Diffie-Hellman is a method of exchanging cryptographic keys through a public domain, with no prior shared knowledge between the client and the server. In fact, the two parties are actually not sharing keys, but creating a new one together.

The standard Diffie-Hellman works as follows1:

• Alice and Bob agree on a shared set of numbers: The prime number p and the generator g which is a primitive root modulo p. The size of p determines the security of the key exchange, and is thus preferrably large.
• Alice generates a random number a and keeps it secretly stored on her computer. She uses this number to generate A = g^a\mod p, which she then shares with Bob.
• Bob also generates a random number b, and shares the number B = g^b \mod p with Alice.
• Both repeat the same proceedure, but replace the generator g with the number they recieved. So for Alice, thats B^a \mod p and for Bob thats A^b\mod p.
• The resulting number is used as the final shared cryptographic key.

This process works because modulo exponents behave in the following way:

(g^a \mod p)^b \mod p = g^{ab} \mod p

(g^b \mod p)^a \mod p = g^{ba} \mod p

Meaning Alice and Bob end up with the same number. Note also that the only numbers that are visible to the public domain are p, g, A and B - none of which a malicious actor could use to get the final key (at least not without brute forcing it, which is why p needs to be large).

As an alternative to the regular Diffie-Hellmann, one can exchange the modulo formula with an elliptic curve to get the Elliptic curve Diffie-Hellman key exchange.

1

Computerphile, & Pound, M. (2017). Diffie Hellman -the Mathematics bit- Computerphile. YouTube. https://www.youtube.com/watch?v=Yjrfm_oRO0w

18:42 Wednesday 24 March 2021