(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.
53
votes
5answers
5k views
Should we MAC-then-encrypt or encrypt-then-MAC?
Most of the time, when some data must be encrypted, it must also be protected with a MAC, because encryption protects only against passive attackers. There are some nifty encryption modes which ...
10
votes
2answers
837 views
Why is H(k||x) not a secure MAC construction?
If H(m) is a secure hash function, can't we implement a MAC using H(k||m)?
However, it seems the more widely used MACs, such as NMAC and HMAC (both originally defined in Keying hash functions for ...
8
votes
1answer
314 views
Is H(k||length||x) a secure MAC construction?
If $H$ is a typical secure hash function, then $(k,x) \mapsto H(k \mid\mid x)$ is not a secure MAC construction, because given a known plaintext $x_1$ and its MAC $m_1$, an attacker can extend $k ...
1
vote
3answers
149 views
Message authentication codes construction
I was reading the paper $[1]$ and came across the scheme that I show below. While I understand the scheme well, I don't understand why they prepend a 0 to the block containing $r$ and a 1 to all other ...
4
votes
2answers
222 views
Is the encryption of a hash a good MAC?
At university we were told that it is a bad idea to implement a MAC by simply concatenating a key with the data to sign and to run it through a hash function (e.g. $s = ...
3
votes
2answers
194 views
Why is h(m||k) insecure?
Here is the post that explains the failure for doing h(k||m) and I understand it.
But I don't understand how h(m||k) is subjected to collison attack, or birthday attack. Please explain?
10
votes
3answers
252 views
Purpose of outer key in HMAC
From what I know, the HMAC constructions has two strength:
It's resistant to length extensions
Since the key is consumed before the message, the attacker does not know the initial state, preventing ...
9
votes
4answers
423 views
Can you make a hash out of a stream cipher?
A comment on another question made me wonder about something:
Assume you're on a rather constrained platform — say, a low-end embedded device — with no built-in crypto capabilities, ...
6
votes
2answers
235 views
Attacks of the MAC construction $\mathcal{H}(m||k)$ for common hashes $\mathcal{H}$?
Consider a common practically-collision-resistant Merkle–Damgård hash function $\mathcal{H}$ (e.g. SHA-1, RIPEMD-160, SHA-256, SHA-512). We define a Message Authentication Code $\mathcal{C}$
$$(k,m) ...
5
votes
1answer
2k views
HMAC vs MAC functions
I've read definitions of MAC and HMAC, but can't say I've completely grasped the differences.
What are principle differences?
When to use one and when the other?(Typical Use Cases)
2
votes
1answer
58 views
Using UMAC with stream cypher
I understand that most stream ciphers, due to being applied with a simple XOR, are specially fragile against data tampering, and must be used with some MAC mechanism. So I am investigating the use of ...
1
vote
1answer
73 views
Non-cryptographic hash function as MAC for stream ciphers
I understand that for a stream cipher to be useful, there must be a way to verify that the message was not tampered with (bits were flipped by an attacker).
So, instead of using some cryptographic ...
1
vote
1answer
174 views
How do unkeyed hash functions (for MDCs) provide security?
Unkeyed hash functions are, by definition, hash functions computed without a key. SHA-1 is an example. MDCs (message digest codes) are a subclass of unkeyed hash functions. How are unkeyed hash ...
-3
votes
0answers
82 views
Why is the basic (fixed-length) CBC MAC construction not a secure MAC when it is used on variable-sized messages? [closed]
Why is the basic (fi xed-length) CBC MAC construction not a secure MAC when it is used on variable-sized messages?
