  This paper establishes a fundamental theory of secure clock synchronization.
Accurate clock synchronization is the backbone of systems managing power
distribution, financial transactions, telecommunication operations, database
services, etc. Some clock synchronization (time transfer) systems, such as the
Global Navigation Satellite Systems (GNSS), are based on one-way communication
from a master to a slave clock. Others, such as the Network Transport Protocol
(NTP), and the IEEE 1588 Precision Time Protocol (PTP), involve two-way
communication between the master and slave. This paper shows that all one-way
time transfer protocols are vulnerable to replay attacks that can potentially
compromise timing information. A set of conditions for secure two-way clock
synchronization is proposed and proved to be necessary and sufficient. It is
shown that IEEE 1588 PTP, although a two-way synchronization protocol, is not
compliant with these conditions, and is therefore insecure. Requirements for
secure IEEE 1588 PTP are proposed, and a second example protocol is offered to
illustrate the range of compliant systems.
