I was reading about DES Triple Encryption,and I found a lot of diffrent combinations of the 3 keys , for example there's : using k1 - k2 - k3 , as there's k1- k2 - k1 , ... My Question is , what is the diffrence between these combinations? and which one is advisable (most used) ?
How many keys does Triple DES encryption need? should provide depth to the following answer:
Using three distinct keys is the best way to go. In fact, it's the only option for the US Government (and US Government contractors):
3.2.1.2 Triple Data Encryption Algorithm (TDEA)
The Triple Data Encryption Algorithm (TDEA), also known as Triple DES, uses the DES cryptographic engine to transform data in three operations. TDEA is specified in SP 800-67.
TDEA encrypts data in blocks of 64 bits, using three keys that define a key bundle. The use of TDEA using three distinctly different (i.e., mathematically independent) keys is approved and is commonly known as three-key TDEA (also referred to as 3TDEA or 3TDES).
Other variations of TDEA, where two or three of the keys are identical, are no longer approved for applying cryptographic protection because of increased computing power or weaknesses in the algorithm.
(Emphasis mine). Source: http://csrc.nist.gov/publications/drafts/800-175/sp800-175b_draft.pdf