Secure and efficient methods and systems of providing multiple access interference constraints for frequency hopped spread spectrum communications are presented. The methods and systems leverage a cryptographic keystream generator and iterative indexing into shuffled lists to generate a constrained collision-free frequency hopping sequence. This flexible frequency hopping access (FFHA) can support varying hop frequency and bandwidth resources including secure operation for applications where the number of channels (hopping sequences) exceeds the number of hop frequency resources available.