  Internet traffic is increasingly encrypted. While this protects the
confidentiality and integrity of communication, it prevents network monitoring
systems (NMS) and intrusion detection systems (IDSs) from effectively analyzing
the now encrypted payloads. Therefore, many enterprise networks have deployed
man-in-the-middle (MitM) proxies that intercept TLS connections at the network
border to examine packet payloads and thus retain some visibility. However,
recent studies have shown that TLS interception often reduces connection
security and potentially introduces additional attack vectors to the network.
In this paper, we present a cooperative approach in which end-hosts as
cryptographic endpoints selectively provide TLS key material to NMS for
decryption. This enables endpoints to control who can decrypt which content and
lets users retain privacy for chosen connections. We implement a prototype
based on the Zeek NMS that is able to receive key material from hosts, decrypt
TLS connections and perform analyzes on the cleartext. The patch is freely
available and we plan to upstream our changes to Zeek once they are mature
enough. In our evaluation, we discuss how our approach conceptually requires
significantly less computational resources compared to the commonly deployed
MitM proxies. Our experimental results indicate, that TLS decryption increases
a runtime overhead of about 2.5 times of the original runtime on cleartext.
Additionally, we show that the latency for transmitting keys between hosts and
the NMS can be effectively addressed by buffering traffic at the NMS for at
least 40ms, allowing successful decryption of 99.99% of all observed TLS
connections.
