Is it dishonest to lie to keep a secret?

Yes it is dishonest, though it would just as bad to tell a secret. The best way to avoid being dishonest is to simply not reply to anything that would revealed the secret. Asking the person you're keeping the secret for is another way of avoiding being dishonest. Let the person you're in a position where it's either lying or keep the secret and you do not want to be dishonest. The friend might just say that it's no worth it and you could avoid dishonesty. If it doesn't work out for the best and the friend wants you to keep the secret, it's basically your choice. Keeping a precious secret might seem like a good reason for telling a lie. The fact is a lie is a lie wether there is a reason for it or not. Once you start arguing that lie you will argue every lie that you ever told or will ever tell. There is always a reason to lie though it may not be as thoughtful as keeping a secret. Everytime a lie is told it's dishonesty, for it not to be would require a change in the meaning of dishonesty.