WinRSA can be downloaded (below) and used free of charge under the terms of the GNU General Public License (see www.gnu.org for details of this license). Of course, this was a student project written under severe time constraints so the usual disclaimers apply. The software is very simple to use and has an explanatory help facility. An edited version of the original project report is available here.
The principal value of this implementation is that it permits the user to specify the required prime length in RSA. A typical secure choice would be primes of length 1024 bits, which translates roughly to a modulus in RSA of about 600 decimal places. Anyone wishing to break encrypted text would then have to factorize a decimal of 600 digits. As far as we know for the foreseeable future it is quite unlikely that this can be done on a time-scale which would be useful. The user has the option to use larger primes which will of course slow down the run times. As machine speeds increase it is recommended that users correspondingly increase the length of the primes. Of course, if quantum computing ever becomes a reality then the only secure system of encryption is likely to be one-time pads. Any file types can be encrypted although, given the practical limitations on file length versus time, most users will find that short messages of a few pages in ASCII are likely to be the principal application.
This software is being distributed in the interests of freedom and the right to individual privacy. As required by the GNU license terms the source code for WinRSA is available and can be downloaded below.
The patent on the RSA encryption algorithm previously held by RSA Data Security Inc has now expired. However, the software also (optionally) uses the IDEA symmetric cipher. IDEA is patented in some countries but the patent holders allow royalty-free use for non-commercial purposes (see the WinRSA licence agreement for more details). However, technically the use of patented algorithms is not allowed in code released under the GNU GPL. For this reason, an IDEA-free implementation of WinRSA is being produced and will soon become the operative version. This version will use only AES, or some other non-patented algorithm, for symmetric encryption.
Latest windows executable (2008 version 2.02) zipped: <here>
Latest windows source (2008 version 2.02 zipped): <here>
Note: As stated in the License preamble, this was originally an undergraduate Computer Science project. It has been improved greatly since then, but essentially is still a toy application. WinRSA should not be treated as a professional-grade cryptographic product, and its use for safeguarding sensitive information is not recommended. Notably, the padding scheme used offers no specific protection against chosen-plaintext and chosen-ciphertext attacks. Sufficient commercial and free software cryptographic programs exist to allow you to safeguard your sensitive data in a secure manner. The only novelty WinRSA has that commercial offerings usually lack is that RSA key modulus lengths are variable: you can specify how big a key you want to generate.