Implementations different from GnuPG that aim for OpenPGP compliance.

OpenPGP.js

http://openpgpjs.org/

License: GNU LGPL v>=2.1.

used by:

• https://www.mailvelope.com/ (License GNU Affero v==3) for Webmail -- extensions for Chrome/Chromium & Firefox
• Mymail-Crypt for Gmail (License: GNU GPL v>=2) -- extension for Chrome/Chromium

Completeness: ?

Compatibility: ?

kbpgp

https://github.com/keybase/kbpgp

Keybase's PGP for JS claims: Public-key crypto and improvements to OpenPGP.js

Chromium Extension End-to-End

https://code.google.com/p/end-to-end/

Completeness: ? Claims to only create ECC certificates (which curve parameters?)

Compatibility: ?

• Google announced a Chromium extension that supports OpenPGP, see http://googleonlinesecurity.blogspot.de/2014/06/making-end-to-end-encryption-easier-to.html
• Heise newsarticle in DE: http://www.heise.de/newsticker/meldung/Google-experimentiert-mit-End-to-End-Verschluesselung-fuer-GMail-2215680.html

Android Privacy Guard

http://thialfihar.org/projects/apg/

Completeness: ?

Compatibility: ?

Integrates with the K-9 Mailer.

Seems to be unmaintained (last release 2014-03, some known vulnerabilities unfixed since (status 2016-04)) with OpenKeychain being the activeley maintained "fork", see ~OpenKeychain FAQ: relation to APG

OpenPGP-Keychain

http://www.openkeychain.org/about/ , Fork of APG. Uses Bouncy Castle.

Completeness: ?

Compatibility: ?

Integrates with the K-9 Mailer.

Bouncy Castle

https://en.wikipedia.org/wiki/Bouncy_Castle_(cryptography) (Homepage with ads: https://bouncycastle.org/)

Completeness: ?

Compatibility: ?

hOpenPGP

http://hackage.haskell.org/package/hOpenPGP

native Haskell implementation of OpenPGP (RFC4880), plus Camellia (RFC5581)

Discussion

• https://news.ycombinator.com/item?id=7843297 Discussion about OpenPGP.js codequality and choice of reimplementation in End-to-End.