You are designing a Group Encrypted Transport Virtual Private Network solution consisting of 30 group members. Which measure helps protect encrypted user traffic from replay attacks?

Counter-based anti-replay requires per-flow sequence number state that cannot be practically shared or synchronized across all group members using the same group SA.

In GET VPN, all group members share the same TEK (Traffic Encryption Key) and Security Association, so counter-based anti-replay cannot be used - it would require per-sender counter state to be synchronized across all 30 members. Time-based anti-replay instead embeds a timestamp in each packet and rejects traffic outside an acceptable time window, providing replay protection without per-member counter synchronization.

Nonce payloads are exchanged during IKE phase negotiations to ensure key freshness and are not a mechanism for protecting encrypted user traffic from replay.

RSA-encrypted nonces are used in IKE Main Mode for peer authentication and do not protect data-plane traffic from replay attacks.

Digital certificates authenticate peers during IKE establishment but provide no protection against replay of already-encrypted user traffic.