Triple DES

Triple DES (also 3DES) is the encrypt-decrypt-encrypt EDE mode of the Data Encryption Standard (DES) cipher algorithm.

The encryption is done by

C = encrypt_k₃(decrypt_k₂(encrypt_k₁(P))).

with

P ... plaintext

C ... ciphertext

k_i ... key #i

encrypt, decrypt ... DES

Because DES is not a group, using it in EDE mode will increase its key size by a factor 3 (or 2 in the case of k₁ = k₃); in the case of DES this totals to 168 (or 112) bits. If k₁ = k₂ or k₂ = k₃, triple DES is equal to DES.

The use of three steps is essential to prevent meet-in-the-middle attacks. The choice of decryption for the middle step (as opposed to encryption) does not affect the security of the algorithm but instead lets tools that implement triple DES interoperate with legacy single DES tools.