does not understand the unilateral authentication mechanism then it can discard the packet option and accept the remainder of the message of course this recipient does not benefit from the authentication mechanism but at least it can understand and use the message contents for clarity 39 s sake the discussion so far depicts all of the authentication information as arriving at the recipient in a single message see for example fig5 and 7 and accompanying discussion this is not necessary for the mechanism to work as public keys can be quite large in some embodiments the sender can shorten its messages by leaving the public key out of each message the sender delivers its public key to a key publishing service note that the key publishing service may in implemented on the same device as the sender when a message is received the recipient requests the sender 39 s public from the publishing service because communications with the publishing service may not be secure nefarious agents may intercept the request and attempt to fool the recipient in response to its request the recipient may receive several invalid public keys along with the one valid key that is not a problem however the recipient uses each received public key in turn in the procedure of fig7 only the valid public key leads to a successful authentication having found the valid key the recipient can store it for future use if none of the received keys works then the recipient discards the message as a general rule of thumb the cryptographic strength of an authentication mechanism that uses a hash increases with the number of bits produced by the hash however the number of.