computer science

GNU Privacy Guard

GNU Privacy Guard is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand GNU Privacy Guard rather than just read about it. In short: GNU Privacy Guard (GnuPG or GPG) is a free-software replacement for Symantec's cryptographic software suite PGP. The software is compliant with the now obsoleted RFC 4880, the IETF standards-track specification of OpenPGP.

Key takeaways

GNU Privacy Guard belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect GNU Privacy Guard to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of GNU Privacy Guard from memory before moving on to harder problems.

Reference excerpt

GNU Privacy Guard (GnuPG or GPG) is a free-software replacement for Symantec's cryptographic software suite PGP. The software is compliant with the now obsoleted RFC 4880, the IETF standards-track specification of OpenPGP. Modern versions of PGP are interoperable with GnuPG and other OpenPGP v4-compliant systems. November 2023 saw two drafts aiming to update the 2007 OpenPGP v4 specification (RFC4880), ultimately resulting in the RFC 9580 standard in July 2024. The proposal from the GnuPG developers, which is called LibrePGP, was not taken up by the OpenPGP Working Group and future versions of GnuPG will not support the current version of OpenPGP. GnuPG is part of the GNU Project and received major funding from the German government in 1999.

Overview GnuPG is a hybrid-encryption software program because it uses a combination of conventional symmetric-key cryptography for speed, and public-key cryptography for ease of secure key exchange, typically by using the recipient's public key to encrypt a session key which is used only once. This mode of operation is part of the OpenPGP standard and has been part of PGP from its first version. The GnuPG 1.x series uses an integrated cryptographic library, while the GnuPG 2.x series replaces this with Libgcrypt. GnuPG encrypts messages using asymmetric key pairs individually generated by GnuPG users. The resulting public keys may be exchanged with other users in a variety of ways, such as Internet key servers. They must always be exchanged carefully to prevent identity spoofing by corrupting public key ↔ "owner" identity correspondences. It is also possible to add a cryptographic digital signature to a message, so the message integrity and sender can be verified, if a particular correspondence relied upon has not been corrupted. GnuPG also supports symmetric encryption algorithms. By default, GnuPG uses the AES symmetrical algorithm since version 2.1, CAST5 was used in earlier versions. GnuPG does not use patented or otherwise restricted software or algorithms. Instead, GnuPG uses a variety of other, non-patented algorithms. For a long time, it did not support the IDEA encryption algorithm used in PGP. It was in fact possible to use IDEA in GnuPG by downloading a plugin for it, however, this might require a license for some uses in countries in which IDEA was patented. Starting with versions 1.4.13 and 2.0.20, GnuPG supports IDEA because the last patent of IDEA expired in 2012. Support of IDEA is intended "to get rid of all the questions from folks either trying to decrypt old data or migrating keys from PGP to GnuPG", and hence is not recommended for regular use. More recent releases of GnuPG 2.x ("modern" and the now deprecated "stable" series) expose most cryptographic functions and algorithms Libgcrypt (its cryptography library) provides, including support for elliptic-curve cryptography (ECDH, ECDSA and EdDSA) in the "modern" series (i.e. since GnuPG 2.1).

Algorithms As of 2.3 or 2.2 versions, GnuPG supports the following algorithms:

Public key RSA, ElGamal, DSA, ECDH (cv25519, cv448, nistp256, nistp384, nistp521, brainpoolP256r1, brainpoolP384r1, brainpoolP512r1, secp256k1), ECDSA (nistp256, nistp384, nistp521, brainpoolP256r1, brainpoolP384r1, brainpoolP512r1, secp256k1), EdDSA (ed25519, ed448) Cipher 3DES, IDEA (for backward compatibility), CAST5, Blowfish, Twofish, AES-128, AES-192, AES-256, Camellia-128, -192 and -256 Hash MD5, SHA-1, RIPEMD-160, SHA-256, SHA-384, SHA-512, SHA-224 Compression Uncompressed, ZIP, ZLIB, BZIP2

History GnuPG was initially developed by Werner Koch. The first production version, version 1.0.0, was released on September 7, 1999, almost two years after the first GnuPG release (version 0.0.0). The German Federal Ministry of Economics and Technology funded the documentation and the port to Microsoft Windows in 2000. GnuPG is a system compliant to the OpenPGP standard, thus the history of OpenPGP is of importance; it was designed to interoperate with PGP, an email encryption program initially designed and developed by Phil Zimmermann. On February 7, 2014, a GnuPG crowdfunding effort closed, raising €36,732 for a new website and infrastructure improvements.

Branches Since the release of a stable GnuPG 2.3, starting with version 2.3.3 in October 2021, three stable branches of GnuPG are actively maintained:

A "stable branch", which currently is (as of 2026) the 2.5 branch. An "old stable branch", which currently is (as of 2026) the 2.4 branch. The old "legacy branch" (formerly called "classic branch"), which is and will stay the 1.4 branch. Before GnuPG 2.3, two stable branches of GnuPG were actively maintained:

"Modern" (2.2), with numerous new features, such as elliptic-curve cryptography, compared to the former "stable" (2.0) branch, which it replaced with the release of GnuPG 2.2.0 on August 28, 2017. It was initially released on November 6, 2014. "Classic" (1.4), the very old, but still maintained stand-alone version, most suitable for outdated or embedded platforms. Initially released on December 16, 2004. Different GnuPG 2.x versions (e.g. from the 2.2 and 2.0 branches) cannot be installed at the same time. However, it is possible to install a "classic" GnuPG version (i.e. from the 1.4 branch) along with any GnuPG 2.x version. Before the release of GnuPG 2.2 ("modern"), the now deprecated "stable" branch (2.0) was recommended for general use, initially released on November 13, 2006. This branch reached its end-of-life on December 31, 2017; Its last version is 2.0.31, released on December 29, 2017. Before the release of GnuPG 2.0, all stable releases originated from a single branch; i.e., before November 13, 2006, no multiple release branches were maintained in parallel. These former, sequentially succeeding (up to 1.4) release branches were:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with GNU Privacy Guard

Start with the simplest possible case. Write down what GNU Privacy Guard claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to GNU Privacy Guard before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about GNU Privacy Guard ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of GNU Privacy Guard

In research: GNU Privacy Guard appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses GNU Privacy Guard in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: GNU Privacy Guard is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1999 software, Cross-platform software, Cryptographic software, so understanding it makes those chapters shorter.
In everyday life: Look for GNU Privacy Guard outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study GNU Privacy Guard in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what GNU Privacy Guard means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain GNU Privacy Guard out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is GNU Privacy Guard in simple terms?

Why does GNU Privacy Guard matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study GNU Privacy Guard?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on GNU Privacy Guard.