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Mac with bloc cipher

  1. Dec 27, 2009 #1
    i have a part of my courses that i don't understand :(
    mac with bloc cipher and initially a vector of initialisation.

    mac k(m) doe's not change if iv xor x1 doe's not change
    mac k(m) doe's not change if x2 doe's not change
    mac k(m) doe's not change if xn doe's not change

    we have two exemple:

    if that mean that the two message have the same mac because we have the same portion x2=x'2?
    thanks for answering and sorry for my english!
  2. jcsd
  3. Dec 27, 2009 #2
    Message block x1 is XOR'ed with what? I can't understand it from your post.
  4. Dec 27, 2009 #3
    x1 xor with iv
    iv = vector of initialization
    sorry if it's not clear :s
    thanks for your attention :)
  5. Dec 27, 2009 #4
    Then the MAC of the two examples will depend of the initialization vector; from your description, they will depend of x1 XOR iv and x'1 XOR iv.
  6. Dec 27, 2009 #5
    they have the same iv and the same key of mac :)
  7. Dec 27, 2009 #6
    because there are at least one same portion in the two message?
  8. Dec 27, 2009 #7
    My apologies: there's a error in my previous post (I misread the first block): the MAC's will be different, because x1 = 00 and x'1 = 11, so when they are both XORed with Iv, this will give different results, resulting in distinct MAC's.
  9. Dec 27, 2009 #8
    thanks, but i have many contradiction in my courses,i have understanded that they will be the same.
    please, have you a link from a good document concerning mac?
    thank you very much :)
  10. Dec 27, 2009 #9
  11. Dec 28, 2009 #10
    thank you :)
    i have found this in the net in the attatchment.
    why x1i must be different of x1j
    please help me cannot understand this :((

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