Editing Triple DES (section)

==Algorithm==
The original DES cipher's [[key size]] of 56&nbsp;bits was considered generally sufficient when it was designed, but the availability of increasing computational power made [[brute-force attack]]s feasible. Triple DES provides a relatively simple method of increasing the key size of DES to protect against such attacks, without the need to design a completely new block cipher algorithm.

A naive approach to increase the strength of a block encryption algorithm with a short key length (like DES) would be to use two keys <math>(K1, K2)</math> instead of one, and encrypt each block twice: <math>E_{K2}(E_{K1}(\textrm{plaintext}))</math>. If the original key length is <math>n</math> bits, one would hope this scheme provides security equivalent to using a key <math>2n</math> bits long. Unfortunately, this approach is vulnerable to the [[meet-in-the-middle attack]]: given a [[known plaintext]] pair <math>(x, y)</math>, such that <math>y = E_{K2}(E_{K1}(x))</math>, one can recover the key pair <math>(K1, K2)</math> in <math>2^{n+1}</math> steps, instead of the <math>2^{2n}</math> steps one would expect from an ideally secure algorithm with <math>2n</math> bits of key.

Therefore, Triple DES uses a "key bundle" that comprises three DES [[Key (cryptography)|keys]], <math>K1</math>, <math>K2</math> and <math>K3</math>, each of 56&nbsp;bits (excluding [[parity bit]]s). The encryption algorithm is:
: <math>\textrm{ciphertext} = E_{K3}(D_{K2}(E_{K1}(\textrm{plaintext}))).</math>
That is, encrypt with <math>K1</math>, <em>decrypt</em> with <math>K2</math>, then encrypt with <math>K3</math>.

Decryption is the reverse:
: <math>\textrm{plaintext} = D_{K1}(E_{K2}(D_{K3}(\textrm{ciphertext}))).</math>
That is, decrypt with <math>K3</math>, <em>encrypt</em> with <math>K2</math>, then decrypt with <math>K1</math>.

Each triple encryption encrypts one [[Block size (cryptography)|block]] of 64&nbsp;bits of data.

In each case, the middle operation is the reverse of the first and last. This improves the strength of the algorithm when using [[#Keying options|keying option]] 2 and provides [[backward compatibility]] with DES with keying option 3.