\documentclass[letterpaper,twocolumn,10pt]{article} \usepackage{usenix} \usepackage[T1]{fontenc} \usepackage[ backend=biber, style=numeric, natbib=true, url=false, doi=true, eprint=false ]{biblatex} \addbibresource{safety-reset.bib} \usepackage{amssymb,amsmath} \usepackage{eurosym} \usepackage{wasysym} \usepackage[binary-units]{siunitx} \DeclareSIUnit{\baud}{Bd} \DeclareSIUnit{\year}{a} \usepackage{commath} \usepackage{graphicx,color} \usepackage{subcaption} \usepackage{array} \usepackage{hyperref} \usepackage{enumitem} \renewcommand{\floatpagefraction}{.8} \newcommand{\degree}{\ensuremath{^\circ}} \newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}} \newcommand{\partnum}[1]{\texttt{#1}} \begin{document} % https://eepublicdownloads.entsoe.eu/clean-documents/pre2015/publications/entsoe/Operation_Handbook/Policy_1_Appendix%20_final.pdf \date{} \title{Ripples in the Pond: Transmitting Information through Grid Frequency Modulation} \author{Jan Sebastian Götte \and Liran Katzir \and Björn Scheuermann} %\institute{TU Darmstadt\\ Communication Networks Lab\\ \email{safetyreset@jaseg.de} %\and Tel Aviv University\\ Faculty of Engineering\\ \email{lirankat@tau.ac.il} %\and TU Darmstadt\\ Communication Networks Lab\\ \email{scheuermann@informatik.hu-berlin.de}} \maketitle %\keywords{Security, privacy and resilience in critical infrastructures \and Security and privacy in ``internet of %things'' \and Cyber-physical systems \and Hardware security \and Network Security \and Energy systems \and Signal theory} \begin{abstract} Previous work has explored the scenario of an attacker compromising a large number of consumer devices, and modulating the power of these devices to cause large load swings at particular resonant frequencies of the electrical grid's control systems that ultimately cause a large-scale outage~\cite{ctap+11,wu01}. Previous work has focused on attacks using smart meters with integrated remote disconnect switches as first proposed in~\cite{anderson01}, but the same attack scenario also applies to large IoT devices such as IoT-equipped air conditioners or central heating systems. Prior work on mitigation of this attack scenario includes generic firmware hardening techniquies % FIXME citation and reducing the susceptibility of the electrical grid towards these resonant oscillation modes~\cite{entsoe01}. In this paper, we will complement these mitigation efforts by considering the recovery process after a successful attack. To transmission system operators (TSOs), the major challenge after such a Smart Meter-triggered outage is that the attacker will likely persist through the outage, and compromised Smart Meters will resume malicious activity after their power is restored. In the event of such an attack, TSOs would need a way to remotely put these compromised devices into a \emph{safe} mode of operation. For this purpose, we propose a remote-controllable \emph{Safety reest} that is designed to remain operational even during a large-scale attack. Given that public telecommunications networks including the internet, cellular networks, and LoRa base stations may also be disrupted during a blackout, the challenging aspect of this \emph{Safety Reset} is the communication channel between TSO and the smart meter. For this purpose, in this paper we propose a simple yet effective communication channel based on modulating grid frequency by modulating the power of a connected load or generator. Our proposed communciation channel (1) requires minimal infrastructure, (2) has a reach spanning the entire power grid and (3) is fully independent of other telecommunication networks and functions even under severe disruption of the grid. The resulting safety reset can be applied to any grid-connected device including smart meters and IoT devices. \end{abstract} \section{Introduction} With the rollout of the smart grid, the IT security of electrical infrastructure has attracted increased attention in the last years. Smart Grid security has two major components: The security of central SCADA systems, and the security of equipment at the consumer premises such as smart meters and IoT devices. While there is previous work on both sides, their interactions have not yet received much attention. In this paper, we consider the previously proposed scenario where a large number of compromised consumer devices is used alone or in conjunction with an attack on the grid's central SCADA systems to destabilize the grid by rapidly modulating the total connected load~\cite{ctap+11,wu01}. Previous work considered compromised smart meters with integrated remote disconnect switches as likely candidates for such an attack, but the same attack can also be performed using compromised IoT devices. Such attacks are hard to mitigate, and existing literature focuses on hardening device firmware to prevent compromise. Despite the infeasibility of perfect firmware security, there is little research on \emph{post-compromise} mitigation approaches. A core issue with post-attack mitigation is that the devices normal network connection may not work due to the attack and as such an out-of-band communication channel is necessary. We propose a \emph{safety reset} controller that is controlled through a novel, resilient, grid-wide powerline communication technique. Our safety reset controller can be fitted into any Smart Meter or IoT device. Its purpose is to await an out-of-band command to put the device into a safe state (e.g. \emph{relay on} or \emph{light on}) that interrupts attacker control over the device. The safety reset controller is separated from the system's main application controller and does not have any conventional network connections to reduce attack surface and cost. We propose a resilient grid-wide broadcast channel based on modulating grid frequency. This channel can be operated by transmission system operators (TSOs) even during black-start recovery procedures and in this situation bridges the gap between the TSO's private network and the consumer devices. To demonstrate our proposed channel, we have implemented a system that transmits error-corrected and cryptographically secured commands. Our approach differs from traditional Powerline Communication (PLC) systems in that it reaches every device within one synchronous area as the signal is embedded into the fundamental grid frequency. Traditional PLC uses a superimposed voltage, which is quickly attenuated across long distances. \begin{figure} \centering \includegraphics[width=0.4\textwidth]{flowchart} \caption{Structural overview of our concept. 1 - Government authority or utility operations center. 2 - Emergency radio link. 3 - Aluminium smelter. 4 - Electrical grid. 5 - Target smart meter.} \label{fig_intro_flowchart} \end{figure} Figure~\ref{fig_intro_flowchart} shows an overview of our concept. Two scenarios for its application are before or during a cyberattack, to stop an attack on the electrical grid in its tracks, and after an attack while power is being restored to prevent a repeated attack. In both scenarios, our concept is independent of telecommunication networks (such as the internet or cellular networks) as well as broadcast systems (such as cable television or terrestrial broadcast radio) while requiring only inexpensive signal processing hardware and no external antennas (such as are needed for satellite communication). A grid frequency-based system can function as long as power is still available, or as soon as power is restored after the attack. One powerful function this allows is ``flushing out`` an attacker from compromised smart meters after an attack, before restoring smart meter internet connectivity. Using simulations we have determined that control of a $\SI{25}{\mega\watt}$ load such as a large aluminium smelter, load bank or photovoltaic farm would allow for the transmission of a crytographically secured \emph{reset} signal within $15$ minutes. We have designed and constructed a proof-of-concept prototype receiver that demonstrates the feasibility of decoding such signals on a resource-constrained microcontroller. \subsection{Motivation} Consumer devices are increasingly becoming \emph{smart}. Large numbers of IoT devices are connected through the public internet, and in several countries internet-connected Smart Meters can disconnect entire households from the grid in case of unpaid bills. The increasing proliferation of smart devices on the consumer side presents an opportunity to grid operators, who rely on forecasts for the cost-optimized control of generation and power flow. The core of the \emph{Smart Grid} vision is that utilities can now gather detailed data for more accurate consumption forecasts, and in some cases can even adjust parameters of large devices like water heaters to smooth out load spikes. However, this increased degree of visibility and control comes with an increased IT security risk. In this paper we focus on scenarios where an attacker compromises a large number of grid-connected remote-controllable devices. This may be simple smart home devices such as IoT light bulbs, but it may also include Smart Meters that are outfitted with a remote disconnect switch as is common in some countries. By rapidly switching large numbers of such devices in a coordinated manner, the attacker has the opportunity to de-stabilize the electrical grid. % FIXME citation Previous work on IoT and Smart Grid security has focused on the prevention of attacks though firmware security measures. While research on prevention is undoubtably important, we estimate that its practical impact will be limited by the vast diversity of implementations found in the field combined with the slow update cycles inherent to non-functional firmware enhancements for consumer devices. We predict that it would be a Sisyphean task to secure sufficiently many devices to deny an attacker the critical mass needed to cause trouble. For this reason, in this paper we focus on recovery after an attack. \subsection{Black-start recovery} The recovery from a large-scale power outage is a complex operational challenge. Large outages are caused by cascading failures. Since all consumers and producers that are connected to the electrical grid are physically coupled through the electromotive force, a fault in one part of the grid affects all devices connected across the grid. To function, the grid relies on a delicate balance between electricity generation, transmission and consumption. When this balance is disturbed, cascading failures can occur. A transmission line shutting off can lead other, nearby lines to overload and shut off. Due to the electromechanical coupling of all machines connected to the grid, a generator or consumer suddenly shutting off causes a transient in the grid's frequency. If the frequency goes too far out of bounds, protection devices take power plants and large industrial loads offline. The recovery from a large-scale outage requires the grid's operators to bring generators and loads back online one by one while continuously maintaining balance between generation and consumption to avoid their protection devices shutting them down again. To coordinate this process, transmission system operators cannot rely on the public internet or cellular networks, as they may not work during a large-scale power outage. Instead, they maintain private communication infrastructure using dedicated lines rented from telecommunciations providers, fibers run along transmission lines, and dedicated radio links. To start from a complete outage, first a number of \emph{black start}-capable power stations that can start by themselves without any external power are brought online. With their help, other power stations and consumers are gradually brought online until a part of the grid has been restored to nominal operation. This process can be performed simultaneously in different parts of the grid. After these \emph{islands} have been restored, they can then be joined to restore the grid to its normal state. \subsection{Contents} Starting from a high level architecture, we have carried out simulations of our concept's performance under real-world conditions. Based on these simulations we implemented an end-to-end prototype of our proposed safety reset controller as part of a realistic smart meter demonstrator. Finally, we experimentally validated our results and we will conclude with an outline of further steps towards a practical implementation. This work contains the following contributions: \begin{enumerate}[topsep=4pt] \item We introduce Grid Frequency Modulation (GFM) as a communication primitive. % FIXME done before in that one paper \item We elaborate the fundamental physics underlying GFM and theorize on the constrains of a practical implementation. \item We design a communication system based on GFM. \item We carry out extensive simulations of our systems to determine its performance characteristics. \end{enumerate} \subsection{Notation} To a computer scientist there is one confusing aspect to the theory of grid frequency modulation. GFM can be seen as a frequency modulation (FM) with a baseband signal in the band below approximately $f_m = \SI{5}{\hertz}$ that is modulated on top of a carrier signal at $f_c = \SI{50}{\hertz}$ in case of the European electrical grid. The frequency deviation $f_\Delta$ that the modulated carrier deviates from its nominal value of $f_m$ is very small at only a few milli-Hertz. When grid frequency is measured by first digitizing the mains voltage waveform, then de-modulating digitally, the FM's SNR is very high and is dominated by the ADC's quantization noise and nearby mains voltage noise sources such as resistive droop due to large inrush current of nearby machines. Note that both the carrier signal at $f_c$ and the modulation signal at $f_m$ both have unit Hertz. To disambiguate them, in this paper we will use \textbf{bold} letters to refer to the carrier waveform $\mathbf{U}$ or frequency $\mathbf{f_c}$ as well as its deviation $\mathbf{f_\Delta}$, and we will use normal weight for the actual modulation signal and its properties such as $f_m$. \section{Related work} \label{sec_related_work} Previous work has analyzed Smart Grid security from numerous angles and made several suggestions towards its improvement. Apart from the critical location that Smart Grid devices occupy, they are computer systems like many others. Thus, for IT security purposes the Smart Grid is simply an aggregation of embedded control and measurement devices that are part of a large control system. These devices share the same security concerns that apply to embedded systems in general. \subsection{Smart Meter Security} Where programmers have been struggling for decades now with issues such as input validation~\cite{leveson01}, the same potential issue raises security concerns in smart grid scenarios as well~\cite{mo01, lee01}. Only, in smart grid we have two complicating factors present: many components are embedded systems, and as such inherently hard to update. Also, the smart grid and its control algorithms act as a large (partially) distributed system making problems such as input validation or authentication harder~\cite{blaze01} and adding a host of distributed systems problems on top~\cite{lamport01}. Given that the electrical grid is essential infrastructure, these issues are significant. Attacks on the electrical grid may have grave consequences~\cite{anderson01,lee01} while the long replacement cycles of various components make the system slow to adapt. Thus, components for the smart grid need to be built to a higher standard of security than e.g.\ IoT devices to live up to well-funded attackers decades down the road. Another implication of their long service life is that their agility w.r.t.\ post-hoc mitigations through firmware updates is limited. %Another fundamental challenge in smart grid implementations is the central role of smart electricity meters in the %smart grid ecosystem. Smart meters are used both for highly-granular load measurement and in some countries also for %load switching~\cite{zheng01}. Smart electricity meters are consumer devices built down to a price. Firmware security research and development budgets are limited by the high degree of market fragmentation that is caused by mutually incompatible national smart metering standards. Landis+Gyr, a large utility meter manufacturer, state in their 2019 annual report that they invested \SI{36}{\percent} of their total R\&D budget on embedded software while spending only \SI{24}{\percent} on hardware R\&D~\cite{landisgyr01,landisgyr02}, which indicates tension between firmware security and the manufacturers's bottom line. % FIXME more sources! \subsection{The state of the art in embedded security} Embedded software security has proven challenging compared to the security of larger computer systems. On one hand, embedded devices usually run highly customized firmware that is rarely updated. On the other hand, embedded devices often lack security mechanisms such as memory management units that are found in higher-power devices. As a result of these factors, even well-funded companies continue to have trouble securing their embedded systems. An example of this difficulty is the 2019 flaw in Apple's iPhone SoC first-stage ROM bootloader that allows for the full compromise of any iPhone older than iPhone X given physical access to the device~\cite{heise01}. iPhone 8, one of the affected models, was still being manufactured and sold by Apple until April 2020. In another instance in 2016, researchers found multiple flaws in Samsung's implementation of ARM TrustZone ``secure world'' firmware that Samsung used for their own mobile phone SoCs. The flaws they found were both architectural flaws such as secret user input being passed through untrusted userspace processes as well as cryptographic flaws such as CVE-2016-1919\footnote{\url{http://cve.circl.lu/cve/CVE-2016-1919}}~\cite{kanonov01}. In a similar way, in 2014, researchers found an integer overflow flaw in the low-level code handling untrusted input in Qualcomm's QSEE firmware\footnote{For an overview of ARM TrustZone including a survey of academic work and past security vulnerabilities of TrustZone-based firmware see~\cite{pinto01}.}~\cite{rosenberg01}. If even companies with R\&D budgets that rival some countries' national budgets at mass-market consumer devices have trouble securing their mass market secure embedded software stacks, what is a much smaller smart meter manufacturer to do? Especially if national standards mandate complex protocols such as TLS that are difficult to implement correctly~\cite{georgiev01}, this manufacturer will be short on options to secure their product. \subsection{Attack surface in the smart grid} From the incidents we outlined in the previous paragraphs we conclude that in smart metering technology, market incentives do not currently provide the conditions for a level of device security that will reliably last for decades after deployment. Considering this tension, in this paragraph we examine the cyberphysical risks that arise from attacks on the smart grid in the first place. These risks arise at three different infrastructure levels. The first level is that of attacks on centralized control systems. This type of attack is often cited in popular discourse and to our knowledge is the only type of attack against an electric grid that has ever been carried out in practice at scale~\cite{lee01}. Despite their severity, these attacks do not pose a strictly \emph{scientific} challenge since they are generic to any industrial control system. Their causes and countermeasures are generally well-understood and the hardest challenge in their prevention is likely to be budgetary constraints. Beyond the centralized control systems, the next target for an attacker may be the communication links between those control systems and other smart grid components. While in some countries such as Italy special-purpose systems such as PLC are common~\cite{ec03}, overall, IP-based technologies have proliferated according to the larger trend towards IP-based communications. This proliferation of IP links brings along the possibility for the application of generic network security measures from the IP world to the smart grid domain. In this way, a standardized, IP-based protocol stack unlocks decades of network security improvements at little cost. Beyond these layers towards the core of the smart grid's control infrastructure, an attacker might also corrupt the network from the edges and target the endpoint devices itself. The large scale deployment of networked smart meters creates an environment that is favorable to such attacks. % FIXME cite RECESSIM landis+gyr protocol hacking wiki/youtube \subsection{Cyberphysical threats in the smart grid} Assuming that an attacker has compromised devices on any of these levels of smart grid infrastructure, what could they do with their newly gained power? The obvious action would be to switch off everything. Of all scenarios, this is both the most likely in practice---it is exactly what happened in the Russian cyberattacks on the Ukranian grid~\cite{lee01}---but it is also the easiest to mitigate since the vulnerable components are few and centralized. Mitigations include the installation of fail safes as well as a defense in depth approach to hardening the grid's cyber infrastructure. Another possible action for an attacker would be to forge energy measurements in an attempt to cause financial mayhem. Both individual consumers as well as the utility could be targeted by such an attack. While such an attack might have localized success, larger-scale discrepancies will likely quickly be caught by monitoring systems. For example, if a large number of meters in an area systematically under- or over-reported their energy readings, meter readings across the affected area would no longer add up with those of monitoring devices in other locations in the transmission and distribution grid. In some countries, smart meter functionality goes beyond mere monitoring devices and also includes remotely controlled switches. There are two types of these switches: Switches to support \emph{Demand-Side Management} (DMS) and cut off-switches that are used to punish defaulting customers. Demand Side Management is when a grid operator can remotely control the timing of large, non-time-critical loads on the customer's premises~\cite{dzung01}. A typical example of this is a customer using an electric water heater: The heater is outfitted with a large hot water storage tank and is connected hooked up to the utility's DSM system. The customer does not care when exactly their water is heated as long as there is enough of it, and the utility offers them cheaper rates for the electricity used for heating in exchange for control over its precise timing. The utility uses this control to even out peaks in the consumption/production imbalance, remotely enabling DSM systems during off-peak times and disabling them during peak hours. In contrast to DSM, cut-off switches are switches placed in between the grid and the entire customer's household such that the utility can disconnect non-paying customers without incurring the expense of sending a technician to the customer's premises. Unlike DSM systems, cut-off switches are not opt-in~\cite{anderson01,temple01}. An attack that uses cut-off switches would obviously immediately cause severe mayhem. Attacks on DSM may have more limited immediate impact as affected consumers may not notice an interruption for several hours. Instead of switching off loads outright, an attack employing DSM switches (and potentially also cut-off switches) could choose to target the grid's stability. By synchronizing many compromised smart meters to switch on and off a large load capacity, an attacker might cause the entire electrical grid to oscillate~\cite{kosut01,wu01,kim01}. As a large system of coupled mechanical systems, the electrical grid exhibits a complex frequency-domain behavior. Resonance effects, colloquially called ``modes'', are well-studied in power system engineering~\cite{rogers01,grebe01,entsoe01,crastan03}. As they can cause issues even under normal operating conditions, a large effort is invested in dampening these resonances. Howewer, fully eliminating them under changing load conditions may not be achievable. \subsection{Communication Channels on the Grid} A core part of intervening with any such cyberattack is the ability to communicate remediary actions to the devices under attack. There is a number of well-established technologies for communication on or along power lines. We can distinguish three basic system categories: systems using separate wires (such as DSL over landline telephone wiring), wireless radio systems (such as LTE) and \emph{Power Line Communication} (PLC) systems that reuse the existing mains wiring and superimpose data transmissions onto the 50 Hz mains sine~\cite{gungor01,kabalci01}. During a large-scale cyberattack, availability of internet and cellular connectivity cannot be relied upon. An attacker may already have disabled such systems in a separate attack, or they may go down along with parts of the electrical grid. Traditional powerline communication systems or an utitly's proprietary wireless systems would work, but at a range of no more than several tens of kilometers reaching all meters in a country would require a large upfront infrastructure investment. \section{Grid Frequency as a Communication Channel} We propose to approach the problem of broadcasting an emergency signal to all smart meters within a synchronous area by using grid frequency as a communication channel. Despite the technological complexity of the grid, the physics underlying its response to changes in load and generation is surprisingly simple. Individual machines (loads and generators) can be approximated by a small number of differential equations and the entire grid can be modelled by aggregating these approximations into a large system of nonlinear differential equations. As a consequence, small signal changes in generation/consumption power balance cause an approximately proportional change in frequency~\cite{kundur01,crastan03,entsoe02,entsoe04}. This \emph{Power Frequency Charactersistic} is about \SI{25}{\giga\watt\per\hertz} for the continental European synchronous area according to European electricity grid authority ENTSO-E. If we modulate the power consumption of a large load such as a multi-megawatt aluminium smelter, this modulation will result in a small change in frequency according to this characteristic. As long as we stay within the operational limits set by ENTSO-E~\cite{entsoe02,entsoe03}, this change will not degrade the operation of other parts of the grid. The advantages of grid frequency modulation are the fact that a single transmitter can cover an entire synchronous area as well as low receiver hardware complexity. To the best of the authors' knowledge, grid frequency modulation has only ever been proposed as a communication channel at very small scales in microgrids before~\cite{urtasun01} and has not yet been considered for large-scale application. Compared to traditional channels such as DSL, LTE or LoraWAN, grid frequency as a communication channel has a large resiliency advantage: If there is power, a grid frequency modulation system is operational. Both DSL and LTE systems not only require power but also require large amounts of centralized infrastructure to operate. Mesh networks such as LoraWAN can cover short distances up to $\SI{20}{\kilo\meter}$ without requiring infrastructure to be available, but for longer distances LoraWAN relies on the public internet for its network backbone. Additionally, systems such as DSL, LTE and LoraWAN are built around a point-to-point communication model and usually do not support a generic broadcast primitive. During times when a large number of devices must be reached simultaneously this can lead to congestion of local cellular towers or gateways. Therefore, during an ongoing cyberattack, grid frequency is promising as a communication channel as only a single transmitter facility must be operational for it to function, and this single transmitter can reach all connected devices simultaneously. After a power outage, it can function as soon as electrical power is restored, even while the public internet and mobile networks are still offline and it is unaffected by cyberattacks that target telecommunication networks. \subsection{Characterizing Grid Frequency} \label{grid-freq-characterization} To collect ground truth measurements for our analysis of grid frequency as a communication channel, we developed a device to safely record mains voltage waveforms. Our system consists of an \texttt{STM32F030F4P6} ARM Cortex M0 microcontroller that records mains voltage using its internal 12-bit ADC and transmits measured values through a galvanically isolated USB/serial bridge to a host computer. We derive our system's sampling clock from a crystal oven to avoid frequency measurement noise due to thermal drift of a regular crystal: \SI{1}{ppm} of crystal drift would cause a grid frequency error of $\SI{50}{\micro\hertz}$. We compared our oven-stabilized clock against a GPS 1 pps reference and found that over a time span of 20 minutes both stayed stable within 5 ppb of each other, which corresponds to the drift specification of a typical crystal oven. In utility SCADA systems, Phasor Measurement Units (PMUs, also called \emph{synchrophasors}) are used to precisely measure grid frequency among other parameters. Details on the inner workings of commercial phasor measurement units are scarce but there is a large amount of academic research on measurement. PMUs employ complex signal analysis algorithms to provide fast and precise measurements even when given a heavily distorted input signal~\cite{narduzzi01,derviskadic01,belega01}. In our application, we do not need the same level of precision. For the sake of simplicity, we use the universal frequency estimation approach of Gasior and Gonzalez~\cite{gasior01}. In this algorithm, the windowed input signal is processed using a Discrete Fourier Transform (DFT), then the signal's fundamental frequency is interpolated by fitting a wavelet to the largest peak in the DFT result. The bias parameter of this curve fit is an accurate estimation of the signal's fundamental frequency. This algorithm is similar to the interpolated DFT algorithm referenced by phasor measurement literature~\cite{borkowski01}. \begin{figure} \centering \includegraphics[width=0.45\textwidth]{../notebooks/fig_out/freq_meas_spectrum_new} \caption{The spectrum of grid frequency variations measured over 24 hours. The raw spectrum is shown in gray, and a smoothed spectrum is shown in red. The blue line is inversely proportional to frequency and illustrates the $1/f$ nature of the spectrum. Distinctive peaks in the spectrum are marked with red crosses, and their locations are given on the bottom of the diagram.} \label{fig_freq_spec} \end{figure} Using our grid frequency recorder, we performed a two-day measurement series of grid frequency. Figure~\ref{fig_freq_spec} shows the frequency spectrum of grid frequency over this two-day span. In this spectrum, we observe a number of features. Across the frequency range, we observe a broad $1/f$ noise. Above a period of $\SI{10}{\second}$, this $1/f$ noise dips to a flat noise floor. We estimate that this low-noise region is caused by the self-regulating effect of loads. %FIXME citation Above a $\SI{10}{\second}$ period, primary control is activated and thus the $1/f$ noise we observe is the result of the interaction between primary control and consumer demand. On top of this $1/f$ behavior, the spectrum shows several sharp peaks at time intervals with a ``round'' number such as $\SI{10}{\second}$, $\SI{60}{\second}$ or multiples of $\SI{300}{\second}$. These peaks are due to loads turning on- or off depending on wall-clock time. Besides the narrow peaks caused by this effect we can also observe two wider bumps at $\SI{7.0}{\second}$ and $\SI{4.7}{\second}$. These bumps closely correlate with continental european synchonous area's oscillation modes at $\SI{0.15}{\hertz}$ (east-west) and $\SI{0.25}{\hertz}$ (north-south)~\cite{grebe01}. \section{Grid Frequency Modulation} A transmitter for grid frequency modulation would be a controllable load of several Megawatt that is located centrally within the grid. A baseline implementation would be a spool of wire submerged in a body of cooling liquid (such as a small lake) which is powered from a thyristor rectifier bank. Compared to this baseline solution, hardware and maintenance investment can be decreased by repurposing a large industrial load as a transmitter. Going through a list of energy-intensive industries in Europe~\cite{ec01}, we found that an aluminium smelter would be a good candidate. In aluminium smelting, aluminium is electrolytically extracted from alumina solution. High-voltage mains power is transformed, rectified and fed into about 100 series-connected electrolytic cells forming a \emph{potline}. Inside these pots alumina is dissolved in molten cryolite electrolyte at about \SI{1000}{\degreeCelsius} and electrolysis is performed using a current of tens or hundreds of Kiloampère. The resulting pure aluminium settles at the bottom of the cell and is tapped off for further processing. Aluminium smelters are operated around the clock, and due to the high financial stakes their behavior under power outages has been carefully characterized. Power outages of tens of minutes up to two hours reportedly do not cause problems in aluminium potlines~\cite{eisma01,oye01}. Recently, even techniques for intentional power modulation without affecting cell lifetime or product quality have been developed to take advantage of variable energy prices~\cite{duessel01,eisma01,depree01}. An aluminium plant's power supply is controlled to constantly keep all smelter cells under optimal operating conditions. Modern power supply systems employ large banks of diodes or thyristors to rectify low-voltage AC to DC to be fed into the potline~\cite{ayoub01}. Potline voltage is controlled through a combination of a tap changer and a transductor. Individual cell voltages are controlled by changing the physical distance between anode and cathode distance. In this setup, power can be electronically modulated using the thyristor rectifier. Since the system does not have any mechanical inertia, high modulation rates are possible. In~\cite{depree01}, the authors describe a setup where a large Aluminium smelter in continental Europe is used as primary control reserve for frequency \emph{regulation}. In this setup, a rise time of $\SI{15}{\second}$ was achieved to meet the $\SI{30}{\second}$ requirement posed by local standards for primary control. In their conclusion, the authors note that for their system, an energy storage capacity of $\SI{7.7}{\giga\watt\hour}$ is possible if all plants of a single operator are used. Given the maximum modulation depth of $\SI{100}{\percent}$ for up to one hour that is mentioned by the authors, this results in an effective modulation power of $\SI{7.7}{\giga\watt}$. Over a longer timespan of $\SI{48}{\hour}$, they have demonstrated a $\SI{33}{\percent}$ modulation depth which would correspond to a modulation power of $\SI{2.5}{\giga\watt}$. We conclude that a modulation of part of an aluminium smelter's power consumption is possible at no significant production impact and at low infrastructure cost. Aluminium smelters are already connected to the grid in a way that they do not pose a danger to other nearby consumers when they turn off or on parts of the plant, as this is commonplace during routine maintenance activities. \subsection{Parametrizing Modulation for GFM} Given the grid characteristics we measured using our custom waveform recorder and using a model of our transmitter, we can derive parameters for the modulation of our broadcast system. The overall network power-frequency characteristic of the continental European synchronous area is about $\SI{25}{\giga\watt\per\hertz}$~\cite{entsoe02}. Thus, the main challenge for a GFM system will be poor SNR due to low transmission power. A second layer of modulation yielding some modulation gain beyond the basic amplitude modulation of the transmitter will be necessary to achieve sufficient overall SNR. The grid's frequency noise has significant localized peaks that might interfere with this modulation. Further complicating things are the oscillation modes. A GFM system must be designed to avoid exciting these modes. However, since these modes are not static, a modulation method that is designed around a specific assumption of their location would not be future proof. Given these concerns, the optimal second-level modulation technique for GFM is a spread-spectrum technique. By spreading signal energy throughout a wide band, both the impact of local noise spikes is minimized and the risk of mode excitation is reduced since spread-spectrum techniques minimize energy in any particular sub-band. In this paper, we chose to perform simulations using Direct Sequence Spread Spectrum for its simple implementation and good overall performance. DSSS chip timing should be as fast as the transmitter's physics allow to exploit the low-noise region between $\SI{0.2}{\hertz}$ to $\SI{2.0}{\hertz}$ in Figure~\ref{fig_freq_spec}. Going past $\approx\SI{2}{\hertz}$ would complicate frequency measurement at the receiver side. We simulated a proof-of-concept modulator and demodulator using data captured from our grid frequency sensor. Our simulations covered a range of parameters in modulation amplitude, DSSS sequence bit depth, chip duration and detection threshold. Figure~\ref{fig_ser_nbits} shows our simulation results for symbol error rate (SER) as a function of modulation amplitude with Gold sequences of several bit depths. From these graphs we conclude that the range of practical modulation amplitudes starts at approximately $\SI{1}{\milli\hertz}$, which corresponds to a modulation power of approximately $\SI{25}{\mega\watt}$~\cite{entsoe02}. Figure~\ref{fig_ser_thf} shows SER against detection threshold relative to background noise. Figure~\ref{fig_ser_chip} shows SER against chip duration for a given fixed symbol length. As expected from looking at our measured grid frequency noise spectrum, performance is best for short chip durations and worsens for longer chip durations since shorter chip durations move our signals' bandwidth into the lower-noise region from $\SI{0.2}{\hertz}$ to $\SI{2}{\hertz}$. %FIXME introduce term "chip" somewhere \begin{figure} \centering \includegraphics[width=0.4\textwidth]{../notebooks/fig_out/dsss_gold_nbits_overview} \caption{Symbol Error Rate as a function of modulation amplitude for Gold sequences of several lengths.} \label{fig_ser_nbits} \end{figure} \begin{figure} \centering \hspace*{-1cm}\includegraphics[width=1.2\textwidth]{../notebooks/fig_out/dsss_thf_amplitude_5678} \caption{SER vs.\ Amplitude and detection threshold. Detection threshold is set as a factor of background noise level.} \label{fig_ser_thf} \end{figure} \begin{figure} \centering \hspace*{-1cm}\includegraphics[width=1.2\textwidth]{../notebooks/fig_out/chip_duration_sensitivity_6} \vspace*{-1cm} \caption{SER vs.\ DSSS chip duration.} \label{fig_ser_chip} \end{figure} \subsection{Parametrizing a proof-of-concept ``Safety Reset'' System Based on GFM} %FIXME introduce scenario Taking these modulation parameters as a starting point, we proceeded to create a proof-of-concept smart meter emergency reset system. On top of the modulation described in the previous paragraphs we layered simple Reed-Solomon error correction~\cite{mackay01} and some cryptography. The goal of our PoC cryptographic implementation was to allow the sender of an emergency reset broadcast to authorize a reset command to all listening smart meters. An additional constraint of our setting is that due to the extremely slow communication channel all messages should be kept as short as possible. The solution we chose for our PoC is a simplistic hash chain using the approach from the Lamport and Winternitz One-time Signature (OTS) schemes. Informally, the private key is a random bitstring. The public key is generated by recursively applying a hash function to this key a number of times. Each smart meter reset command is then authorized by disclosing subsequent elements of this series. Unwinding the hash chain from the public key at the end of the chain towards the private key at its beginning, at each step a receiver can validate the current command by checking that it corresponds to the previously unknown input of the current step of the hash chain. Replay attacks are prevented by recording the most recent valid command. Keys revocation is supported by designating the last key in the chain as a \emph{revocation key} upon whose reception the client devices advance their local hash ratchet without taking further action. This simple scheme does not afford much functionality but it results in very short messages and removes the need for computationally expensive public key cryptography inside the smart meter. % FIXME add more precise/formal description of crypto % FIXME add description of targeting/scope function? % FIXME somewhere above descirbe entire reset system architecture????!!! % FIXME add description of disarm message (replay protection) \subsection{Experimental results} \begin{figure} \centering \includegraphics[width=0.6\textwidth]{prototype.jpg} \caption{The completed prototype setup. The board on the left is the safety reset microcontroller. It is connected to the smart meter in the middle through an adapter board. The top left contains a USB hub with debug interfaces to the reset microcontroller. The cables on the bottom left are the debug USB cable and the \SI{3.5}{\milli\meter} audio cable for the simulated mains voltage input.} \label{fig_proto_pic} \end{figure} For a realistic proof of concept, we decided to implement our signal processing chain from DSSS demodulator through error correction up to our simple cryptography layer in microcontroller firmware and demonstrate this firmware on actual smart meter hardware, shown in Figure~\ref{fig_proto_pic}. In our proof of concept a safety reset controller is connected to the main application microcontroller of a smart meter. The reset controller is tasked with listening for authenticated reset commands on the voltage waveform, and on reception of such a command resetting the smart meter application controller by flashing a known-good firmware image to its memory. The signal processing chain of our PoC is shown in Figure~\ref{fig_demo_sig_schema}. To interoperate with existing implementations of SHA-512 and reed-solomon decoding, this implementation was written in the C programming language. To demonstrate an application close to a field implementation, we chose an Easymeter \texttt{Q3DA1002} smart meter as our reset target. This model is popular in the German market and readily available second-hand. The meter consists of three isolated metering ASICs connected to a data logging and display PCB through infrared optical links. To demonstrate the safety reset's firmware reset functionality, we connected our safety reset microcontroller to the Texas Instruments \texttt{MSP430} microcontroller on the meter's display and data logging board through the JTAG debug interface that the board's vendor had conveniently left accessible. We ported part of \texttt{mspdebug}\footnote{\url{https://dlbeer.co.nz/mspdebug/}} to drive the meter microcontroller's JTAG interface and wrote a piece of demonstrator code that overwrites the meter's firmware with one that displays an identifying string on the meter's display after boot-up. \begin{figure} \centering \includegraphics[width=\textwidth]{prototype_schema} \caption{The signal processing chain of our demonstrator.} \label{fig_demo_sig_schema} \end{figure} To measure grid frequency in our demonstrator, we ported the same code we used in Section~\label{grid-freq-characterization} to our demonstrator, again using the voltage measured using the microcontroller's internal ADC but using a regular crystal instead of a crystal oven for the microcontroller's system clock. Since we did not have an aluminium smelter ready, we decided to feed our proof-of-concept reset controller with an emulated grid voltage sine wave from a computer's headphone jack. Where in a real application this microcontroller would take ADC readings of input mains voltage divided down by a long resistive divider chain, we instead feed the ADC from a $\SI{3.5}{\milli\meter}$ audio input. For operational safety, we disconnected the meter microcontroller from its grid-referenced capacitive dropper power supply and connected it to our reset controlller's debug USB power supply. We performed several successful experiments using a signature truncated at 120 bit and a 5 bit DSSS sequence. Taking the sign bit into account, the length of the encoded signature is 20 DSSS symbols. On top of this we used Reed-Solomon error correction at a 2:1 ratio inflating total message length to 30 DSSS symbols. At the \SI{1}{\second} chip rate we used in other simulations as well this equates to an overall transmission duration of approximately \SI{15}{\minute}. To give the demodulator some time to settle and to produce more realistic conditions of signal reception we padded the modulated signal unmodulated noise on both ends. \section{Lessons learned} For our proof of concept, before settling on the commercial smart meter we first tried to use an \texttt{EVM430-F6779} smart meter evaluation kit made by Texas Instruments. This evaluation kit did not turn out well for two main reasons. One, it shipped with half the case missing and no cover for the terminal blocks. Because of this some work was required to get it electrically safe. Even after mounting it in an electrically safe manner the safety reset controller prototype would also have to be galvanically isolated to not pose an electrical safety risk since the main MCU is not isolated from the grid and the JTAG port is also galvanically coupled. The second issue we ran into was that the development board is based around a specific microcontroller from TI's \texttt{MSP430} series that is incompatible with common JTAG programmers. Our initial assumption that a development kit would be easier to program than a commercial meter did not prove to be true. Contrary to our expectations the commercial meter had JTAG enabled allowing us to easily read out its stock firmware without either reverse-engineering vendor firmware update files nor circumventing code protection measures. The fact that its firmware was only available in its compiled binary form was not much of a hindrance as it proved not to be too complex and all we wanted to know we found with just a few hours of digging in Ghidra\footnote{\url{https://ghidra-sre.org/}}. In the firmware development phase our approach of testing every module individually (e.g. DSSS demodulator, Reed-Solomon decoder, grid frequency estimation) proved useful particularly for debugging. The modular architecture allowed us to directly compare our demodulator implementation to our Jupyter/Python prototype, where we found that our C implementation outperformed the Python prototype. Despite the algorithms's complexity, the microcontroller C implementation has no issues processing data in real-time due to the low sampling rate necessary. \section{Conclusion} \label{sec_conclusion} \subsection{Applicability to IoT devices} \subsection{Discussion} During an emergency in the electrical grid, the ability to communicate to large numbers of end-point devices is a valuable tool for restoring normal operation. When a resilient communcation channel is available, loads such as smart meters and IoT devices can be equipped with a supervisor circuit that allows for a remote ``safety reset'' that puts the device into a safe operating state. Using this safety reset, an attacker that uses compromised smart meters or IoT devices to attack grid stability can be interrupted before the can conclude their attack. During recovery from an outage, a safety reset can be used to reduce stress on the system during a black start by temporarily disabling non-essential loads such as air conditioners. In this paper we have developed an end-to-end design for a safety reset system that provides these capabilities. Our novel broadcast data transmission system is based on intentional modulation of global grid frequency. Our system is independent of normal communication networks and can operate during a cyberattack. We have shown the practical viability of our end-to-end design through simulations. Using our purpose-designed grid frequency recorder, we can capture and process real-time grid frequency data in an electrically safe way. We used data captured this way as the basis for simulations of our proposed grid frequency modulation communication channel. In these simulations, our system has proven feasible. From our simulations we conclude that a large consumer such as an aluminium smelter at a small cost can be modified to act as an on-demand grid frequency modulation transmitter. We have demonstrated our modulation system in a small-scale practical demonstration. For this demonstration, we have developed a simple cryptographic protocol ready for embedded implementation in resource-constrained systems that allows triggering a safety reset with a response time of less than 30 minutes. In this demonstration we use simulated grid frequency data to trigger a commercial microcontroller to perform a firmware reset of an off-the-shelf smart meter. The next step in our evaluation will be to conduct an experimental evaluation of our modulation scheme in collaboration with an utility and an operator of a multi-megawatt load. The safety reset controller does not require any peripherals except for an ADC. Thus we expect code size to be the main factor affecting per-unit cost in an in-field deployment of our concept. At around \SI{64}{\kilo\byte}, our demonstrator firmware implementation is viable on low-end microcontrollers. Thus, we expect safety reset controllers to be commercially viable. Source code and EDA designs are available at the public repository listed at the end of this document. \printbibliography[heading=bibintoc] \center{ \center{This is version \texttt{\input{version.tex}\unskip} of this paper, generated on \today. The git repository can be found at:} \center{\url{https://git.jaseg.de/safety-reset.git}} } \end{document}