+ * 128bit block we have used is contained in *num, and the
+ * encrypted counter is kept in ecount_buf. Both *num and
+ * ecount_buf must be initialised with zeros before the first
+ * call to AES_ctr128_encrypt().
+ *
+ * This algorithm assumes that the counter is in the x lower bits
+ * of the IV (ivec), and that the application has full control over
+ * overflow and the rest of the IV. This implementation takes NO
+ * responsability for checking that the counter doesn't overflow
+ * into the rest of the IV when incremented.