Chaff n Winnow

Code:

Welcome to Chaff n Winnow
Your goal is to decrypt the message, but unfortunately, you'll need to figure out whether a byte corresponding to a letter, are valid. If the byte is valid, then the letter makes up the secret message.
There are rules to which bytes are valid but I won't tell you. You'll just have to write a message with a byte, and I'll tell you whether it's valid or not.

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