Is the one-time pad secure if the key is computed so the ciphertext will consist only of "A"? I recently learned about the one-time pad method, which has perfect security if the following conditions are met :

The key must be truly random.
The key must be at least as long as the plaintext.
The key must never be reused in whole or in part.
The key must be kept completely secret.

As a thought experiment, I wondered : what if you choose the key so the ciphertext will only be &quot;A&quot;s (or whatever other letter) ? This would break the rule n°1, since the plaintext is not random and the key is linked to it. Does that mean that such a ciphertext can be broken ?
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.