Security of AES in CTR mode with fixed nonce (but different key) If I am using AES in CTR mode and use a unique key every time but the same nonce, is this secure?

If so, does it make a difference if the nonce is public?
Yes, if you use a different key for each message, then it's secure, and it's not a problem if you use a public (or fixed) nonce.

One example where this comes up is the Integrated Encryption Scheme, which uses a symmetric system to encrypt the actual message, and which internally generates a key that will be used only for that message.  Note that in IES, you'll also need to implement integrity protection as well (e.g. a MAC)