This facility is "built in" to Rails, with MessageEncryptor and MessageVerifier. Before you consider writing your own "apply an HMAC-SHA1 hash to a string" library in Ruby, just take those classes from ActiveSupport instead. They've been reasonably well tested.
This is a MUCH WORSE IDEA. Don't do this. Even this Python wrapper library does a better job providing HMAC services to users than OpenSSL's HMAC primitive does. Leave OpenSSL:* to the suckers^H^H^H^H^H^H crypto library implementors.
