There are many Crypto API available out there. Search for "crypto api" and "crypto library" using searchSecurity. You'll find quite a few. Check to make sure they work with your application development language (C++? Java?). The top two who come to mind are Baltimore Technologies and RSA Security. Sun might have one. Yes, Microsoft does, but for something as important as crypto, I'd go with someone in the security business.
Once you have a Crypto library, you'll be able to use the documentation that comes with it to learn how to use it.
For more information on this topic, visit these other searchSecurity resources:
Best Web Links: Cryptography
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This was first published in February 2002