Yes, so I googled "Curve25519 libsodium" and I still have absolutely no clue how to get this going.
I mean the first hit starts with this sentence: "Ed25519 keys can be converted to X25519 keys, so that the same key pair can be used both for authenticated encryption (crypto_box) and for signatures (crypto_sign)."
The first example shows both sides of the communication. The public keys are shared (but you want to make sure that you have the correct one, for example by meeting the recipient in person and comparing the keys).
The library defaults in libsodium are already chosen to be secure (using Curve25519 and secure algorithms).
Maybe it's not your Googlefu. Maybe it's because the article isn't a video on YouTube, or some sort of social media post, nor is the page heavily weighted with ad. It is a web page full of useful/on-topic information, and the current algo at Google seems to make those types of pages lower ranked. At least that's how it feels
If I write something that uses libsodium, am I violating the near universal "don't roll your own crypto" admonishment, or does libsodium sufficiently hide all the scary dangerous cryptographic stuff so that mere mortals can safely use it?
By using libsodium, you're not rolling your own crypto. Rolling your own crypto would mean
-trying to find new one way functions for public key crypto
-trying to implement RSA from textbook
-trying to implement RSA-OAEP from papers, RFCs, books etc.
Using a library is not anywhere near those. There are other ways to fail cryptography too, from not doing public key authentication, to storing private keys in insecure places.
> By using libsodium, you're not rolling your own crypto.
That's debatable. Libsodium does not have a proper authenticated key exchange. The key exchange does the job somewhat, but it has worse security properties than a properly crafted interactive protocol.
Problem is, designing your own key exchange protocol is more delicate than most construction. I know, I designed my own, and made several serious mistakes in the process (one of which voided an important security property).
Granted, rolling your own constructions is generally less error prone than rolling your own primitive. But it still shouldn't be done without at least having followed and fully understood an introductory course in cryptography (I recommend https://www.crypto101.io/). I mean, I did quite a bit more than that, and I still don't fully trust myself.
TL;DR - you're mostly avoiding the admonishment by using libsodium
There are still some dangerous things you can do (e.g. mismanage keys, reuse nonces).
I wrote a library called Halite for PHP developers that wraps libsodium and makes it even harder to misuse. My philosophy was, "You shouldn't even need to know what a nonce is to use it securely."
From elsewhere in the thread, PyNaCl takes a similar approach. So how dangerous "just using libsodium" is, with respect to the "don't roll your own crypto" guidance, depends a little bit on which binding you're using.
Also, a lot of tasks might require a specific protocol (PAKEs, VPN protocols, searchable encryption, etc.) that libsodium isn't suitable for.
I just read the article a second time and I really like it. This is precisely the starting point that I was craving for and if I had known about it I might not have written this rant :-)
My only SEO advice is patience. Your articles are great, people will link to it increasingly often.
That said, why the sole PHP focus? The starting point you just linked to is pretty much independent from PHP, and there's libsodium bindings in plenty languages. If your goal is simply to make good crypto easier to do for "normal" developers then the PHP association may do more harm than good (particularly because PHP is not very hip right now)
Then have another developer or consultant handle that part of your project. It seems like you misread a non-existent imperative in the article that "anyone can crypto with these 3 easy steps". They made no such claim.
Where do those consultants/developers come from? Are they born from Crypto eggs with all the knowledge in their brain? Or how do those mighty people come to be?
There are good formal cryptography courses, though some of them focus on aspects that aren't relevant to what I'd call "cryptographic engineering". For example, differential cryptanalysis is very interesting, but basically nobody except the IOTA community is silly enough to try and implement their own primitives, so that's not a common problem.
I authored Crypto 101. One of my cofounders co-authored Cryptopals and founded the cryptographic practice at Matasano (as well as co-founding Matasano itself ;)). So for cryptographic engineering, I think the answer is "historically oral, and increasingly written, tradition"?
> A random person on the internet advising which curve to use can be insecure as well.
First, the advise does not come from "a random person".
* The recommendation of Curve25519 is originally concluded by its author after evaluating all available elliptic-curve standards under a set of well-defined, objective criteria. The research is published here [0].
* It's not an one-man's claim. This conclusion has been peer-reviewed by many leading researchers and institutions in the past 10 years. There have been enormous amount of discussions made in various communities, including cryptographic developers working on community projects, like OpenBSD [1], OpenSSH [2], Tor [3], industry leaders such as Apple [4], Google [5], Microsoft [6], CloudFlare [7], as well as Internet institutions such as IETF [7], now it's officially part of TLSv1.3.
Nearly everyone agreed the security claims by Curve25519 is valid and it offers many desirable properties compared to previous standards.
> And even if it's a secure choose now it may not be in a few years.
The theoretical security margin is comparable to any other 256-bit ECC encryption, yet it offers a more conservative and robust design against known attacks than other curves, including a good complex-multiplication field discriminant D against potential speedups to Rho Method, immunity against invalid-curve attack, indistinguishability from uniform random strings, and allowing easier, more robust implementations of constant-time (anti side-channel) addition and multiplication.
In other words, in theoretical cryptography, there is no known, computationally-feasible way to attack it. Perhaps in the future someone could find a way, but ECC has been thoroughly analyzed in the past 20 years and it's reasonable to believe a major weakness is very unlikely. "You cannot be sure that it cannot be attacked in the future" is not valid logic in applied cryptography. The valid logic is "You only choose something with a high-level of confidence".
Currently, the only way to attack is using a quantum computer, but when they come, all the public-key algorithms deployed on the Internet are vulnerable without exceptions (it has been claimed that ECC needs fewer qubits to crack than RSA, that's true. But when quantum computers are large enough to attack ECC, attacking RSA is only a matter of time in the short-term. Giving up ECC entirely today and facing all the potential problems of RSA with long keys for 10-20 years, just to buy a few years of time from quantum attackers is a very questionable sacrifice). Therefore, the research on Post-Quantum Cryptography has already started [8], with possible candidates like McEliece and NTRU. They are expected to replace current standard in the next decade.
