Module DES3

Triple DES symmetric cipher

Triple DES (or TDES or TDEA or 3DES) is a symmetric block cipher standardized by NIST. It has a fixed data block size of 8 bytes. Its keys are 128 (Option 1) or 192 bits (Option 2) long. However, 1 out of 8 bits is used for redundancy and do not contribute to security. The effective key length is respectively 112 or 168 bits.

TDES consists of the concatenation of 3 simple DES ciphers.

The plaintext is first DES encrypted with K1, then decrypted with K2, and finally encrypted again with K3. The ciphertext is decrypted in the reverse manner.

The 192 bit key is a bundle of three 64 bit independent subkeys: K1, K2, and K3.

The 128 bit key is split into K1 and K2, whereas K1=K3.

It is important that all subkeys are different, otherwise TDES would degrade to single DES.

TDES is cryptographically secure, even though it is neither as secure nor as fast as AES.

As an example, encryption can be done as follows:

>>> from Crypto.Cipher import DES
>>> from Crypto import Random
>>> from Crypto.Util import Counter
>>>
>>> key = b'-8B key-'
>>> nonce = Random.new().read(DES.block_size/2)
>>> ctr = Counter.new(DES.block_size*8/2, prefix=nonce)
>>> cipher = DES.new(key, DES.MODE_CTR, counter=ctr)
>>> plaintext = b'We are no longer the knights who say ni!'
>>> msg = nonce + cipher.encrypt(plaintext)