david wong

Hey! I'm David, a security engineer at the Blockchain team of Facebook, previously a security consultant for the Cryptography Services of NCC Group. I'm also the author of the Real World Cryptography book. This is my blog about cryptography and security and other related topics that I find interesting.

Authentication What The Fuck: Part II posted February 2020

(part 1 is here)

Writing about real world cryptography, it seems like what I end up writing a lot about is protocols and how they solve origin/identity authentication.

Don't get me wrong, confidentiality has interesting problems to (e.g. how to bring confidentiality to a blockchain), but authentication is most of what applied cryptography is about, for realz.

Do I need to convince you?

If you think about it, most protocols are about finding ways to provide authentication to different scenarios. And that's why they can get complicated!

I'll take my life for example, here is the authentication problems and solutions that I use:

  • insecure → one-side authenticated. Every day I use HTTPS, which uses the web public-key infrastructure (web PKI) to allow my browser to authenticate any websites on the web. It's a mess, but that's how you scale machine-to-machine authentication nowadays.
  • one-side authenticated → mutually-authenticated. Whenever I log into a website, over a secure HTTPS connection, this is what happens. A machine asks me to present some password (in clear, or oblivious via an asymmetric password-authenticated key exchange), or maybe a one-time password (via TOTP), or maybe I'll have to press my thumb on a yubikey (FIDO 2), or maybe I'll have to do a combination of several things (MFA). These are usually machine authenticating humans-type of flow.
  • insecure → mutually-authenticated. Whenever I talk to someone on Signal, or connect to a new WiFi, or pair a bluetooth device (like my phone with a car), I go from an insecure connection to a mutually-authenticated connection. There is a bit more nuance here, as sometimes I'll authenticate a machine (a WiFi access point for example) and sometimes I'll authenticate a human (end-to-end encryption). So different techniques work best depending on the type of peer you're trying to talk to.

In the end, I think these are the main three big categories of origin authentication. Can you think of a better classification?


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