When Diffie and Hellman introduced public-key cryptography, they proposed a modified telephone directory in which you could find public keys. Instead of name, address, and phone number, it would have name, address, and public key. If you wanted to find John Robinson's public key you would look him up in the directory, get his public key and send him a message for his eyes only using that public key. This might have worked with the Stanford Computer Science Department phone directory in 1976, but how many John Robinsons are in the New York City phone book, much less in a hypothetical phone book for the global Internet?