it will decrypt the message for you.