The Threat of Covert Channels in Network Time Synchronisation Protocols

Abstract

Synchronized clocks are vital for most communication scenarios in networks of Information Technology (IT) and Operational Technology (OT). The process of time synchronisation requires transmission of high-precision timestamps often originating from external sources. In this paper, we analyze how time synchronization protocols impose a threat by being leveraged as carrier for network covert channels. This paper is an extended version version of our open-access paper [15] in which we performed an in-depth analysis of the Network Time Protocol (NTP) in regards to covert channels. In this extended version, we broaden the view and take a look and time synchronisation in a more general way as we provide two comprehensive threat scenarios regarding covert channels and discuss the applicability of such covert channels to another time synchronisation protocol, namely the Precision Time Protocol, PTP. While the Network Time Protocol (NTP) is the most prevalent protocol for synchronizing clocks in IT networks, the Precision Time Protocol (PTP) is mostly found in networks of Industrial Control Systems (ICS) due to higher demands regarding accuracy and resolution. To illustrate the threat of covert channels in such protocols we describe two threat scenarios, one for the Network Time Protocol and one for the Precision Time Protocol. For NTP we perform a systematic in-depth analysis of covert channels. Our analysis results in the identification of 49 covert channels, by applying a covert channel pattern-based taxonomy. The summary and comparison based on nine selected key attributes show that NTP proofs itself as a plausible carrier for covert channels. The analysis results are evaluated in regards to common behavior of NTP implementations in six major operating systems. Two channels are selected and implemented to be evaluated in network test-beds. By hiding encrypted high entropy data in a high entropy field of NTP we show in our first assessment that practically undetectable channels can be implemented in NTP, motivating the required further research. In our evaluation, we analyze 40,000 NTP server responses from public NTP server providers and discuss potential countermeasures. Finally, we discuss the relevance, applicability and resulting threat of these findings for the Precision Time Protocol.

Covert Channels in Network Time Security

“…The complete transfer of the text (169kb) takes roughly 12 hours in the test environment (256 bit per UID, 62 seconds between time requests using chrony). First tests with statistical methods (entropy, runs-test, chi-squared) and rand-test 9 we were not able to differentiate between original and embedded UIDs. In the following, we describe how to overcome this issue and design an active warden, capable of mitigating the covert channels described in this paper.…”

“…With the decrypted client-to-server 𝐾 𝐶2𝑆 the server verifies the signature of the NTS time request (8). The server generates a NTP response (9), inserts the same Unique Identifier from the request, generates and encrypts new cookies using its secret server key 𝐾 𝑆 (10) and inserts these into the NTS response (11). To secure the transmission, the new cookies are encrypted using the server-to-client key 𝐾 𝑆2𝐶 (12) (so that only the authenticated client can decrypt the cookies).…”

“…For retrieval on the server side, the server seeds the PRNG with the shared secret 𝐾 𝑆𝑡𝑒𝑔 (8) and generates a pseudorandom ID as well. To retrieve the plaintext the server extracts the UID from the clients NTS time request (9) and performs an XOR operation on the extracted UID and the pseudo-random ID it generated with the stego key (10). To retrieve the next message block the server seeds the PRNG with the previous pseudo-random ID (11), generates a new pseudo-random ID, extracts the next UID from the clients time request and performs again an XOR operation on both the UID from the client and the newly generated UID from the server (12).…”

“…This paper aims at closing that gap. In the past related protocols, namely NTP and PTP were analysed in regards to their potential to be used as carrier for covert channels [7,9], for example to covertly infiltrate or exfiltrate information out of network, or to establish Command-and-Control channels (shortened C2 channel). Therefore, in this paper we want to shed light on the threat of covert channels in the novel Network Time Security protocol as the use of cryptography adds further options to establish covert channels and makes their detection even harder.…”

“…However, these form a sub-protocol on their own and do not play a role in NTS. In [9] Lamshöft et al investigate networked time synchronisation protocols as carrier for hidden communication and propose covert channels for the Precision Time Protocol (PTP). In this paper, we focus specifically on covert channels to be found in the NTS specification.…”