Best Crypto – On the Impending Crypto Monoculture
On the Impending Crypto Monoculture ===================================
A number of IETF standards groups are currently in the process of applying the second-system effect to redesigning their crypto protocols. A major feature of these changes includes the dropping of traditional encryption algorithms and mechanisms like RSA, DH, ECDH/ECDSA, SHA-2, and AES, for a completely different set of mechanisms, including Curve25519 (designed by Dan Bernstein et al), EdDSA (Bernstein and colleagues), Poly1305 (Bernstein again) and ChaCha20 (by, you guessed it, Bernstein).
What’s more, the reference implementations of these algorithms also come from Dan Bernstein (again with help from others), leading to a never-before-seen crypto monoculture in which it’s possible that the entire algorithm suite used by a security protocol, and the entire implementation of that suite, all originate from one person. How on earth did it come to this?
The Underlying Problem ———————-
It would be easy to dismiss the wholesale adoption of Bernstein algorithms and code as rampant fanboyism, and indeed there is some fanboyism present. An example of this is the interpretation of the data formats to use as “whatever Dan’s code does” rather than the form specified in widely-adopted standards like X9.62 (“Additional Elliptic Curves (Curve25519 etc) for TLS ECDH key agreement”, TLS WG discussion), something that hasn’t been seen since the C language was defined as “whatever the pcc compiler accepts as input”. The underlying problem, though, is far more complex. In adopting the Bernstein algorithm suite and its implementation, implementers have rejected both the highly brittle and failure-prone current algorithms and mechanisms and their equally brittle and failure-prone implementations. Consider the simple case of authenticated encryption as used in the major Internet security protocols TLS, SSH, PGP, and S/MIME (the remaining protocol would be IPsec, but I’ve never written an IPsec implementation so I don’t have sufficient hands-on experience with it to comment on it in practice). S/MIME has an authenticated-encryption mode (encrypt-then-MAC or EtM) that’s virtually never used or even implemented, PGP has a sort-of integrity-check mode that encrypts a hash of the plaintext in CFB mode, and both TLS and SSH use the endlessly failure-prone MAC-then-encrypt (MtE) mode, with an ever- evolving suite of increasingly creatively-named attacks stretching back 15 years or more (TLS recently adopted, after a terrific struggle on their mailing list, an option to use EtM, but support in some major implementations is still lagging). What are the (standardised) alternatives? Looking through a recent paper from Real World Crypto (“The Evolution of Authenticated Encryption”, Phil Rogaway), we see the three options GCM, CCM, and OCB. The GCM slide provides a list of pros and cons to using GCM, none of which seem like a terribly big deal, but misses out the single biggest, indeed killer failure of the whole mode, the fact that if you for some reason fail to increment the counter, you’re sending what’s effectively plaintext (it’s recoverable with a simple XOR). It’s an incredibly brittle mode, the equivalent of the historically frighteningly misuse-prone RC4, and one I won’t touch with a barge pole because you’re one single machine instruction away from a catastrophic failure of the whole cryptosystem, or one single IV reuse away from the same. This isn’t just theoretical, it actually happened to Colin Percival, a very experienced crypto developer, in his backup program tarsnap. You can’t even salvage just the authentication from it, that fails as well with a single IV reuse (“Authentication Failures in NIST version of GCM”, Antoine Joux). Compare this with old-fashioned CBC+HMAC (applied in the correct EtM manner), in which you can arbitrarily misuse the IV (for example you can forget to apply it completely) and the worst that can happen is that you drop back to ECB mode, which isn’t perfect but still a long way from the total failure that you get with GCM. Similarly, HMAC doesn’t fail completely due to a minor problem with the IV. Then there’s CCM, which is two-pass and therefore an instant fail for streaming implementations, which is all of the protocols mentioned earlier (since CCM was designed for use in 802.11 which has fixed maximum-size packets this isn’t a failure of the mode itself, but does severely limit its applicability). The remaining mode is OCB, which I’d consider the best AEAD mode out there (it shares CBC’s graceful-degradation property in which reuse or misuse of the IV doesn’t lead to a total loss of security, only the authentication property breaks but not the confidentiality). Unfortunately it’s patented, and even though there are fairly broad exceptions allowing it to be used in many situations, the legal minefield that ensues makes it untouchable for most potential users. For example does the prohibition on military use cover the situation where an open-source crypto package is used in a vendor library that’s used in a medical insurance app that’s used by the US Navy, or where banking transactions protected by TLS may include ones of a military nature (both of these are actual examples that affected decisions not to use OCB). Since no-one wants to call in lawyers every time a situation like this comes up, and indeed can’t call in lawyers when the crypto is several levels away in the service stack, OCB won’t be used even though it may be the best AEAD mode out there. (The background behind this problem can be found in Phil Rogaway’s excellent essay “The Moral Character of Cryptographic Work”, which discusses aligning crypto work with principles like the Buddhist concept of right livelihood, applying it in an ethical manner. Unfortunately, in the same way that the current misguided attempts by politicians to limit mostly non-existent use of crypto by terrorists and other equestrians only affects legitimate users (the few terrorists who may actually bother with encryption won’t care), so the restriction of OCB, however well-intentioned, have the effect that a beautiful AEAD mode that should be used everywhere is instead used almost nowhere). The implementations of the algorithms aren’t much better. Alongside brittle, failure-prone crypto modes and mechanisms, we also have brittle, failure-prone implementations. The most notorious of these is OpenSSL, which powers a significant part of the world’s crypto infrastructure not only directly (as a TLS/SSL implementation) but also indirectly, when it’s used as a component of other applications like OpenSSH. In fact one of the reasons given for OpenSSH’s adoption of the chacha20-poly1305 crypto mechanisms (alongside Curve25519 and others) was that it finally allowed them to remove the last vestiges of OpenSSL from their code. The Reason for the Monoculture —————————— Anyone who works with crypto on the Internet has had to endure 15-20 years of constant breakage of the crypto they use, both of the algorithms and mechanisms and of the implementations. It’s not even possible to give references for this because the list of breakage is so long and extensive that it would take pages and pages just to enumerate it all. Take for example an organisation like Google. Every single time that there’s been some break in a crypto mechanism, Google gets hit. Again and again, year in, year out. So when they look to moving to ChaCha20 and Poly1305, it’s not Bernstein fanboyism, it’s an attempt to dig themselves out of the current hole where they get hit with a new attack every couple of months, and the breakage just keeps recurring, endlessly. What implementers are looking for is what Bernstein has termed boring crypto, “crypto that simply works, solidly resists attacks, never needs any upgrades” (“Boring crypto”, Dan Bernstein). Bernstein and colleagues offer a silver bullet, something that appears better than anything else that’s out there at the moment. In this they have no real competition. There’s no AEAD mode that’s usable, the ECC algorithms and parameters that we’re supposed to use are both tainted due to NSA involvement and riddled with side-channels (the Bernstein algorithms and mechanisms have been specifically designed to deal with both of these issues), and so on. Consider being lost in an endless desert. If you see an oasis in the distance, you head towards it even if the water is brackish and has camel dung floating in it. Bernstein et al are the oasis (or perhaps the mirage of an oasis), in an endless desert of cryptosystems and implementations of cryptosystems that keep breaking. So the (pending) Bernstein monoculture isn’t necessarily a vote for Dan, it’s more a vote against everything else. Acknowledgements —————- This essay came about as the result of a discussion at AsiaCrypt 2015, and was then developed with significant input from Lucky Green. Prior to publication, further input was provided by some of the people whose work is mentioned in it. _______________________________________________ The cryptography mailing list firstname.lastname@example.org http://www.metzdowd.com/mailman/listinfo/cryptography