  Clock synchronization has become essential to modern societies since many
critical infrastructures depend on a precise notion of time. This paper
analyzes security aspects of high-precision clock synchronization protocols,
particularly their alleged protection against delay attacks when clock
synchronization traffic is encrypted using standard network security protocols
such as IPsec, MACsec, or TLS. We use the Precision Time Protocol (PTP), the
most widely used protocol for high-precision clock synchronization, to
demonstrate that statistical traffic analysis can identify properties that
support selective message delay attacks even for encrypted traffic. We
furthermore identify a fundamental conflict in secure clock synchronization
between the need of deterministic traffic to improve precision and the need to
obfuscate traffic in order to mitigate delay attacks.
  A theoretical analysis of clock synchronization protocols isolates the
characteristics that make these protocols vulnerable to delay attacks and
argues that such attacks cannot be prevented entirely but only be mitigated.
Knowledge of the underlying communication network in terms of one-way delays
and knowledge on physical constraints of these networks can help to compute
guaranteed maximum bounds for slave clock offsets. These bounds are essential
for detecting delay attacks and minimizing their impact. In the general case,
however, the precision that can be guaranteed in adversarial settings is orders
of magnitude lower than required for high-precision clock synchronization in
critical infrastructures, which, therefore, must not rely on a precise notion
of time when using untrusted networks.
