@public is correct; OpenSSL's assumption in counter-based modes is that you have provided an n-bit integer (where n is the block size of the underlying algorithm). It monotonically increases it and does not prevent it from wrapping around assuming you encrypt sufficient blocks.

At this time we don't have any code for creating counter objects. If we eventually decide to go this route it will be difficult (impossible?) to use with some backends (like OpenSSL) that want to manage the counter themselves.