Is the one-time pad secure if the key is computed so the ciphertext will consist only of "A"?
You're missing the goal of a one-time pad: it's a key generated and distributed ahead of message transmission.
When using a one-time pad, the only secure time to exchange the pad is ahead of time. The message itself is encrypted with the pad to make up for the lack of a safe communications channel.
By computing a &quot;one-time pad&quot; such that the ciphertext is a fixed content, you are moving pad generation from &quot;ahead of transmission&quot; (when we have a secure channel) to &quot;at transmission time&quot; (when we do not).
You now need to send your &quot;pad&quot; instead of your &quot;all As&quot; message. This means that you have turned your pad into a message and have no secure channel to send it.