What if your base os is compromised and you use a livecd whilst being offline to store the coins.....can this make you unsafe?

How did they decide to shorten it by so much 

I know how to sign a message with bitcoin core but was wondering how to do it with my hot wallet which is on blockchain.info I would rather not get the backup & import it into bitcoin core...that takes too long to do.

Does blockchain.info wallet allow you to sign a message? If it does how? step by step would be helpful.

Do you really believe that? Why?