mkcert: valid HTTPS certificates for localhost

The web is moving to HTTPS, preventing network attackers from observing or injecting page contents. But HTTPS needs TLS certificates, and while deployment is increasingly a solved issue thanks to the ACME protocol and Let's Encrypt, development still mostly ends up happening over HTTP because no one can get an… »

Touch-to-operate password-store with YubiKey 4

There are dozens of tutorials on how to fight GnuPG to use YubiKeys for everything, but my favorite overlooked feature of the YubiKey 4 is "touch to operate", where each cryptographic operation takes a physical touch of the gold surface. That pairs particularly well with password-store, a PGP backed password… »

Making a Gmail bot with Apps Script and TypeScript

Google Apps Script is one of the best hidden features of Gmail. Did you ever want just a bit more flexibility from a filter? Maybe the ability to remove a label, or match on a header, or just decide the order they are applied in. Apps Script can do all… »

Easy Windows and Linux cross-compilers for macOS

tl;dr: you can install cross-compiler toolchains to compile C/C++ for Windows or Linux from macOS with these two Homebrew Formulas. brew install FiloSottile/musl-cross/musl-cross brew install mingw-w64 hr { margin: 2em 0; } Cross-compiling C and C++ is dreadful. While in Go you just need to set an environment… »

Live streaming Cryptopals

tl;dr: I'm livecoding the Cryptopals in Go on Twitch, one set every Sunday. The recordings are on YouTube. Oh, wow. I love the idea. Would anyone here seriously watch 20 to 40 hours of me doing crypto, math and Go? Mic, screen, and everything.—… »

The scrypt parameters

The recommended scrypt parameters in the Go docs were recently brought up for discussion given they haven't changed since 2009. Even if at this point I memorized the three numbers (N=16384, r=8, p=1) I only have a vague understanding of their meaning, so I took some time… »

We need to talk about Session Tickets

More specifically, TLS 1.2 Session Tickets. Session Tickets, specified in RFC 5077, are a technique to resume TLS sessions by storing key material encrypted on the clients. In TLS 1.2 they speed up the handshake from two to one round-trips. Unfortunately, a combination of deployment realities and three… »

A secure captive portal browser with automatic DNS detection

Captive portals are the worst. Flaky detection. The OS and browser try to detect these annoying network features but fail quite often, leaving you with broken connections. DID YOU KNOW that probe-based captive portal detection really doesn't work very well, with ~30% FP *and* ~30% FN rate in… »

Playing with kernel TLS in Linux 4.13 and Go

Linux 4.13 introduces support for nothing less than... TLS! The 1600 LoC patch allows userspace to pass the kernel the encryption keys for an established connection, making encryption happen transparently inside the kernel. The only ciphersuite supported is AES-128-GCM as per RFC 5288, meaning it only supports TLS version… »

restic cryptography

tl;dr: this is not an audit nor an endorsement and I take no responsibility, but I had a quick look at the crypto and I think I'm going to use restic for my personal backups. I keep hearing good things about restic. I am redoing my storage solution, and… »

rustgo: calling Rust from Go with near-zero overhead

[русский] Go has good support for calling into assembly, and a lot of the fast cryptographic code in the stdlib is carefully optimized assembly, bringing speedups of over 20 times. However, writing assembly code is hard, reviewing it is possibly harder, and cryptography is unforgiving. Wouldn't it be nice if… »

Cleaning up my GOPATH with Homebrew

tl;dr: use the script at the bottom to go get into the Homebrew "Cellar" and keep your GOPATH clean. I personally like GOPATH and import paths, but while trying to reduce my laptop to a thin reproducible client, I felt the pain of keeping track of the hundreds of… »

Reproducing Go binaries byte-by-byte

Fully reproducible builds are important because they bridge the gap between auditable open source and convenient binary artifacts. Technologies like TUF and Binary Transparency provide accountability for what binaries are shipped to users, but that's of limited utility if there is no way (short of reverse engineering) of proving that… »

Setting a custom FileVault (macOS FDE) passphrase

FileVault 2 is the full-disk encryption system of macOS. Normally, it's turned on from System Preferences, and locks the disk with the passwords of all the users allowed to unlock the machine. Overloading the login/unlock/sudo password is an understandable UX simplicity choice, but makes it very hard to… »

Finding Ticketbleed

Ticketbleed (CVE-2016-9244) is a software vulnerability in the TLS stack of certain F5 products that allows a remote attacker to extract up to 31 bytes of uninitialized memory at a time, which can contain any kind of random sensitive information, like in Heartbleed. If you suspect you might be affected… »

Go Time #32 - Hellogopher, whosthere?

I joined Erik St. Martin, Carlisia Pinto and Brian Ketelsen for episode #32 of the Go Time podcast to chat about Hellogopher, whosthere (, $GOPATH, TLS 1.3, Cloudflare's secret reverse proxy, and more. Go Time #32 — Hellogopher, whosthere? with Filippo Valsorda hellogopher — "just clone and make" The… »

TLS 1.3 at 33c3

Nick Sullivan and I gave a talk about TLS 1.3 at 33c3, the latest Chaos Communication Congress. Here's the Fahrplan entry. We spoke about the flow of TLS 1.2 vs. TLS 1.3, how it manages to save a round trip, resumption and 0-RTT, forward secrecy and replays,… »

How to protect yourself from the WebEx extension

On Monday, Tavis Ormandy of Project Zero revealed that the Cisco WebEx Chrome extension (20M users) has a critical vulnerability. OMFG🔥 The WebEx Chrome extension has a trivial code execution vulnerability: any website could just install malware on your machine silently— Filippo Valsorda (@FiloSottile) 23… »

So you want to expose Go on the Internet

I was asked to contribute a post to the excellent Gopher Academy advent series. I took the occasion to write down what I learned deploying a Go service on the Cloudflare edge. The result is a catalogue of what you need to know before you drop NGINX from in front… »

I'm giving up on PGP

After years of wrestling GnuPG with varying levels of enthusiasm, I came to the conclusion that it's just not worth it, and I'm giving up. At least on the concept of long term PGP keys. This is not about the gpg tool itself, or about tools at all. Many already… »