GPG: the Best Free Crypto You Aren't Using, Part I of II

Ten years after Phil Zimmermann released PGP v.1.0 (Pretty Good Privacy), PGP has evolved from an underground tool for paranoiacs to the gold standard, even an internet standard, for e-mail encryption. GnuPG, the GNU Privacy Guard, is a 100% free alternative to commercial PGP and is included in most Linux distributions. And yet, not nearly as many people who need it (and already have it) use it.

Are you among the many GnuPG procrastinators of the world? Hopefully you won't be after this and next months' columns. After you've generated your personal keys, sent your first encrypted e-mail and finally verified the security signature of that cool software package you downloaded, you'll be glad you took the trouble to master the multifunctional marvel that is GnuPG.

This month we'll begin with PGP/GnuPG background, concepts and quick-start. Next month we'll dig deeper into file and e-mail encryption, key management and graphical user interfaces.

Back in 1991, when the US Congress appeared to be on the brink of outlawing all private use of cryptographic software, Phil Zimmermann released PGP v.1.0. This originally free tool allowed ordinary users of consumer-grade computer systems to encrypt their personal data and communications effectively enough to thwart even determined and well-financed eavesdroppers (such as, for example, the US government).

Phil Zimmermann's story is important and compelling and can be read in Simson Garfinkel's book and on Phil's own home page (see Resources). But for now suffice it to say that despite government investigation, patent complications and the tribulations of corporate assimilation, PGP has continued to improve and grow in serving Zimmermann's vision of protecting people's and organizations' privacy worldwide.

When I say that, however, I really mean PGP in the broadest sense, including OpenPGP and GnuPGP. The emergence of the latter have, respectively, resulted in PGP's key and message formats becoming an internet standard in RFC 2440, and in providing users worldwide with a completely free and unencumbered (by patents) implementation of PGP.

Although Mr. Zimmermann is, by any reasonable standard, one of the true pioneers of and contributors to open-source software, Network Associates Inc.'s (NAI) product PGP is problematic for Linux users in specific and open-source adherents in general. First and most obviously, commercial PGP runs only on Windows and Mac OS.

Second, even PGP Freeware is free only to noncommercial users, that is, users in educational and nonprofit settings. Third, NAI has decided to reduce greatly the amount of PGP source code it makes available for peer review and public scrutiny, even for Freeware PGP.

This last development apparently contributed to Phil Zimmermann's resignation from PGP Security and has cast doubt on the advisability of fully trusting NAI's version of PGP. Considering the US government's hostility toward PGP and various governmental efforts to require “back doors” in cryptographic software (including key-escrow), it's all too easy to imagine NAI quietly bowing to governmental pressure and creating such a back door. Without public scrutiny of PGP's full source code, we have no means of validating assertions by NAI that this is not and will never be the case.

GnuPG, on the other hand, is a 100% open-source and 100% free package that does most of what PGP does (it lacks certain features such as virtual private networking and disk-volume encryption that are included in PGP Desktop). In a short time, GnuPG has become the preferred e-mail and data encryption tool for Linux users and is packaged with most current Linux distributions. The GnuPG Project is headed, and its code primarily developed, by Werner Koch.

GNU Privacy Guard consists of a single-binary executable, gpg. Actually there's an additional binary, gpgv, but since gpgv merely provides a convenient subset of gpg's functionality you can still think of gpg as essentially self-contained. Therefore, it's perfectly okay to use the terms GnuPG and gpg interchangeably—I'm going to do so for the remainder of this article. I'm also going to be loose with the term PGP—rather than the specific commercial product by Network Associates, I'll henceforth use PGP to refer to the protocols, mechanisms and Web of Trust common to PGP, OpenPGP and GnuPG.

GnuPG performs four basic functions to which all of its other functions are supplemental: encrypting data, decrypting data, cryptographically signing data and cryptographically verifying digital signatures. It is also used to create and manage keys, activities that, although subordinate to the four listed above, are absolutely essential in performing those functions securely.

In real terms, this means that people generally use GnuPG to encrypt files, especially e-mail; decrypt mail or files that have been sent to them; digitally sign documents, source code distributions and other electronic files; validate others' digital signatures to determine whether a given file matches its accompanying signature (i.e., wasn't tampered with at any point) and whether the file was verifiably signed by the person who claims to have signed it; and maintain keyrings (key databases) containing their personal key or keys (their secret keyring) and the public keys of their friends, colleagues, business partners, etc., (their public keyring).

Obviously, then, you need GnuPG if you wish to exchange encrypted messages and files with other GnuPG users (and users of other OpenPGP-compliant software). It's also one of your options if you wish to encrypt data that is stored locally but in a not-altogether trusted place, e.g., the hard drive of a laptop computer you travel with and are resigned to the possibility of being stolen.

But even if none of your friends use GnuPG or PGP, and even if you feel that none of your data is worth protecting with encryption, there's still one very compelling reason to learn at least a little about using GnuPG: software-distribution signing. Thanks to several high-profile break-ins at public FTP sites on the Internet in which software packages were replaced with compromised (Trojaned) versions, it has become common practice for developers of security software to distribute digital signatures of their software distributions.