Emulate 3DES EDE using only DES

Question

My device doesn't support full 3DES (EDE). How can I emulate one using standard DES? Encryption mode is CBC.

Answer 1

You start by picking three independent DES keys which are not related to each other in any way.

You will want to put DES into ECB mode, not CBC mode. You also need to ensure that each encryption and decryption operation is done only on 64-bit blocks and nothing more or less. Padding schemes and the likes will cause a vulnerability in the implementation and will lead to the discovery of the block content via brute force faster than a brute force against each key.

Using the first key, encrypt your plaintext. Using the second key, decrypt that value. Using the third key, encrypt the value for your full block. It looks like this:

Encrypt(k3, Decrypt(k2, Encrypt(k1, plaintext)))

Decryption is the other way around and looks like this:

Decrypt(k1, Encrypt(k2, Decrypt(k3, ciphertext)))

When you encrypt your blocks with 3DES you then need to apply your mode of operation like CBC or CTR and apply padding if needed.

Be careful.