There is MS info on HMAC with C program examples at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/security/hh/crypto/usingcrypto_9wiv.asp

>We have a need to decrypt URL parameters that is encrypted by HMAC-MD5.
>Are there functions available for Visual Foxpro? Can anyone point me in the right direction?
>
>Thank you,
>Steve McMahon
>ZDI
>San Diego