DES properties and multiple encryptions

Have 2 small questions...

suppose the round function in the the 16 rounds of DES is replaced with a function that sets all the bits to zero:


Now what function would DES compute now?

And why is double encrypting with DES (2DES = DESk1(DESk2(m))) not efficient? and how it may be broken...

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andydisConnect With a Mentor Commented:

i cannot answer your 1st question but i know tripe DES is now an industry standard becuase..

Triple-DES is normally preferred to double-DES since the latter is vulnerable to a meet-in-the-middle attack. The EFF’s "Deep Crack" custom key-search machine found a 56-bit single-DES key from a single plaintext/ciphertext pair in under 56 hours.
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