Practical Cryptography With Go
leanpub.com
leanpub.com
http://pastebin.com/raw.php?i=NXgU30xK
(as a Github gist:
https://gist.githubusercontent.com/anonymous/3cc34251e501c2c...)
The more important recommendation, which this book misses, is NaCl (or its repackaging in libsodium). If you can use NaCl, use NaCl, and nothing else.
I'd recommend adding some more metadata (when was it posted, maybe a nick/Anonymous) and a clear distinction between "frame" and "content".
But extra meta-data I've been loathe to add so far because I see it as a barrier. I'm open to persuasion though.
One minor nit; I believe there is a typo in the review which caused parsing problems on my end:
The contact forward secrecy provides a design is...
s/contact/contract ?
A mistake in crypto can invalidate your entire system, not just make it unreliable or crash, and those mistakes don't have to be something obvious, there are many insidious little things that can happen as well. That's my take on it.
That said, I believe that software engineers should learn basic crypto and fiddle around with their own ideas, _with the understanding that there is always someone smarter than them_ in order to understand some of the problems they'll be facing.
I've been reading about this recently, but I'm confused. Wasn't the PS3 issue that the key was static? That's not the opposite of randomized, because there could be a deterministic, yet message-specific scheme for generating the secret key. Or is that known to be unacceptable?
> there is concern that the NIST curves are backdoored and should be disfavored and replaced with Curve25519 and curves of similar construction.
I've heard the opposite, that the curves are not the problem but the choice of basepoint. E.g. [1] states: "...the vulnerabilities in Dual-EC have precisely nothing to do with the specifics of the NIST standard elliptic curves themselves. The 'back door' in Dual-EC comes exclusively from the relationship between P and Q -- the latter of which is published only in the Dual-EC specification."
Is it still a matter of dispute whether the curves themselves are suspect? Or is the referenced blog post outdated?
[1]: http://blog.cryptographyengineering.com/2013/09/the-many-fla...
Dual_EC isn't a curve standard, it's a random number generator standard. The NIST P- curves are thought to possibly be backdoored. Dual_EC is all but certainly a backdoored.
Second, if NSA backdoored a curve standard, they probably did it in a fashion that only allows them privileges. Google [NOBUS NSA]. Dual_EC is a NOBUS backdoor, unless you can efficiently solve the ECDLP, in which case the backdoor doesn't matter anyways.
Finally, even if you stipulate for argument that a curve was backdoored in such a way that a researcher might find the backdoor, who's to say that curve researchers care that much about Bitcoin?
And I can't find anyone anywhere giving evidence of mathematical insecurity of any NIST standard curves. Evidence might come in the form of a (significantly more than state of the art)-subexponential but still superpolynomial time algorithm. Dual_EC has something even better: a proof of insecurity.
This is just reinforcing my point. There is nothing known to be mathematically wrong with the standard curves. Bernstein just warns against all the (admittedly many) pitfalls in implementations, and that the Weierstrass normal form makes it easier to run into problems than the normal form he proposes. This is the only reason he says NIST doesn't guarantee security, and of course they don't guarantee against engineering errors.
But that's extremely different from saying NSA planted backdoored curves intentionally. The only thing in Bernstein's analysis that I could possibly construe as suggesting malicious behavior is that the NIST curves are outdated (the suggestion being that they are intentionally left outdated).
In response to Jabbles, this is from his (work in progress) book. Page 7.
// Implement the standard padding scheme for block ciphers. This
// scheme uses 0x80 as the first non-NULL padding byte, and 0x00 to
// pad out the data to a multiple of the block length as required.
// If the message is a multiple of the block size, add a full block
// of padding. Note that the message is copied, and the original
// isn't touched.
I personally have purchased this book as well as reported errors to Kyle.
I'd encourage anyone with questions or comments to reach out to Kyle. He is quick to respond.
For example, this function stores the length of a slice in a variable, creates a new slice of the same length, copies all the bytes from the old to the new, then checks that the length of the new one is the same as the old (stored variable) - since it cannot have changed this check never fails. Hardly inspires confidence.
https://github.com/kisom/gocrypto/blob/master/chapter2/chat/...
Is this an example of what _not_ to do?
https://github.com/kisom/gocrypto/blob/master/chapter5/auths...
This is the authenticate-then-decrypt-anyway pattern. 100% Vaudenay compliant.
It happens that in this library, with padding errors delivered as error return values, checking a MAC and then obliviously decrypting won't leak the broken pad values --- presuming your code (a) uses the author's idiosyncratic 80h+00h padding scheme, or (b) uses PKCS7 padding like every other system and carefully checks to make sure that the pad bytes don't run it off the end of a slice --- doesn't fatally compromise the security of the system.
But in virtually every other language in which people implement cryptography, the pattern the author uses snatches defeat from the jaws of victory by creating the opportunity for padding exceptions even after the code has verified for itself that the message it's decrypting can't possibly be valid.
Obviously, the bigger problem is that you're more than halfway through the book before you find out that the example from Chapter 2 is totally hosed.
If it had been CBC with ISO7816 or PKCS#5 padding then I do think this would be attackable. The unpadding errors from decrypt trump the MAC error thanks to the "err == nil && !match" expression.
They start as complete masters of the field in their teenage years?
One could know all about buffer overflows, memory layout arcana and whatever, but not actually now much about cryptography, primes, et al.
The history of cryptography can basically be described as a series of assumptions which turn out to be invalid. This is either because they were never valid to begin with (and it just took time for problems to be discovered and shaken out), or because the facts on the ground change, making them invalid.
I'm not a cryptographer. I also don't really call myself a hacker anymore (because the loaded assumptions around that word make it useless as a descriptor, you wind up having to explain it in so much detail that it's just easier to start out with a different word), but I do know some cryptographers. They have all described the process in a similar way, which is you start by learning about the pitfalls of everything that's come before you.
Much like if you were learning to build bridges, you'd spend time learning about past bridges that didn't hold up.
So you start with the oldest crypto systems, and learn why those fell out of fashion. One nice benefit to this approach is that it makes it fairly apparent how brittle these constructions are. I don't think I've ever met a cryptographer who isn't suitably hesitant about designing cryptographic systems. Also, a good portion of the time spent as a cryptographer (maybe most of it? I'd love to hear a dissenting view from an actual cryptographer), is in breaking cryptographic systems (initially other people's, and than later, your own).
Cryptography isn't like web frameworks in the sense that everyone is making their own. New crypto systems (at least ones that come from cryptographers) don't spring up out of the ether every week.
One problem that seems to come up in cryptography is that cryptographers themselves "seem" to be mostly only concerned with the primitives. They leave implementation as "an exercise to the reader". This is a problem because I'd say that the overwhelming majority of actual security problems that stem from cryptography aren't problems with the primitives. They're problems with the construction necessary to do anything useful with those primitives. That's why when people heed advice like "Use AES" they're probably screwed.
Additionally, sane people do not use building blocks that they don't trust. If you need a block cipher you use AES (which is partially backed by security proofs) instead of designing your own (unless you have very specific requirements). This saves immense time in both design and analysis. Same with modes (Feistel structure, Sponges, HMAC, etc). Proofs play a big role here, although it is important to understand what they say, instead of blindingly assume they mean whatever you use them for is secure.
You are right that implementation is generally left as an afterthought. This is slowly changing, and primitives are increasingly being designed for implementation and side-channel friendliness.
It assumes that you aren't looking for cryptographic
theories, but practical ways to use cryptography in your
projects.
https://www.schneier.com/crypto-gram-9910.html
Including his self study: https://www.schneier.com/paper-self-study.pdf
Any Golang program that implements crypto will also need to implement an incomplete and poorly-specified version of NaCl.
(That's true of virtually every other programming language as well.)
Yes you can still use them wrong, and you're not much more advanced. But the original comment seemed to suggest it was about implementing the low-level algorithms.