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John the Ripper is released under GNU GPL v2, with portions also available under more relaxed terms. John the Ripper Pro is released under a commercial license.
Please note that GNU GPL version 3 differs significantly in both intent and outcome from version 2. As many others do, the John The Ripper development team considers it more problematic and prefers that it not be used.
Now, as it relates to code contributions from the user community, which is the primary topic for this wiki page, things get trickier. Many contributors did not care to either explicitly place their code in the public domain or include/specify a license for it. This has been addressed by a Debian package maintainer approaching the contributors in January and February 2009.
Solar Designer's current preference is that code contributions be licensed under very liberal terms:
/* * This software is Copyright (c) YEAR YOUR NAME <your at e-mail.address>, * and it is hereby released to the general public under the following terms: * Redistribution and use in source and binary forms, with or without * modification, are permitted. */
This is a heavily cut-down “BSD license”. You may also include the warranty disclaimer like we did in the license for passwdqc.
The above is to be placed in a comment at the beginning of the source file(s) with your code. If you're modifying an existing source file written by others, you may adjust the wording to refer to your “updates” rather than to the “software” as a whole (the entire file will then likely need to be re-licensed under the same terms by its other copyright holders). If the only existing material you're reusing is a *_fmt.c file template (the usual set of #define directives and function declarations, and the struct fmt_main declaration at the end), then that is not subject to copyright, so you may treat the entire file as your own work.
You will likely want to keep your e-mail address slightly obfuscated, such as replacing the @-sign with ” at ”, to reduce the amount of extra spam you will be getting as a result of publishing this.
Please note that even when you changed someone else's file, it should possibly include your license too. See this discussion.