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Mac with bloc cipher

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

    "m=x1||x2||....xn
    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:
    m=0011
    m=x1||x2
    x1=00
    x2=11
    and

    m'=1111
    m'=x'1||x'2
    x'1=11
    x'2=11
    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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