Digital signature Ruby code?

Does anyone know where I can find a Ruby module (preferably written in
pure Ruby, so that I can easily install it on different platforms) for
generating and verifying digital signatures? I looked in the RAA, but
nothing was obvious (I could only find private key encryption

A signature is a string of bits generated from a message and a private
key (which only the signer has). The validity of the signature can be
verified using the public key (which everyone has). The signature
proves that the signer wrote the message.