Excuse the prolix comment; I'm not feeling well today.

I think people who follow both me and Colin on HN know that I have a lot of respect both for him and for Tarsnap, the service he runs, which is the only encrypted backup service I have ever recommended to anyone and which is to this day my go-to recommendation for people looking to safely store data in the cloud. Colin has built one of the very few modern cryptosystems I actually trust.

First, let me dodge Colin's whole post. My Twitter post was:

If you’re not learning crypto by coding attacks, you might not actually be learning crypto.

(I was cheerleading people doing our crypto challenges [http://www.matasano.com/articles/crypto-challenges/] and didn't think much of my twerp; I didn't exactly try to nail it to the door of the All Saints Church).

Note the word "might". I specifically chose the word "might" thinking "COLIN PERCIVAL MIGHT READ THIS". Colin, "might" means "unless you're Colin".

Anyways: I think the point Colin is making is valid, but is much more subtle than he thinks it is.

Here's what's challenging about understanding Colin's point: in the real world, there are two different kinds of practical cryptography: cryptographic design and software design. Colin happens to work on both levels. But most people work on one or the other.

In the world of cryptographic design, Colin's point about attacks being irrelevant to understanding modern crypto is clearly valid. Modern cryptosystems were designed not just to account for prior attacks but, as much as possible, to moot them entirely. A modern 2010's-era cryptosystem might for instance be designed to minimize dependencies on randomness, to assume the whole system is encrypt-then-MAC integrity checked, to provide forward secrecy, to avoid leaking innocuous-seeming details like lengths, &c.

While I think it's helpful to understand the attacks on 1990's-era crypto so you can grok what motivates the features of a 2010's-era ("Crypto 3.0") system, Colin is right to point out that no well-designed modern system is going to vulnerable to a (say) padding oracle, or an RSA padding attack (modern cryptosystems avoid RSA anyways), or a hash length extension.

In this sense, learning how to implement a padding oracle attack (which depends both on a side channel leak of error information and on the failure to appropriately authenticate ciphertext, which would never happen in a competent modern design) is a little like learning how to fix a stuck carburator with a pencil shaft.

The deceptive subtlety of Colin's point comes when you see how cryptography is implemented in the real world. In reality, very few people have Colin's qualifications. I don't simply mean that they're unlike Colin in not being able to design their own crypto constructions (although they can't, and Colin can). I mean that they don't have access to the modern algorithms and constructions Colin is working with; in fact, they don't even have intellectual access to those things.

Instead, modern cryptographic software developers work from a grab-bag of '80s-'90s-era primitives. A new cryptosystem implemented in 2013 is, sorry to say, more likely to use ECB mode AES than it is to use an authenticated encryption construction. Most new crypto software doesn't even attempt to authenticate ciphertext; cryptographic software developers share a pervasive misapprehension that encryption provides a form of authentication (because tampering with the ciphertext irretrievably garbles the output).

I think it's telling that Colin breaks this out into '90s-crypto and 2010's-crypto. For instance:

http://www.daemonology.net/blog/2011-01-18-tarsnap-critical-...

This is an AES CTR nonce reuse bug in Colin's software from 2011. Colin knew about this class of bug long before he wrote Tarsnap, but, like all bugs, it took time for him to become aware of it. Perhaps he'd have learned about it sooner had more people learned how cryptography actually works, by coding attacks, rather than reading books and coding crypto tools; after all, Colin circulates the code to Tarsnap so people can find exactly these kinds of bugs. Unfortunately, the population of people who can spot bugs like this in 2010's-era crypto code is very limited, because, again, people don't learn how to implement attacks.

But I'll push my argument further, on two fronts.

First: Colin should account for the fact that there's a significant set of practical attacks that his approach to cryptography doesn't address: side channels. All the proofs in the world don't help you if the branch target buffer on the CPU you share with 10 other anonymous EC2 users is effectively recording traces of your key information.

Second: Colin should account for the new frontiers in implementation attacks. It's easy for Colin to rely on the resilience of "modern" 2010's-era crypto when all he has to consider is AES-CTR, a random number generator, and SHA3. But what about signature systems and public key? Is Colin so sure that the proofs he has available to him account for all the mistakes he could make with elliptic curve? Because 10 years from now, that's what everyone's going to be using to key AES.

So, I disagree with Colin. I think it's easy for him to suggest that attacks aren't worth knowing because (a) he happens to know them all already and (b) he happens to be close enough to the literature to know which constructions have the best theoretical safety margin and (c) he has the luxury of building his own systems from scratch that deliberately minimize his exposure to new crypto attacks, which isn't true of (for instance) anyone using ECC.

But more importantly, I think most people who "learn crypto" aren't Colin. To them, "learning crypto" means understanding what the acronyms mean well enough to get a Java application working that produces ciphertext that looks random and decrypts to plaintext that they can read. Those people, the people designing systems based on what they read in _Applied Cryptography_, badly need to understand crypto attacks before they put code based on their own crypto decisions into production.