Understanding MAC with Block Cipher & Initialization Vector

[ranell]

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!
:)

 

[JSuarez]

Message block x1 is XOR'ed with what? I can't understand it from your post.

 

[ranell]

x1 xor with iv
iv = vector of initialization
sorry if it's not clear :s
thanks for your attention :)

 

[JSuarez]

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.

 

[ranell]

they have the same iv and the same key of mac :)

 

[ranell]

because there are at least one same portion in the two message?

 

[JSuarez]

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.

 

[ranell]

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 :)

 

[JSuarez]

The better source would Stinson's book or one similar. Online here are fewer good sources; try:

http://www.cs.cornell.edu/courses/cs513/2005fa/NL20.hashing.html

 

[ranell]

thank you :)
i have found this in the net in the attatchment.
why x_1ⁱ must be different of x_1^j
please help me cannot understand this :((