According to the documentation OpenSSL::HMAC.digest
Returns the authentication code an instance represents as a binary string.
If you have a problem using that maybe you need a hex encoded form provided by OpenSSL::HMAC.hexdigest
Example
key = 'key'
data="The quick brown fox jumps over the lazy dog"
digest = OpenSSL::Digest.new('sha256')
OpenSSL::HMAC.digest(digest, key, data)
#=> "\xF7\xBC\x83\xF40S\x84$\xB12\x98\xE6\xAAo\xB1C\xEFMY\xA1IF\x17Y\x97G\x9D\xBC-\x1A<\xD8"
OpenSSL::HMAC.hexdigest(digest, key, data)
#=> "f7bc83f430538424b13298e6aa6fb143ef4d59a14946175997479dbc2d1a3cd8"