Again, your H(counter+seed) solution for generating unique IDs is perfectly fine. This is clean, neat and fast. There are no subtle issues whatsoever. An HMAC w

Again, your H(counter+seed) solution for generating unique IDs is perfectly fine. This is clean, neat and fast. There are no subtle issues whatsoever. An HMAC will not give you any extra security in this case. (The issues with H(message+secret) only exist when the attacker has some control on "message", but he does not in your case --I am familiar with this, I do crypto reviews for a living.) I cannot update my comment (HN only lets you edit posts < 2 hours old), but I would not want to update it anyway since what I say is true :) However I think you should edit your blog post and mention that the order of concatenation of the counter and seed is important. If you want say something along the lines of "HMAC is traditionally the best practice but in this case H(counter+seed) is fine".