Securing the IoT: Part 1 - Public key cryptography

Yann Loisel and Stephane di Vito, Maxim Integrated

January 11, 2015

Yann Loisel and Stephane di Vito, Maxim IntegratedJanuary 11, 2015

Security of electronic devices is a must in today’s interconnected world of the Internet of Things (IoT). Electronic devices range from smart connected refrigerators to uranium centrifuge control systems. When the security of a device is compromised we can no longer rely on the device for secure data exchange, processing, or storage. If electronic transactions, critical systems such as nuclear plants, or implantable medical devices are hacked, then the global trust would be impacted dramatically.

This is the first article in a two-part series on security for the Internet of Things (IoT). In Part 1 we describe how to identify and then assess the security risks for a connected electronic device. We explain how the best, proven security is designed into electronic devices. Our focus is on countermeasures, specifically public key-based algorithms.

In Part 2 we focus on the importance of a secure boot and the “root of trust", which are the cornerstones of an electronic device’s trustworthiness. We will demonstrate how device security can be implemented conveniently and how devices can be updated in the field. DeepCover secure microcontrollers will serve as example trust-enabling components to secure the IoT.

The connected world reaches out
Our lives are increasingly surrounded by interconnected electronic devices in what is now called the IoT or even the Internet of Everything. The IoT and all secure portable devices as well as industrial and medical equipment have software running within the hardware. They ease our days, answer our needs, control electrical functions in our households, protect our lives in medical equipment, and provide us utility services (water, gas, electricity) through smart grids or by controlling power plants.

Secure personal devices and the IoT have altered personal behavior for many of us. The technology extends our arms, our wills, and our minds beyond our bodies to help us communicate and consume. Manufacturers and many industries are embracing the IoT for business efficiencies and data tracking (i.e., Industry 4.0). Energy and water utilities are realizing the efficiencies and intelligence that they will gather with data management and data mining from remote access to smart meters [1] on an IoT network. Banks and payment processors now enable fast transactions with smart cards, at any time and any place, using free (or almost free), colorful, touch terminals. Home health with the IoT—ECG monitoring, glucose dispensers, or insulin pumps—is improving lives and saving time and money for both patients and medical facilities. Projections estimate that there will be 88M mobile POS connections in 2018 [2]. Clearly, the connected electronic devices have definite value, but they have definite vulnerabilities too.

Recognize the security risks

It has become so easy, so comfortable surfing on the web from almost everywhere with our smartphones that we have forgotten about our old 56k modem. But today’s connected devices and the instant accessibility to a bright world also give us a misguided sense of confidence. We should remember a sad but simple truth: the investments, connections, and transactions over the Internet or IoT whet the appetite of hackers.

The security risks come from competitors, lone predators, and criminal organizations. Competitors are more inclined to duplicate/clone technology—the magical smartphones or the ink cartridges—often saving them years of R&D efforts, The others will be more interested in stealing payment cards, PIN codes, keys in payment terminals, or in blackmailing individuals, perhaps by sabotaging an account or remotely shutting down a portable medical device. We can also imagine terrorist threats by remote hacking of energy smart meters for energy distribution at industrial plants or hospitals [3]. There is no need for more examples here. Suffice it to say that the security risks are all around us.

The risks to the stakeholders are numerous:

  • Loss of reputation. “The battery that you (manufacturer ‘x’) claimed as genuine has exploded in my laptop.”
  • Loss of IP. “The terrific algorithm I’ve developed in my video decoder during the last five years has been copied and duplicated. And I did not patent it to avoid disclosure of my tricks!”
  • Loss of money. “Tens of payment terminals are hacked in my retail chain store, so fake transactions are performed and/or cardholder sensitive data are stolen. Customers are going to blame me and I will need to identify the hackers.”
  • Loss of goods. “I just read about the hack of an energy meter published on the web and already thousands of dishonest subscribers are implementing it to pay a lower bill.”
  • Loss of health. “My insulin pump does not dispense any more, or it dispenses too much. Who ordered a change in delivery times?”
  • Loss of control of vital infrastructures. “Who turned the lights off in the whole city?

Obviously, any provider of electronic devices must have two objectives: first, deliver new, powerful and cost-effective devices or services; and, second, be totally committed to the robustness, liability, and security of their product. This is, in fact, the only way for them to keep the confidence of users, stakeholders, and consumers.

Analyze the risks

The above objectives, ambitious to be sure, are the sine qua non conditions for the longevity of a business. But recognizing security risks is only a first step in delivering secure products. Each provider must also employ a strict process of ongoing risk analysis.

Risk analysis, in its most simple form, is a three-step process. It starts by evaluating the assets, the goods to be protected, for their strengths and weaknesses. Then evaluate any potential attackers and profile their possible methods. Finally, examine any possible attack paths. Any consistency among the assets, attack method, and attack paths puts the device (the asset) at risk.

Consider a possible scenario. If hacking an energy smart meter with a simple Bluetooth connection saves someone 20% on a monthly bill, there is a high risk of massive, even wide-spread fraud. Similarly, if it costs very little to acquire the binary code of an application that controls the water and energy usage of household appliances, some dishonest competitors (or suppliers?!) would do it.

The options for action following a risk analysis require case-by-case decisions.
  • Take the legal/contractual approach. This avenue is always cost effective and worth setting up. A device manufacturer can easily ask subcontractors (e.g., manufacturing plants) to sign a nondisclosure agreement (NDA) and to promise to be honest and faithful [4].
  • Implement technical countermeasures. These steps will protect devices against dishonest partners, subcontractors, and outlaw/unreachable attackers. Technical countermeasures guarantee that a device’s expected behavior and functions are controlled, sealed, and sanctioned by the manufacturer. Nothing can then either modify defined operation or access protected functions.

When a manufacturer uses legal contracts with suppliers and technical countermeasures in a device, it is protecting its own assets and safeguarding the device against unauthorized tampering and theft of IP. A manufacturer is also ensuring the safe, reliable operation of the device for a user.

Makes sense so far, but how do you really implement countermeasures in a device? Part of the answer is cryptography in the software. We will now see how cryptography can be used as a toolbox to ensure device security and provide the trust and confidence for both the manufacturer and end user.

A secure boot? We are not going to say a great deal about a secure boot in this article because it is a major focus in Part 2 of this article. Nonetheless, we cannot discuss cryptography without some mention of a secure boot.

Electronic devices are composed of a set of electronic components mounted on a printed circuit board (PCB) with usually one (or more) microcontrollers that run embedded software. The software is seen as digital content and stored in memory in a binary, executable format. Enabling trust in the executed software is a fundamental expectation, and this trust is enabled thanks to the secure boot.

A secure boot is a process involving cryptography that allows an electronic device to start executing authenticated and therefore trusted software to operate.

Public key-based signature verification
Existing public key cryptography schemes [5] verify, conveniently and securely, the integrity and authenticity of digital content. Integrity means that the digital content has not been modified since it was created. Authenticity means that the same digital content has been released by a well-identified entity. These two fundamental characteristics are provided by the digital signature scheme and is required so the digital content (i.e., the binary executable code) can be trusted by an electronic device.

The integrity of digital content is guaranteed by a mechanism called the ‘message digest’, i.e., a secure hash algorithm like the famous SHA-1, SHA-256, and most recently the SHA-3. A message digest is like a "super cyclic redundancy check (CRC)” [6] but produces more bytes in the output. For instance, the SHA-256 algorithm produces a 32-byte output; a CRC-32 produces only 4 bytes. There is an important, fundamental property of a secure hash algorithm: it is impossible to forge digital content that produces a predefined hash value.

The corollary is that two different random digital contents produce two different hash values. (The probability of having two different digital contents producing the same hash value is virtually zero.) Consequently, if some bytes of the digital content are changed, the hash value of the digital content changes. In addition, unlike a CRC, it is not possible to append some bytes to the modified digital content so that the resulting hash value will match the original, non-modified digital content’s hash value. Therefore, with a hash algorithm guarding the digital content, it is not possible to secretly modify that digital content. Lastly, computing a hash is like computing a CRC: no cryptographic keys are involved.

The authenticity of digital content is guaranteed by the public key-based digital signature scheme itself (i.e., a cryptographic recipe). Public-key cryptography is based on pairs of keys. Anyone can possess a pair of keys: one private key stored secretly (e.g., KPRIV), and one public key (e.g., KPUB) publicly available to anyone. The private key can be used to sign a digital content. The issuer of the digital content uses its own secretly held private key to identify itself as the issuer. The public key can be used by anyone to verify a digital content’s signature. Those two keys are tied together. Indeed, signing content with KPRIV produces digital signatures that can be successfully verified by KPUB only. No other public key can work. Conversely if a signature is successfully verified using KPUB, then it was unquestionably signed by KPRIV and no other private key.

Digital signature generation involves two steps. The first step consists of hashing the digital content and producing a hash value with the properties explained above. In the second step the former hash value is “signed” using the uniquely owned, undisclosed private key of the digital content author. This second step produces a value (the 'signature') that is attached to the original digital content.

Now anyone who wants to verify the digital content signature has to perform the two following steps. In the first step the digital content is hashed again, as in the signature generation process. Then in a second step, the resulting reconstructed hash value is used as an input to the signature verification algorithm, together with the signature attached to the digital content and the public key. If the algorithm determines that the signature is authentic, this proves that the digital content is identical to the original digital content (the integrity), and that the author of this digital content is really who he claimed to be (the authenticity) (Figure 1).

Figure 1: A diagram of a digital signature, how it is applied and verified.

Public key-based digital signature schemes work because the private key can be used for signing content only by the owner of this private key and no one else. Therefore, the private key has to be kept secret in good hands. Yet the public key need not be confidential because anyone can verify a digital content’s signature. The only fundamental requirement for a public key is trustworthiness.

Please note that 'public' here does not mean insecure. The public key is freely accessible because it gives no indication about the private key; one cannot calculate the private key knowing the public key. Moreover, the public key does not allow anyone to perform personally identifiable actions like signing digital content.

Nevertheless, as anyone can generate a pair of keys there must be a mechanism to verify the identity of the public key owner. Suppose that a public key has no strong binding with an identity. Then if you successfully verify the digital signature of a digital content with that public key, you still cannot trust this digital content because you do not know who actually signed this digital content.

Therefore, public key integrity, authenticity, and identity must all be guaranteed. This can be done in different ways.

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