Understanding MAC with Block Cipher & Initialization Vector

  • Thread starter ranell
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In summary, the conversation discusses the concept of MAC (Message Authentication Code) and its relationship with the initialization vector (IV) and message blocks. It is explained that the MAC of a message depends on the IV and the XOR'ed result of each message block with the IV. Examples are given to illustrate this, with the conclusion that if the first block of two messages are different, then their MACs will also be different. The conversation ends with a request for a good source on the topic of MAC.
  • #1
ranell
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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!
:)
 
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  • #2
Message block x1 is XOR'ed with what? I can't understand it from your post.
 
  • #3
x1 xor with iv
iv = vector of initialization
sorry if it's not clear :s
thanks for your attention :)
 
  • #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.
 
  • #5
they have the same iv and the same key of mac :)
 
  • #6
because there are at least one same portion in the two message?
 
  • #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.
 
  • #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
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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1. What is MAC?

MAC stands for Message Authentication Code. It is a cryptographic technique used to ensure the integrity and authenticity of a message. It is often used to detect any unauthorized changes or alterations made to a message during transmission.

2. How does a block cipher work?

A block cipher is a type of encryption algorithm that works by dividing a message into fixed-sized blocks and encrypting each block separately. It uses a secret key to perform a series of mathematical operations, such as substitution and permutation, on each block to produce a ciphertext. The same key is used to decrypt the ciphertext and retrieve the original message.

3. What is an Initialization Vector (IV)?

An Initialization Vector (IV) is a random or predetermined value used in conjunction with a key to initialize a block cipher and add randomness to the encryption process. It is typically used to ensure that the same plaintext blocks do not produce the same ciphertext blocks, making it more difficult for an attacker to break the encryption.

4. How does MAC with Block Cipher & IV provide security?

MAC with Block Cipher & IV provides security by combining the strengths of both techniques. The block cipher provides confidentiality by encrypting the message, while the MAC provides integrity and authenticity by generating a unique code for each message that can only be verified by a recipient with the correct key. The IV adds an additional layer of security by preventing the same message from producing the same MAC and making it more difficult for an attacker to guess the key.

5. What are the limitations of MAC with Block Cipher & IV?

One limitation of MAC with Block Cipher & IV is that it does not provide protection against replay attacks. If an attacker intercepts and saves a valid MAC, they can resend it at a later time to impersonate the legitimate sender. This can be mitigated by using a timestamp or nonce in the message. Another limitation is that it does not protect against insider attacks, where an authorized user intentionally modifies the message. This can be addressed by implementing proper access controls and monitoring systems.

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