(or message authentication code), a short piece of information used to authenticate a message, and the algorithm to create and check such information, using a secret key.

learn more… | top users | synonyms

2
votes
1answer
196 views

Why is this MAC based on secure PRF with ordering and randomization insecure?

Is this MAC secure: To authenticate a message $m=m_1||\ldots||m_l$ where $m_i \in \{0,1\}^{n/2}$, choose $r \leftarrow \{0,1\}^n$ at random, compute $$t:=F_k(r)\oplus F_k(\langle 1\rangle || ...
3
votes
1answer
84 views

OMAC : Padding on empty string

I started to implement some MAC since last week with the specifications given here. I'm currently testing the OMAC (one-key CBC) with test vectors. In the OMAC specifications at page 4, they explain ...
0
votes
1answer
101 views

Why is appending the key to a mesage and then hashing that insecure if the hash isn't weakly collision resistant? [duplicate]

Suppose I have H(M|K) and that H is not weakly collision resistant. If I have a message mac pair (M,MAC), how is it possible to find another message mac pair (M2,MAC2)? My thinking for this problem is ...
6
votes
2answers
171 views

Why don't we use MACs to store passwords?

These days, the best practice for storing passwords is to use a scheme like scrypt or bcrypt. The goal of these schemes is to make dictionary attack inefficient for an attacker but it also slows down ...
5
votes
1answer
118 views

How would you encrypt-then-MAC when using pen-and-paper and a Caesar cipher?

I'll probably get shot for asking this, but I've got some kids (aged 8-10) in my neighbourhood that I've been showing/teaching the simple pen-and-paper Caesar cipher and they're successfully playing ...
2
votes
1answer
352 views

How is digital signature different from a message authentication code (MAC)?

I know the definitions of both but can't specifically tell how they are different and if one is better than the other. Please help.
3
votes
2answers
212 views

Is there a companion algorithm for OTP to ensure integrity and/or authentication?

One-Time Padding is (theoretically) perfect encryption algorithm. Let's assume that these problems are solved: Fast generation of independent and identically distributed random variables Perfect key ...
0
votes
2answers
212 views

What block cipher is used for CBC-MAC?

What block cipher is used for CBC-MAC? DES, AES, 3DES? Or it doesn't matter?
1
vote
3answers
166 views

Does MAC provide origin authentication? Why not just use symmetric encryption?

Let's assume Bob and Alice has a secret key for MAC. If MAC is constructed by the key and the message, that means that both Alice and Bob can construct the exact same message and MAC. So, Bob can ...
4
votes
1answer
162 views

Proof that MACing a hash of the message is also a secure MAC

I found a theorem that says: Let $MAC = (S,V)$ be a MAC for short messages over $(K,M,T)$. Let $H: M^{big} → M$. Define $MAC^{big} = (S^{big},V^{big})$ over $(K,M^{big},T)$ as: $S^{big}(k,m) = ...
3
votes
2answers
227 views

Does encrypting a hash of the message yield a secure MAC?

Is $mac = enc(hash(x))$ a secure MAC for hash = SHA256 and enc AES in CBC mode or ECB mode. Normally AES-CBC is malleable. But in this case I use a hash function for the one-way property. For ...
4
votes
2answers
371 views

What is the definition of “security beyond the birthday paradox”?

I'm reading a paper about MACs and I would like to be sure about the meaning of a security beyond the birthday paradox. Is there a definition?
2
votes
1answer
112 views

Is it possible to find a preimage for a reduced size hash?

Given a message $M$ and a 256 bit key $K$, perform HMAC($K$, $M$) with a 256 bit hash function resulting in: $a$ = ...