Answer: Secret Code #200

The secret message says:

BALL

The shift was 18.

The secret note decoded: BALL
The note, decoded.

Letter by letter

Secret letterDST
Real letterLAB

Decode it with Python

This program does the decoding for you. Change the secret and the shift to crack other messages!

secret = "TSDD"
shift = 18
message = ""

for letter in secret:
    if letter.isalpha():
        number = (ord(letter) - ord("A") - shift) % 26
        message = message + chr(number + ord("A"))
    else:
        message = message + letter

print(message)

When you run it, it prints:

BALL

Did you know? Shift codes are very old. A Roman writer called Suetonius said that Julius Caesar wrote secret letters by shifting the alphabet by three. That’s why it’s often called the Caesar cipher.