Introduction on OpenPGP.foo

Intro

Mon, 01 Jan 0001 00:00:00 +0000

Intro: Looking at the basics of OpenPGP #

This series of articles aims to teach the concepts of OpenPGP, mostly from the perspective of a user.

The main goal of these texts is empowerment. My underlying belief is that OpenPGP is much less complex and hard to learn than it might seem.

Please get in touch, if any of the content here is confusing, or if there’s material you wish existed, but is currently missing!

rsop - Stateless OpenPGP (SOP)

Mon, 01 Jan 0001 00:00:00 +0000

rsop #

rsop is a Stateless OpenPGP (SOP) CLI tool: https://crates.io/crates/rsop/

Like all SOP tools, rsop enables users to perform common OpenPGP operations with a common simple CLI interface.

For an introduction to SOP, see the “Learn” section of this site, which walks through a range of operations and how they can be performed with SOP tools.

Architectural layering of rsop #

rsop is a modular project. Its two main components are rPGP and rpgpie.

Stateless OpenPGP ("SOP")

Mon, 01 Jan 0001 00:00:00 +0000

Stateless OpenPGP (SOP) #

The “Stateless OpenPGP Command Line Interface” is an evolving specification of a vendor-agnostic way to use OpenPGP. The SOP specification outlines a “generic stateless command-line interface for dealing with OpenPGP messages, certificates, and secret key material”.

Many tools, based on a wide range of OpenPGP libraries, implement the SOP standard.

In this text we will often use SOP tools to explore OpenPGP concepts, since they are (by design) both easy to use and easy to reason about.

Hello World in OpenPGP - encrypting a message

Mon, 01 Jan 0001 00:00:00 +0000

“Hello World” in OpenPGP #

In programming language introductions, it’s customary to show a program that prints “Hello World”.

For this series of articles, we’ll start with the equivalent for OpenPGP: We’ll create a new key, encrypt a message to it, and decrypt it again.

Making a new “private key” #

To make a new key with SOP, we can run a SOP command like this:

rsop generate-key alice > alice.tsk

This produces an output file named alice.tsk, which contains a new OpenPGP private key. The parameter alice is used as an identity that is associated with the key, and contained in it.

oct - OpenPGP card tool

Mon, 01 Jan 0001 00:00:00 +0000

openpgp-card-tool “oct” #

The oct CLI tool for inspecting, configuring and using OpenPGP card devices.

https://crates.io/crates/openpgp-card-tools/

OpenPGP card devices #

OpenPGP card devices are a type of hardware security module, specifically aimed at use with OpenPGP.

Commonly used OpenPGP card brands and devices include Nitrokey, YubiKey and the Free Software/Open Hardware Gnuk project.

The point of OpenPGP card devices is to handle private key material outside the host computer. This ensures that the private key material as such can’t be exfiltrated even when the user’s host computer is fully compromised.

rsop-oct - SOP for OpenPGP card

Mon, 01 Jan 0001 00:00:00 +0000

rsop-oct (rsoct) #

A Stateless OpenPGP (SOP) CLI tool based on rPGP, for use with OpenPGP card devices

https://crates.io/crates/rsop-oct/

rsoct is a sibling project of rsop. It supports private key operations backed by an OpenPGP card device.

Specifically, rsoct can currently be used for decrypt, sign and inline-sign operations backed by private key material on an OpenPGP card device.

When running rsoct, users can provide their OpenPGP certificate (aka “public key”) instead of a transferable secret key (TSK). The tool then automatically finds and uses the matching OpenPGP card device with the suitable key material.

Types of OpenPGP data

Mon, 01 Jan 0001 00:00:00 +0000

The stuff OpenPGP is made of #

In the previous chapter we made a new private key, extracted the public-facing certificate, and encrypted a message.

This already covers most of the existing types of OpenPGP data. In this article we’ll take stock of all the types of artifacts that exist in the world of OpenPGP.

Key material: Certificates and private keys #

The central element that all OpenPGP operations revolve around is the (asymmetric) cryptographic key material that OpenPGP public keys (certificates) and private/secret keys (often in the form of “transferable secret keys”) are made of.

Exploring signatures

Mon, 01 Jan 0001 00:00:00 +0000

Data Signatures #

(Data) signatures are a cryptographic tool to demonstrate the authenticity of some data.

In the narrowest sense, a data signature shows that some signer has used their key to issue a cryptographic statement about some data (such as an email message, or a source code archive).

Typically, OpenPGP data signatures signal either authorship (such as “this email has been written by me”), or approval (such as “this project certifies that this source code archive contains an official release of ours”)

ocsa - SSH Agent for OpenPGP cards

Mon, 01 Jan 0001 00:00:00 +0000

openpgp-card-ssh-agent #

A standalone SSH Agent for use with OpenPGP cards.

https://crates.io/crates/openpgp-card-ssh-agent

This SSH Agent tool aims for simple and enjoyable UX (including desktop notifications for touch confirmation). It serves the same purpose as the ssh agent feature in the gpg-agent tool. However, this tool always uses pcscd to access cards (and never locks them for exclusive use).

See the project documentation above to learn about setting up and using openpgp-card-ssh-agent.

ASCII Armor

Mon, 01 Jan 0001 00:00:00 +0000

“ASCII Armor” #

One of the properties of OpenPGP that is recognizable to many is its “armored” format.

For example, certificates (aka public keys) are often seen in armored form like this:

ASCII armored vs. binary #

Almost all OpenPGP data can be represented in two formats, which contain the exact same information, and are at least in theory totally interchangeable:

oct-git - Git signing with OpenPGP cards

Mon, 01 Jan 0001 00:00:00 +0000

oct-git #

Git signing and verification with a focus on OpenPGP cards.

https://crates.io/crates/openpgp-card-tool-git

oct-git can act as a replacement for one mode of operation of the gpg tool, specifically as it is combined with the git version control tool.

Signing #

git can optionally use an external tool (such as oct-git, or historically gpg) to produce cryptographic signatures for “commits” or “tags”.

Issuing signatures is a private-key based operation, which oct-git can perform on an OpenPGP card.

OpenPGP mechanisms

Mon, 01 Jan 0001 00:00:00 +0000

OpenPGP mechanisms #

OpenPGP acts as a toolkit that offers a variety of operations, which may be combined to achieve a variety of goals.

Encryption #

Encryption, which provides confidentiality, is one central facility of OpenPGP.

In a way, encryption is the most non-negotiable element of OpenPGP: When a message is encrypted, only actors who have access to a suitable secret are able to access the contained data.

File encryption with SOP/OpenPGP (and age)

Wed, 03 Jun 2026 00:00:00 +0000

This article explores file encryption with Stateless OpenPGP (SOP) tools, and contrasts this use of SOP with the age tool.

SOP is a CLI standard for (vendor-agnostic) OpenPGP tools. There are numerous independent implementations of SOP, based on a variety of OpenPGP implementations in different languages.

The msop implementation of SOP #

In this article, we’ll use the msop implementation of SOP. It is based on the very small minipgp6 Rust library. minipgp6 supports only a narrow modern set of algorithms, with v6 keys as specified in RFC 9580 and comes with native support for PQC.

rsop now decrypts GnuPG-proprietary messages

Thu, 25 Sep 2025 00:00:00 +0000

This is a short writeup about a new feature in the rsop CLI tool:

Since version 0.8.0, rsop can decrypt a GnuPG-proprietary message encryption format.

GnuPG sometimes produces non-standard encrypted messages #

Unfortunately, under some circumstances, GnuPG produces encrypted messages that are not compatible with the OpenPGP standard. This issue started in its 2.4.x series. GnuPG calls its proprietary format “OCB encrypted data”.

However, note that practically all big Linux distributions disable these GnuPG non-standard formats in their versions of the GnuPG package by default. So on most Linux distributions, if you follow along with the CLI calls in this article, you will end up producing a standard OpenPGP encrypted message (and not the problematic format I’m demonstrating here). Windows users of GnuPG on the other hand will usually use upstream GnuPG, without these safeguards!

Inspecting OpenPGP certificates

Mon, 25 Aug 2025 00:00:00 +0000

This article outlines using the new rpgp CLI tool to inspect OpenPGP certificates (aka “public keys”)¹.

The rpgp tool is available as part of the rpgpie crate. This article describes the tool as of rpgpie version 0.6.6.

rpgp can be installed with the Rust cargo tool like this: cargo install rpgpie --features=cli).

Different views of OpenPGP certificate internals #

The rpgp tool can show information about OpenPGP certificates in two different modes:

Using a second OpenPGP card for my primary key

Mon, 14 Jul 2025 00:00:00 +0000

Personal blog: One more OpenPGP card #

This is a writeup about my OpenPGP card setup.

It’s a mix of general observations about OpenPGP card devices and a report about setting up a secondary OpenPGP card for myself.

Disclaimer #

This article describes a relatively involved setup consisting of two OpenPGP card hardware security devices.

I’m describing this setup merely because it is what I currently use, not because I think anyone should mimic it. If you’d rather not use hardware cryptographic devices, then more power to you!¹

Post-quantum cryptography

Mon, 12 May 2025 00:00:00 +0000

Post-quantum cryptography (PQC) #

Post-quantum cryptography (PQC) is a branch of cryptography that develops algorithms which are thought to be secure against attacks using hypothetical quantum computers (which don’t exist right now, but may be developed in the future).

Development and deployment of such algorithms defends against potential future attacks (in case relevant quantum computers materialize).

Defense against potential future attacks with quantum computers is of particular interest for encryption. In particular when it is relevant if an attacker might store encrypted communication and decrypt it in the future. Defending against such an attack requires deployment of countermeasures well before the attack becomes practical.