It refers to the idea that most asymmetric cryptosystems (which, generally speaking means that each user has a public key and a private key) allow for a user to

It refers to the idea that most asymmetric cryptosystems (which, generally speaking means that each user has a public key and a private key) allow for a user to create a 'signature' using their private key, which can be verified using their public key. See here: http://stackoverflow.com/questions/454048/what-is-the-difference-between-encrypting-and-signing-in-asymmetric-encryption http://stackoverflow.com/questions/454048/what-is-the-differ...