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a) I did some research and there are two opinions.. some say it cracks in several years and others say it doesn't crack for billions of years USING BRUTE FORCE ATTACK...

Vodafone Smart
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    3DES doesn't use 128bit key. – axapaxa Jan 20 '17 at 20:41
  • thanks for the answer.. i am studying information security and there is a case exercise in the book where it says that a person used 128 bit 3DES key to encrypt the message. – Vodafone Smart Jan 20 '17 at 20:45
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    Then your book is wrong. There is no 128bit 3DES variant, only 56, 112 and 168bit variants. Also time spent bruteforcing is always same [bound] for same size of key= 2^keysize. So your calculations like 64bit*2 are incorrect. – axapaxa Jan 20 '17 at 20:52
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    128-bit triple-DES actually has a 112-bit key because the 16 parity bits are ignored (or deducible from the others), and no more than 111-bit resistance (assuming chosen plaintext) due to the complementation property; see this question about its resistance to better attacks. 128-bit double-DES (not triple) is vulnerable to meet-in-the-middle attack, and better attacks needing much less memory. – fgrieu Jan 20 '17 at 20:54
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    The reason @fgrieu mentions double DES is that is what you have described (encrypt with key 1, decrypt with key 2). Triple DES, or 3DES, is typically an encrypt-decrypt-encrypt operation with 2 keys for a total of 112 bits of key material. If you + your hardware can make X tests a year then it will take $2^{111} / X$ years on average to break. – Thomas M. DuBuisson Jan 20 '17 at 21:55
  • @fgrieu How much time to crack that 128 bit triple DES? Is it billions? – Vodafone Smart Jan 20 '17 at 22:24
  • @VodafoneSmart I don't want to sound insulting, but please slow down, read, and think more than knee-jerk respond. Quoting from above "There is no 128bit 3DES variant". If this is homework I suggest you note that in your answer then make your computation for the 112 bits instead of 128. Being up-front is usually best with graders and professors. – Thomas M. DuBuisson Jan 21 '17 at 02:44

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