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Review
Ralph Merkle helped create the framework for public key cryptography. Despite the drawbacks to his key distribution approach, Merkle designed an ingenious way to securely distribute secret keys over an insecure public communication channel. Although Merkle's initial idea didn't give people the desired competitive advantage over their adversaries, he worked with Martin Hellman and Whitfield Diffie to create a mathematically feasible system. It's now accepted that British cryptographers developed public key cryptography before Diffie, Hellman, and Merkle, even though the Stanford trio were the first to patent a public key system.
The Diffie-Hellman-Merkle public key system (commonly referred to as Diffie-Hellman, or DH), implements the strategy of making a problem that has at least two solution paths: an easy one and a very difficult one. The idea is to give your friends the easy, less time-consuming path and force your adversaries to solve the difficult, more time-consuming version of the same problem. All public key cryptography uses this principle.
The Diffie-Hellman-Merkle key agreement method didn't provide all the versatility and assurances needed to fuel the digital age. Authentication was still a problem. The idea of asymmetric ciphers, in which one key is used to encrypt and another is used to decrypt, solves the problem of authentication. Rivest, Shamir, and Adleman built on the public key foundation built by Diffie, Hellman, and Merkle to create an asymmetric cipher known as RSA.