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15. Security Considerations

NTP security requirements are stringent due to the circular dependency between authentication and time synchronization. Reliable time synchronization requires cryptographic keys valid over time intervals, but time intervals can only be enforced with reliable synchronization.

Attack Types​

1. Archive attacks: Intruders can intercept and store packets indefinitely.

2. Resource exhaustion: Generating packets faster than processing capability, especially with expensive crypto operations.

3. Wiretap attacks: Intercept, modify, and replay packets without breaking transmission.

4. Middleman/masquerade attacks: Positioned between server and client, can block original packets but lacks server private keys.

Built-in Defenses​

  • On-wire timestamp exchange inherently resistant to spoofing, packet-loss, and replay attacks
  • Clock filter, selection, and clustering algorithms defend against Byzantine traitors
  • Multi-server associations with agreement algorithms distinguish truechimers from falsetickers

Security Recommendations​

Use cryptographic authentication, maintain secure key distribution, configure multiple independent time sources, implement access control lists, monitor anomalies, and regularly update keys.

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