Just as with symmetric and public-key encryption, we can group attacks on hash functions and MACs into two categories: brute-force attacks and cryptanalysis.

**Security of Hash Functions and
Macs**

Just as
with symmetric and public-key encryption, we can group attacks on hash
functions and MACs into two categories: brute-force attacks and cryptanalysis.

**Brute-Force Attacks**

The
nature of brute-force attacks differs somewhat for hash functions and MACs.

**Hash Functions**

The
strength of a hash function against brute-force attacks depends solely on the
length of the hash code produced by the algorithm. Recall from our discussion
of hash functions that there are three desirable properties:

·
One-way: For any given code h, it is
computationally infeasible to find x such that H(x) = h.

·
Weak collision resistance: For any given block x,
it is computationally infeasible to find y x with H(y) = H(x).

·
Strong collision resistance: It is computationally
infeasible to find any pair (x, y) such that H(x) = H(y).

·
For a hash code of length n, the level of effort
required, as we have seen is proportional to the following:

**Message Authentication Codes**

A
brute-force attack on a MAC is a more difficult undertaking because it requires
known message-MAC pairs.. To attack a hash code, we can proceed in the
following way. Given a fixed message x with n-bit hash code h = H(x), a
brute-force method of finding a collision is to pick a random bit string y and
check if H(y) = H(x). The attacker can do this repeatedly off line. To proceed,
we need to state the desired security property of a MAC algorithm, which can be
expressed as follows:

Computation
resistance: Given one or more text-MAC pairs (x_{i}, C_{K}[x_{i}]),
it is computationally infeasible to compute any text-MAC pair (x, C_{K}(
x)) for any new input x ≠x_{i}.

In other
words, the attacker would like to come up with the valid MAC code for a given
message x. There are two lines of attack possible: Attack the key space and
attack the MAC value. We examine each of these in turn.

To summarize,
the level of effort for brute-force attack on a MAC algorithm can be expressed
as min(2^{k}, 2^{n}). The assessment of strength is similar to
that for symmetric encryption algorithms. It would appear reasonable to require
that the key length and MAC length satisfy a relationship such as min(k, n) ≥N,
where N is perhaps in the range of 128 bits.

**Cryptanalysis**

As with
encryption algorithms, cryptanalytic attacks on hash functions and MAC
algorithms seek to exploit some property of the algorithm to perform some
attack other than an exhaustive search.

Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail

Cryptography and Network Security : Security of Hash Functions and Macs |

**Related Topics **

Privacy Policy, Terms and Conditions, DMCA Policy and Compliant

Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.