Diffie–Hellman key exchange

General overview[edit]

Illustration of the idea behind Diffie–Hellman key exchange

Diffie–Hellman key exchange establishes a shared secret between two parties that can be used for secret communication for exchanging data over a public network. The following conceptual diagram illustrates the general idea of the key exchange by using colors instead of very large numbers.

The process begins by having the two parties, Alice and Bob, agree on an arbitrary starting color that does not need to be kept secret (but should be different every time^[3]); in this example the color is yellow. Each of them selects a secret color that they keep to themselves. In this case, orange and blue-green. The crucial part of the process is that Alice and Bob now mix their secret color together with their mutually shared color, resulting in orange-tan and light-blue mixtures respectively, then publicly exchange the two mixed colors. Finally, each of the two mix together the color they received from the partner with their own private color. The result is a final color mixture yellow-brown that is identical to the partner's color mixture.

If a third party listened to the exchange, it would be computationally difficult for them to determine the secret colors. In fact, when using large numbers rather than colors, this action is computationally expensive for modern supercomputers to do in a reasonable amount of time.

Cryptographic explanation[edit]

The simplest and the original implementation of the protocol uses the multiplicative group of integers modulo p, where p is prime, and g is a primitive root modulo p. These two values are chosen in this way to ensure that the resulting shared secret can take on any value from 1 to p–1. Here is an example of the protocol, with non-secret values in blue, and secret values in red.

Alice and Bob agree to use a modulus p = 23 and base g = 5 (which is a primitive root modulo 23).
Alice chooses a secret integer a = 4, then sends Bob A = g^a mod p
- A = 5⁴ mod 23 = 4
Bob chooses a secret integer b = 3, then sends Alice B = g^b mod p
- B = 5³ mod 23 = 10
Alice computes s = B^a mod p
- s = 10⁴ mod 23 = 18
Bob computes s = A^b mod p
- s = 4³ mod 23 = 18
Alice and Bob now share a secret (the number 18).

Both Alice and Bob have arrived at the same value s, because, under mod p,

{\displaystyle {\color {Blue}A}^{\color {Red}b}{\bmod {\color {Blue}p}}={\color {Blue}g}^{\color {Red}ab}{\bmod {\color {Blue}p}}={\color {Blue}g}^{\color {Red}ba}{\bmod {\color {Blue}p}}={\color {Blue}B}^{\color {Red}a}{\bmod {\color {Blue}p}}}

^[8]

More specifically,

{\displaystyle ({\color {Blue}g}^{\color {Red}a}{\bmod {\color {Blue}p}})^{\color {Red}b}{\bmod {\color {Blue}p}}=({\color {Blue}g}^{\color {Red}b}{\bmod {\color {Blue}p}})^{\color {Red}a}{\bmod {\color {Blue}p}}}

Note that only a, b, and (g^ab mod p = g^ba mod p) are kept secret. All the other values – p, g, g^a mod p, and g^b mod p – are sent in the clear. Once Alice and Bob compute the shared secret they can use it as an encryption key, known only to them, for sending messages across the same open communications channel.

Of course, much larger values of a, b, and p would be needed to make this example secure, since there are only 23 possible results of n mod 23. However, if p is a prime of at least 600 digits, then even the fastest modern computers cannot find a given only g, p and g^a mod p. Such a problem is called the discrete logarithm problem.^[3] The computation of g^a mod p is known as modular exponentiation and can be done efficiently even for large numbers. Note that g need not be large at all, and in practice is usually a small integer (like 2, 3, ...).