MITM on 3TDES and why is the effective key lenght 112 bits

Discussion in 'Computer Security' started by Saqib Ali, Sep 7, 2006.

  1. Saqib Ali

    Saqib Ali Guest

    Can anyone explain how the MITM works on 3TDES (three distinct keys)?

    I am typically interested in finding out why 3TDES has effective
    key-length of 112-bit

    In Peace,
    Saqib Ali
    http://www.full-disc-encryption.com
    Saqib Ali, Sep 7, 2006
    #1
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  2. Saqib Ali

    nemo_outis Guest

    nemo_outis, Sep 7, 2006
    #2
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  3. Saqib Ali

    Saqib Ali Guest

    Saqib Ali, Sep 7, 2006
    #3
  4. Saqib Ali

    Ludovic Joly Guest

    I think you should post your question to sci.crypt...

    Kind regards
    Ludovic
    Ludovic Joly, Sep 7, 2006
    #4
  5. Saqib Ali

    Jon Guest

    Saqib Ali wrote:
    > Can anyone explain how the MITM works on 3TDES (three distinct keys)?
    >
    > I am typically interested in finding out why 3TDES has effective
    > key-length of 112-bit
    >


    I wrote this ages ago for TechTarget, but it answers your question.

    <http://searchsecurity.techtarget.com/ateQuestionNResponse/0,289625,sid14_cid591441_tax292741,00.html>

    Jon
    Jon, Sep 7, 2006
    #5
  6. Saqib Ali

    Saqib Ali Guest

    Jon wrote:
    <http://searchsecurity.techtarget.com/ateQuestionNResponse/0,289625,sid14_cid591441_tax292741,00.html>

    Thank you Sir. This is what I was looking for. :)

    Another good explanation was given by Mark Wooding on sci.crypt:

    ----------------
    So, triple DES involves three keys, K1, K2, K3. Write
    single-DESencryption with a key K and plaintext block x as E(K, x), and
    decryption as D(K, x). Triple DES encryption is E(K3, D(K2, E(K1,
    x))).

    Suppose you're given a plaintext block x and corresponding ciphertext
    y. For each possible K3, compute D(K3, y), and store the result in a
    table. This takes about 2^56 work, and uses 2^56 blocks of memory.

    Now, for each pair K1, K2, compute D(K2, E(K1, x)). If this matches
    one of the values in the table, find the corresponding K3, and test the
    whole key against some other plaintext/ciphertext pairs. Continue
    until you're done. This step takes no extra memory and requires 2^112
    time.
    --------------------

    In Peace,
    Saqib Ali
    http://www.full-disc-encryption.com
    Saqib Ali, Sep 7, 2006
    #6
  7. Saqib Ali wrote:

    >> http://en.wikipedia.org/wiki/Triple_DES

    >
    > This has NO explanation of how the MITM works or why the effective key
    > lenght is reduced to 112 bits.


    It contains a link to the article about MITM.
    Sebastian Gottschalk, Sep 8, 2006
    #7
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