Triple DES - Algorithm

Algorithm

Triple DES uses a "key bundle" which comprises three DES keys, K₁, K₂ and K₃, each of 56 bits (excluding parity bits). The encryption algorithm is:

ciphertext = E_K3(D_K2(E_K1(plaintext)))

I.e., DES encrypt with K₁, DES decrypt with K₂, then DES encrypt with K₃.

Decryption is the reverse:

plaintext = D_K1(E_K2(D_K3(ciphertext)))

I.e., decrypt with K₃, encrypt with K₂, then decrypt with K₁.

Each triple encryption encrypts one block of 64 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 option 2, and provides backward compatibility with DES with keying option 3.