I'm setting up some external hardware for OTP encryption to take care of all the issues that can arise with key storage, re-use etc. I'm using the ISAAC secure PRNG to get my stream of numbers which I read is cryptographically secure (so far). The problem is that although most streams are very large, there are some keys that will produce runs of only 2^40 before repeating. As it is impossible to tell how long the sequence will be from a particular key my question is would the OTP still be secure if the key starts again after 2^40 iterations? Of course it would be a different message and would be unlikely to be using the same sequence.