But if we rephrase the requirement to indicate the reasons behind it, we find that the concern is still valid. And it may also be noted that public-key cryptography, which allows the two participants to avoid having to exchange their private keys, and which allows them to use a fresh session key for each message, contributes to the ease of meeting this requirement. Enciphered messages should be in a form suitable to transmission by means of whatever communications medium is intended to be used, or convenient to use. This may mean the Internet or a fiber-optic link instead of the telegraph, but the principle remains sound. In order for a cipher to satisfy the first rule, it seems impossible to avoid having to use a piece of apparatus for encipherment, the digital computer.
Computers certainly do exist that are portable and which are easily used by one person today.
Again, it seems that for a cipher to remain unbreakable by today's standards, the algorithm used would have to be intricate and complicated. However, it is also true that we now have computers to do all the hard work. One of the reasons that a cipher should not be too complicated is to avoid problems caused by error in the encipherment of messages. Hence, this dictum could be considered to recommend that ciphers with unfavorable error-propagation characteristics should be avoided, since transmission errors can also make it necessary to retransmit a message. And this relates more directly to the cipher itself than to simply note that any encryption program, like any other computer program, should have a good user interface.