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Triple-DES question

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  • 08-11-2005 3:40pm
    #1
    sed
    Closed Accounts Posts: 13 ✭


    hey. i have this triple des problem below. now i dont require the solution but the last question puzzles me.

    You are given the following information:


    A triple DES system has encrypted a message using a DESede key with no mode and no padding giving the following ciphertext,

    IkUO7dHZXN4JQYkRE6mMUs3Yg25U/eyE/u0z+7gae04pSDM2RtwRGQ==

    It is known that the key generator is weak with all keys following the pattern below

    11:11:11:11:11:11:00:00:XY:XY:XY:XY:XY:XY:00:00:WZ:WZ:WZ:WZ:WZ:WZ:00:00

    where XY and WZ are unknown bytes, for example XY could be 11110001.


    Your task is to find the key and decrypt the ciphertext.

    Also more than one key may decrypt, can you explain this??

    When is it possible to have more than one key able to decrypt if k1,k2,k3 are all different???


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    #2
    crashedmind
    Closed Accounts Posts: 59 ✭✭crashedmind


    sed wrote:
    hey. i have this triple des problem below. now i dont require the solution but the last question puzzles me.

    You are given the following information:


    A triple DES system has encrypted a message using a DESede key with no mode and no padding giving the following ciphertext,

    IkUO7dHZXN4JQYkRE6mMUs3Yg25U/eyE/u0z+7gae04pSDM2RtwRGQ==

    It is known that the key generator is weak with all keys following the pattern below

    11:11:11:11:11:11:00:00:XY:XY:XY:XY:XY:XY:00:00:WZ:WZ:WZ:WZ:WZ:WZ:00:00

    where XY and WZ are unknown bytes, for example XY could be 11110001.


    Your task is to find the key and decrypt the ciphertext.
    Bruteforce should be quick enough since there are only 64K keys (ignoring parity you have 256 possible value of XY and 256 values of WZ). This assumes you can recognise the decrypted message when you get it e.g. it's an ascii string.
    sed wrote:
    Also more than one key may decrypt, can you explain this??

    When is it possible to have more than one key able to decrypt if k1,k2,k3 are all different???

    Lost of answers depending on how these statements are interpreted:
    1. Any key can decrypt - but you need the right one to get the right plaintext :)
    2. Keys could be the same except for parity bits.
    3. Keys could be weak/semiweak.


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