Security Engineering. Ross Anderson
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computations will become easy [1728]. So far only very small quantum devices have been built; although there are occasional claims of ‘quantum supremacy’ – of a quantum computer performing a task sufficiently faster than a conventional one to convince us that quantum superposition or entanglement is doing any real work – they seem to lead nowhere. I am sceptical (as are many physicists) about whether the technology will ever threaten real systems. I am even more sceptical about the value of quantum cryptography; it may be able to re-key a line encryption device that uses AES for bulk encryption on a single uninterrupted fibre run, but we already know how to do that.
What's more, I find the security proofs offered for entanglement-based quantum cryptography to be unconvincing. Theoretical physics has been stalled since the early 1970s when Gerard ’t Hooft completed the Standard Model by proving the renormalisability of Yang-Mills. Since then, a whole series of ideas have come and gone, such as string theory [2035]. Quantum information theory is the latest enthusiasm. Its proponents talk up the mystery of the Bell tests, which are supposed to demonstrate that physics cannot be simultaneously local and causal. But alternative interpretations such as ’t Hooft's cellular automaton model [918] and Grisha Volovik's superfluid model [1971] suggest that the Bell tests merely demonstrate the existence of long-range order in the quantum vacuum, like the order parameter of a superfluid. Since 2005, we've had lab experiments involving bouncing droplets on a vibrating fluid bath that demonstrate interesting analogues of quantum-mechanical properties relevant to the Bell tests [1560]. This book is not the place to discuss the implications in more detail; for that, see [312]. There is a whole community of physicists working on emergent quantum mechanics – the idea that to make progress beyond the Standard Model, and to reconcile the apparent conflict between quantum mechanics and general relativity, we may need to look at things differently. Meantime, if anyone claims their system is secure ‘because quantum mechanics’ then scepticism may be in order.
I think it more likely that a major challenge to public-key cryptography could come in the form of a better algorithm for computing discrete logarithms on elliptic curves. These curves have a lot of structure; they are studied intensively by some of the world's smartest pure mathematicians; better discrete-log algorithms for curves of small characteristic were discovered in 2013 [169]; and the NSA is apparently moving away from using elliptic-curve crypto.
If quantum computers ever work, we have other ‘post-quantum’ algorithms ready to go, for which quantum computers give no obvious advantage. In 2020, NIST began the third round of public review of submissions for the Post-Quantum Cryptography Standardization Process. The 65 initial submissions have been cut to 15 through two rounds of review12. One or more algorithms will now be chosen and standardised, so ciphersuites using them could be dropped into protocols such as TLS as upgrades. Many protocols in use could even be redesigned to use variants on Kerberos. If elliptic logarithms become easy, we have these resources and can also fall back to discrete logs in prime fields, or to RSA. But if elliptic logs become easy, bitcoins will become trivial to forge, and the cryptocurrency ecosystem would probably collapse, putting an end to the immensely wasteful mining operations I describe in section 20.7. So mathematicians who care about the future of the planet might do worse than to study the elliptic logarithm problem.
5.7.9 What else goes wrong
Very few attacks on systems nowadays involve cryptanalysis in the sense of a mathematical attack on the encryption algorithm or key. There have indeed been attacks on systems designed in the 20th century, mostly involving keys that were kept too short by export-control rules, clueless designs or both. I already discussed in section 4.3.1 how weak crypto has facilitated a wave of car theft, as all the devices used for remote key entry were defeated one after another in 2005–15. In later chapters, I give examples of how the crypto wars and their export control rules resulted in attacks on door locks (section 13.2.5), mobile phones (section 22.3.1) and copyright enforcement (section 24.2.5).
Most attacks nowadays exploit the implementation. In chapter 2, I mentioned the scandal of NIST standardising a complicated random number generator based on elliptic curves that turned out to contain an NSA backdoor; see section 2.2.1.5. Poor random number generators have led to many other failures: RSA keys with common factors [1142], predictable seeds for discrete logs [1679], etc. These vulnerabilities have continued; thanks to the Internet of Things, the proportion of RSA certs one can find out there on the Internet that share a common factor with other RSA keys has actually risen between 2012 and 2020; 1 in 172 IoT certs are trivially vulnerable [1048].
Many of the practical attacks on cryptographic implementations that have forced significant changes over the past 20 years have exploited side channels such as timing and power analysis; I devote Chapter 19 to these.
In Chapter 20, I'll discuss a number of systems that use public-key mechanisms in intricate ways to get interesting emergent properties, including the Signal messaging protocol, the TOR anonymity system, and cryptocurrencies. I'll also look at the crypto aspects of SGX enclaves. These also have interesting failure modes, some but not all of them relating to side channels.
In Chapter 21, I'll discuss protocols used in network infrastructure such as DKIM, DNSSec versus DNS over HTTP, and SSH.
5.8 Summary
Many ciphers fail because they're used badly, so the security engineer needs a clear idea of what different types of cipher do. This can be tackled at different levels; one is at the level of crypto theory, where we can talk about the random oracle model, the concrete model and the semantic security model, and hopefully avoid using weak modes of operation and other constructions. The next level is that of the design of individual ciphers, such as AES, or the number-theoretic mechanisms that underlie public-key cryptosystems and digital signature mechanisms. These also have their own specialised fields of mathematics, namely block cipher cryptanalysis and computational number theory. The next level involves implementation badness, which is much more intractable and messy. This involves dealing with timing, error handling, power consumption and all sorts of other grubby details, and is where modern cryptosystems tend to break in practice.
Peering under the hood of real systems, we've discussed how block ciphers for symmetric key applications can be constructed by the careful combination of substitutions and permutations; for asymmetric applications such as public key encryption and digital signature one uses number theory. In both cases, there is quite a large body of mathematics. Other kinds of ciphers – stream ciphers and hash functions – can be constructed from block ciphers by using them in suitable modes of operation. These have different error propagation, pattern concealment and integrity protection properties. A lot of systems fail because popular crypto libraries encourage programmers to use inappropriate modes of operation by exposing unsafe defaults. Never use ECB mode unless you really understand what you're doing.
There are many other things that can go wrong, from side channel attacks to poor random number generators. In particular, it is surprisingly hard to build systems that are robust even when components fail (or are encouraged to) and where the cryptographic mechanisms are well integrated with other measures such as access control and physical security. I'll return to this repeatedly in later chapters.
The moral is: Don't roll your own! Don't design your own protocols, or your own ciphers; and don't write your own crypto code unless you absolutely have to. If you do, then you not only need to read this book (and then read it again, carefully); you need to read up the relevant