thedecryptionequationin3deswithtwokeysis

In 1978, a triple encryption method using DES with two 56-bit keys was proposed by Walter Tuchman; in 1981, Merkle and Hellman proposed a more secure triple-key version of 3DES with 112 bits of security.

TDEA involves using three 64-bit DEA keys (K1, K2, K3) in Encrypt-Decrypt-Encrypt (EDE) mode, that is, the plain text is encrypted with K1, then decrypted with K2, and then encrypted again with K3.

Two versions of triple DES are in use today: triple DES with two keys and triple DES with three keys. In triple DES with two keys, there are only two keys: K1 and K2.

Decryption: Apply the DES algorithm three times to the encrypted ciphertext using the three keys in reverse sequence: decrypt with K3, encrypt with K2, and decrypt again with K1.

Double DES is a encryption technique which uses two instance of DES on same plain text. In both instances it uses different keys to encrypt the plain text. Both keys are required at the time of decryption.

簡單的說，Triple DES 系統是連續使用 DES 編碼器三次，如只用到兩把 56 個位元的鑰匙（112 bits），則稱為 Two-keys 3DES（如圖 3-2 所示）。

In other words, user encrypt plaintext blocks with key K 1, then decrypt with key K 2, and finally encrypt with K 1 again. Therefore, 2TDES has a key length of 112 bits.