It's a secret (16)

So what's up with the three-pass method - why hasn't it become the procedure of choice?  Well, the first problem is in the name - it requires the message to be passed three times - A to B, B back to A and then A back to B.  If it's just you and me messing about it doesn't matter, but the use of a cipher usually implies that the matter is of some urgency (like our "get out now" message) and the delay may be unacceptable.  And then, the most common reason for ciphers to fail is human error - someone wrongly transcribing something or looking something up incorrectly, and of course the three pass method triples the possibility of this happening.  But the most crucial failing is that it is fundamentally insecure.  This seems counter-intuitive, but think about it - if someone is intercepting the communications between A and B then by comparing what is sent with what is sent back he/she has all the information necessary to work out what system is being used and what the individual keys are. Not that it would necessarily be easy to work out, but all the information is there.  So the three-pass method never really caught on.  But now cryptography was entering the age of the computer...