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A high-level view
Why OpenPGP?
OpenPGP is a widely recognized, IETF-standardized set of cryptographic operations. It is broadly used in securing communications, for example, in encrypted text messages and email, and enjoys a vast ecosystem of libraries, tools, and community support forums. Moreover, its robustness and versatility has made OpenPGP a security choice for other use cases in which encryption is important. These include file transfer applications, password managers, and secure data storage.
There are other compelling reasons for why you might consider using OpenPGP in your project:
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Decentralized trust model: OpenPGP's decentralization defines mechanisms for authentication that allow individuals and entities to create and manage their own cryptographic identities. Unlike centralized trust models, decentralized trust models empower individuals and entities to manage their own identities, fostering a community-driven web of trust instead of relying on a centralized authority, thus reducing single points of failure.
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End-to-end encryption: OpenPGP provides a robust framework for implementing end-to-end encryption. Content remains confidential, verifiable, authenticated, and protected against unauthorized access, even when the communication channel itself might be otherwise compromised. Encryption is crucial in a myriad of scenarios, particularly when transmitting sensitive information such as financial data, personally identifiable information (PII), or proprietary business data.
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Anonymity and pseudonymity: In sensitive and volatile situations where identity protection is crucial, OpenPGP can be used to provide a level of anonymity or pseudonymity that helps protect user identities. For example, OpenPGP has been used alongside other privacy tools, such as Tor and VPNs, to provide secure and anonymous communication for whistleblowers, human rights lawyers, activists in repressive regimes, and journalists, reducing their risks for retaliation and state violence.
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Interoperability: OpenPGP is a well-structured and standardized protocol, widely adopted by various public and private entities but not tied to any particular vendor's technology. It supports all major operating systems, such as Windows, macOS, GNU/Linux, Android, and iOS. Because of standardization, wide adoption, cross-platform compatibility, and adaptability, OpenPGP's interoperability significantly contributes to reducing development time, costs, and technical hurdles.
A very brief history
The OpenPGP standard has evolved over time, and remains under active development.
(Also see https://www.openpgp.org/about/history/)
Pretty Good Privacy (PGP)
The origins of OpenPGP can be traced back to Pretty Good Privacy (PGP), a software program written by Phil Zimmermann and first released in 1991.
The original PGP software played a role in the political struggles sometimes referred to as the "Crypto Wars" (also see "Crypto: How the Code Rebels Beat the Government Saving Privacy in the Digital" (2002), which includes some of PGP's history).
The original PGP software was never under a Free Software license, despite its source code being widely published by its author. PGP's ownership has changed over the years, and PGP's scope and suite of products have expanded.
Standardizing OpenPGP
While PGP was first developed as commercial software, the owner at the time, PGP Inc., started a standardization effort with the IETF, first publishing RFC 1991 "PGP Message Exchange Formats" in August 1996.
In July 1997, a process to produce an open standard under the then new name OpenPGP was started, resulting in RFC 2440 "OpenPGP Message Format", published in November 1998.
The name OpenPGP can be used freely by implementations, unlike the name PGP, which is a registered trademark.
GnuPG, an early Free Software implementation
First released 1997-12-20 by Werner Koch, a German computer programmer, GNU Privacy Guard (GnuPG) is a free and open-source implementation of the OpenPGP standard.
GnuPG was a major early implementation of OpenPGP. Over the years, the importance of GnuPG has grown significantly as it became a foundational tool for email security, software signing, and more. It played an important (and successful) role in the release of NSA documents by Edward Snowden.
Because the GnuPG program binary is called "gpg," "GnuPG" and "gpg" are often used interchangeably.
The RFC 4880 era
OpenPGP version 4
In 2007, the IETF published RFC 4880, which defines version 4 OpenPGP artifacts. As of late 2023, version 4 is the most commonly used version.
An extension for Elliptic Curve Cryptography was defined in RFC 6637.
Some implementations explored other non-standardized extensions. Notably, algorithms based on Curve 25519 were tentatively defined in the rfc4880bis document. These algorithms are widely used, even though rfc4880bis has never been finalized as a new version of the standard.
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Major implementations of OpenPGP
Today, multiple implementations of OpenPGP play important roles:
- Proton Mail, which provides email encryption services for a large number of users, uses and maintains OpenPGP.js as well as GopenPGP, an OpenPGP wrapper library written in golang.
- The Mozilla Thunderbird email software uses RNP, a C++ implementation of OpenPGP.
- The RPM Package Manager software includes an OpenPGP backend based on Sequoia PGP, a modern OpenPGP implementation written in Rust. The Fedora operating system, Fedora Linux 38, uses Sequoia PGP in rpm.
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Interoperability
OpenPGP was standardized in 1997 to encourage development of interoperable implementations. This has already been a success early on, but in recent years, there has been much development of new implementations.
Historically, interoperability has only been tested in an adhoc manner. Since 2019, the Sequoia project is maintaining and operating the "OpenPGP interoperability test suite", for more rigorous and systematic testing. The test suite has identified numerous issues.
The road ahead
Software and protocol development sometimes skips version numbers due to reasons like internal testing, significant changes, avoiding confusion, marketing decisions, or technical issues. The official successor to OpenPGP version 4 is OpenPGP version 6, detailed below.
OpenPGP version 6
As of this writing (in 2023), version 6 of OpenPGP is approaching publication as an RFC. The IETF OpenPGP working group is focused on updating the cryptographic mechanisms, adding new algorithms, and the deprecation of obsolete algorithms.
This document describes OpenPGP version 6, while pointing out differences to previous versions that are relevant to application developers.
Significant support for OpenPGP version 6 has already been achieved for multiple implementations, including:
Additionally, initial work on support for version 6 has been done for the PGPainless and RNP implementations.
Post-quantum cryptography in OpenPGP
There is ongoing work to standardize and add support for post-quantum public-key algorithms in OpenPGP. This project is funded by the german "BSI". Goals include adding support for post-quantum cryptography to Thunderbird and GnuPG. A presentation was given at IETF 113.
Zooming in: Internal structure of OpenPGP data
OpenPGP data is internally structured as "packets." We'll look into examples of this internal structure in a series of chapters at the end of this document.
Getting familiar with the internal format of OpenPGP data provides practical insight into the RFC, which describes the internal structure of OpenPGP packets in full detail, and may also come in handy for debugging issues.
(Most of the time, however, we will look at OpenPGP artifacts at a higher level of abstraction.)