# Mac with bloc cipher

1. Dec 27, 2009

### 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!
:)

2. Dec 27, 2009

### JSuarez

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

3. Dec 27, 2009

### ranell

x1 xor with iv
iv = vector of initialization
sorry if it's not clear :s

4. Dec 27, 2009

### 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.

5. Dec 27, 2009

### ranell

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

6. Dec 27, 2009

### ranell

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

7. Dec 27, 2009

### 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.

8. Dec 27, 2009

### ranell

thanks, but i have many contradiction in my courses,i have understanded that they will be the same.
thank you very much :)

9. Dec 27, 2009

### JSuarez

10. Dec 28, 2009

### ranell

thank you :)
i have found this in the net in the attatchment.
why x1i must be different of x1j

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