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 "A"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 ?