Cryptography related question

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    • #5500
      nadeemvirk
      Participant

      DES was replaced by NIST after it was broken through brute fore attack. Reason -> shorter key length and key space. The algorithm itself was never broken but key was found through brute force. Later symmetric algorithms that replaced DES such as AES, 3DES, and IDEA etc etc increased the key length.  some also supports different key lengths such as AES uses 128, 192, and 256. Now as a quick fix to the problem 3 DES was introduced that also has different variants and uses 2 keys normally that has increased the key length to 112. Now what was the reason or limitation that we couldn’t increase the key length in DES to make it more secure and why we had to add more keys in 3DES to increase the key length???

    • #34752
      sil
      Participant

      I’ll take a stab at this however, you’re better off posting this on a crypto-forum for a better answer. DES as an algorithm wasn’t broken however, the fact that you can brute-force attack it shows the limits of they keyspace: From wiki: Even if a symmetric cipher is currently unbreakable by exploiting structural weaknesses in its algorithm, it is possible to run through the entire space of keys in what is known as a brute force attack. Since longer symmetric keys require exponentially more work to brute force search, a sufficiently long symmetric key will prevent this line of attack.

      So let’s think about this for a moment. These demonstrations were done 12 years ago and the power of computing has increased phenomenally. No one is attempting to attack DES anymore because its been established that you CAN bruteforce the keyspace. Meaning, there aren’t enough keys generated. Now… 3DES (TDEA, pick your poison) differs in the sense that its still based on DES (56bit key) but re-(d)encrypts.

      With 3DES we have DES-EEE and DES-EDE (encrypt-encrypt-encrypt and enc-dec-enc)… And if you ask me I would say its just DES over DES if you break it down to a logical term of what it does and why it does it.

      EEE
      Key1 –> Encrypt
      Key2 –> Encrypt the output of key1
      Key3 –> Encrypt the output of key2

      EDE
      Key1 –> Encrypt
      Key2 –> Decrypt the output of key1
      Key3 –> Encrypt the output of key2

      All keys differ but guess what… All keys are still the same @ 56bits. The end result is 112bits (56×3) since (drum roll) DES was broken. Lop off 56 bits from 168. Because someone can attack the keyspace of DES, the result is something (that I interpret) similar to

      EEE
      Key1 –> Encrypt
      [s:2xqpwtdp]Key2 –> Someone will break this so we do it again[/s:2xqpwtdp]
      Key3 –> Encrypt the output of key2

      EDE
      Key1 –> Encrypt
      [s:2xqpwtdp]Key2 –> Someone will break this so we do it again[/s:2xqpwtdp]
      Key3 –> Encrypt the output of key2

      I’m not a crypto specialist/expert in fact… Don’t care much for what’s under the hood although I’ll read about it from time to time to satisfy my own curiousity and brutally (mis)interpret what I read. I suggest posting the question to IACR though or digging through FreeTextbooks (http://www.freetechbooks.com/information-security-f52.html) NOTE: FreeTextBooks is actually linked from IACR in case someone is thinking “free warez puppy site” or something.

      http://www.iacr.org/
      Mountains of crypto material enough to make you puke algorithms for months: http://eprint.iacr.org/

    • #34753
      former33t
      Participant

      Like sil said, post this on a crypto forum and you’re bound to get better answers.  I took a crypto class in college a couple of years ago though and I seem to remember that there was a keyspace reduction attack against triple DES.  I remember the question coming up in class why not increase the key length and the instructor went through a proof about how if you could reduce the space on triple DES (at least for some keys) then you could do it for any incremental implementation of DES (like 6-DES or whatever we would go to next).

      This paper explains a little about the idea, I’m sure there are more recent examples.  If I were at home, I could pull out my books/notes and get you something better, but I’m on the road.  Hope this helps.

      http://www.google.com/url?sa=t&source=web&cd=5&ved=0CCcQFjAE&url=http%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fdownload%3Fdoi%3D10.1.1.91.555%26rep%3Drep1%26type%3Dpdf&rct=j&q=DES%20key%20reduction&ei=rLt1TIXAO8P98Qak0-GjDg&usg=AFQjCNEFIQXNXmldbWyxgBef3v_DsAascw&cad=rja

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