  In the past decades, Ethernet has become an alternative technology for the
field buses traditionally used in industrial control systems and distributed
measurement systems. Among different transmission media in Ethernet standards,
optical fiber provides the best bandwidth, excellent immunity to
electromagnetic interference, and less signal loses than other wired media. Due
to the absence of a standard that provides security at the physical layer of
optical Ethernet links, the main motivation of this paper is to propose and
implement the necessary modifications to introduce encryption in Ethernet
1000Base-X standard. This has consisted of symmetric streaming encryption of
the 8b10b symbols flow at physical coding sublayer level, thanks to a keystream
generator based on chaotic algorithm. The overall system has been implemented
and tested in an field programmable gate array and Ethernet traffic has been
encrypted and transmitted over an optical link. The experimental results show
that it is possible to cipher traffic at this level and hide the complete
Ethernet traffic pattern from passive eavesdroppers. In addition, no space
overhead is introduced in data frames during encryption, achieving the maximum
throughput.
