[Haskell-cafe] Exporting a Type Class for Type Signatures

In the crypto package, I have two functions

encrypt :: AESKey a => a -> Word128 -> Word128 decrypt :: AESKey a => a -> Word128 -> Word128

which are exported. I also have

class (Bits a, Integral a) => AESKey a

instance AESKey Word128 instance AESKey Word192 instance AESKey Word256

unexported which stops you using invalid keys. Someone has asked me to export AESKey as they want to write an explicit type signature for a function they are creating e.g. foo :: AESKey a => a -> Word128 -> Word128 foo x y = encrypt x y but this generates an error. Is there a way of allowing someone to use AESKey in a type signature but not allow them to declare new instances? Thanks, Dominic.