Suppose I use a public key authentication scheme to communicate with a collaborator, Bob. To avoid any possible failures of technology, we've developed a simpli

Suppose I use a public key authentication scheme to communicate with a collaborator, Bob. To avoid any possible failures of technology, we've developed a simplified scheme that can be done with pencil and paper and sent through the mail. When Bob receives the message, he uses his private key to decrypt the message, writing each letter in the plaintext on a piece of paper so he can read it. After reading, he burns the paper completely with a small fire. Are you going to insist that this last step, burning the plaintext copy, is not "valid security"? If you do, haven't we so mangled the meaning of security that it's not even intelligible any more? I believe that "Let's not keep around unencrypted / minimally protected copies of our communications after we're done using them" is a perfectly valid security measure for two people to use. No, it doesn't solve "trust issues", in that one of them could simply refuse to delete the messages, but neither is it intended to.