The answer for several thousand years was: no. So the question is, can Alice and Bob talk to each other in a way that will allow them to foil Eve, without first establishing a secret key that only they know? Remember, once a secret key is known to Alice and Bob, but not Eve, a cipher can be used to scramble and unscramble the data.
(By the way, these conventional names that are seen in many discussions of cryptography were introduced by the same paper.) Figure 1.
In Figure 1 we see the idealized layout of things: Alice and Bob want to talk to each other securely, but they have to assume that a malicious actor, Eve (as in “eavesdropper”) has access to the channel as well. Let’s get a closer look at how it works.įirst, consider the process in theory. The ability to securely establish a key between unknown parties was a cryptographic earthquake.
Once the shared secret key is in place, we can switch to traditional symmetric encryption for speed. This secret is known as a shared secret key. The answer in Diffie-Hellman is that, by using one-way functions, two parties can arrive at a secret number that they both know, but that any eavesdropping party cannot determine. We might be in the position of having to FedEx a cipher key to each other whenever we wanted to make an online purchase. Without Diffie-Hellman, it’s difficult to picture what the internet would look like. The question is, given two parties without prior arrangement, how do we establish a secure channel and verify identity? As you can readily imagine, such a capability is essential for the fundamental operation of the then-nascent ARPANET, soon to become our ubiquitous internet. When Diffie and Hellman first introduced their idea of asymmetric keys, they began their paper with the promise, “We stand today on the brink of a revolution in cryptography.” They meant it, and history has borne them out. A key is used to scramble messages, and the same key is used to unscramble them. Until the 1970s, the development of secure communications entailed ever more sophisticated symmetric ciphers.
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