Btw I just learned that signing such a generic message may be a mistake because someone else could use it to impersonate me...:
If you just put Bitcoin address or PGP key, it is easy to impose another person. A signed message is better.
False.
In reality is it the other way around.
The Bitcoin address or public PGP key cannot be use to impose another person.
Most user have this information public available in their signature, contact information, keybase.io etc.
But you need it to published somewhere in bitcoin talk in order to be able to retrieve your account if it gets stolen.
Publishing a generic signed message is not cleaver, it can be used by an imposter.
If you sign something, the message must include the purpose of the signed message and include the time/date.
This is an example of a not so cleaver signed message:
Message: "This is TookDk from Bitcointalk"
Signature:
This is much better:
Message: "Date: 2015.03.19. The purpose of this message is to prove that TookDk from Bitcointalk is in control of bitcoin address 1TookDkVTaqsCn56Xo7aMfUMAUN3NhRjN at this point in time"
Signature:
Makes sense. So should I sign the same address again with a better message?
Always add the current date and even time if you wish, but I would follow the above link to the letter. You could sign another here and send Cyrus the link.