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Notes on OpenPGP
An introduction to the concepts of OpenPGP, aimed mainly at software developers who are looking to use OpenPGP functionality in their projects.
This document describes OpenPGP version 6, with occasional remarks about differences to earlier versions.
This text is not intended as a guide for end-users who use OpenPGP-related software.
What is OpenPGP?
OpenPGP is an open standard for cryptographic operations. It has grown out of the "Pretty Good Privacy (PGP)" software.
OpenPGP is an open standard, there are many widely used (and interoperable) implementations.
Goals of this document
There are three groups of people who interact with OpenPGP:
- End-Users, who use software that contains OpenPGP functionality (e.g., the Thunderbird email software)
- Software developers who build applications that contain OpenPGP functionality
- Implementers of libraries or software that handles the processing of internal OpenPGP data structures
This document is focused at the second of these groups: software developers who use OpenPGP functionality in their software projects.
It is not intended for end-users who use software that contains OpenPGP functionality.
This text aims to describe OpenPGP at the "library-level": we teach the concepts that will help you get started as a user of any implementation (such as OpenPGP JS, Sequoia PGP, ...)
Requirements
We presuppose solid knowledge in both software development concepts, and of general cryptographic concepts.
OpenPGP is a system based on well-understood cryptographic building blocks. We describe the properties of the OpenPGP system, and how to use it.
A companion for the OpenPGP RFC
The RFC explains lots of details (which bit goes where) that are crucial
for implementers, but unimportant for software developers who use OpenPGP
through a library.
The OpenPGP RFC defines "the message formats used in OpenPGP" to "provide encryption with public-key or symmetric cryptographic algorithms, digital signatures, compression and key management".
The RFC, as a standards document, is mainly aimed at the third group: Implementers of software that handles internal OpenPGP data structures. In that context, the nitty-gritty of which bit of data goes where is crucial.
For software developers using OpenPGP through a library, however, it is not. This document describes OpenPGP concepts at the "library" level of abstraction, and ignores most details about how OpenPGP artifacts are encoded at the lowest level.
The idea is to go over various common OpenPGP artifacts, as they are currently used, to get an overview.
Covering versions
We will mainly cover v6 of OpenPGP, but occasionally point out differences to previous versions.
Version 4 of OpenPGP will remain relevant for a number of years, and some OpenPGP version 3 artifacts are still in use as of this writing (in 2023).
For example, the RFC states that implementations MAY accept version 3 signatures. Handling version 3 artifacts is relevant in some contexts, where dealing with historical OpenPGP material is required.
Where differences between versions may be relevant to application developers, we will point them out.