41
votes
Accepted
HMAC vs ECDSA for JWT
The distinction is that ECDSA solves a problem that HMAC does not. If you need that problem solved, then you need to do ECDSA rather than HMAC; if you do not, then HMAC works just as well (and is a ...
- 139k
40
votes
Accepted
Key size for HMAC-SHA256
Short answer: 32 bytes of full-entropy key is enough.
Assuming full-entropy key (that is, each bit of key is chosen independently of the others by an equivalent of fair coin toss), the security of ...
- 132k
36
votes
Accepted
Why is HMAC-SHA1 still considered secure?
In the first section of this answer I'll assume that through better hardware or/and algorithmic improvements, it has become routinely feasible to exhibit a collision for SHA-1 by a method similar to ...
- 132k
24
votes
HMAC-SHA1 vs HMAC-SHA256
I would use HMAC-SHA256.
While poncho's answer that both are secure is reasonable, there are several reasons I would prefer to use SHA-256 as the hash:
Attacks only get better. SHA-1 collision ...
- 31.8k
24
votes
Accepted
Encrypt-then-MAC: Do I need to authenticate the IV?
In short: You must authenticate the IV.
Which particular attacks apply if you don't depends on the block cipher mode; I will give two common examples.
In CTR mode, an attacker who fiddles with the IV ...
- 11.9k
22
votes
What size should the HMAC key be with SHA-256?
The only rule for the key is that it should at least contain 256 bits of randomness. If the key is smaller you may not get the full security of HMAC-SHA-256. The full security of HMAC is basically ...
- 89.2k
20
votes
Why does HMAC use two different keys?
Alas, there is no simple satisfactory answer to this question. What I can offer is a very strong property that $m \mapsto H\bigl(k \mathbin\| H(k \mathbin\| m)\bigr)$ fails to achieve; a more ...
19
votes
Why is HMAC-SHA1 still considered secure?
When people say HMAC-MD5 or HMAC-SHA1 are still secure, they mean that they're still secure as PRF and MAC.
The key assumption here is that the key is unknown to the attacker.
$$\mathrm{HMAC} = \...
- 24.6k
19
votes
HMAC vs ECDSA for JWT
To add to poncho's answer (since they beat me to it!), there are several advantages to choosing HMAC over ECDSA (or RSA) if you can get away with it:
Insanely better performance: signing and ...
- 2,744
17
votes
Why not authenticate full-disk encryption?
Authentication and probabilistic encryption are two desirable features which each take up a small amount of extra space. And you are absolutely right that the percentage of space consumed is of no ...
- 1,377
15
votes
Could an HMAC be used as a hash?
HMAC remains unbroken with MD5 and SHA1 because it has a secret key that the attacker doesn't know. Therefore, the attacker cannot carry out huge computations on itself (as is required for finding ...
- 27.3k
15
votes
Accepted
HMAC-SHA256 vs HMAC-SHA512 for JWT API authentication
Both algorithms provide plenty of security, near the output size of the hash. So even though HMAC-512 will be stronger, the difference is inconsequential. If this ever breaks it is because the ...
- 89.2k
14
votes
Accepted
How is HMAC(message,key) more secure than Hash(key1+message+key2)
The construction you are proposing is called the "envelope" or "sandwich" MAC, it predates HMAC, and it is in fact secure—provided the key and message are appropriately padded. That is,
$$
\text{...
- 12.1k
14
votes
Why not authenticate full-disk encryption?
So, are there reasons for not using authentication that I'm missing?
I believe that the real reason is not actually space, but time.
As you said, storing the tags would not require that much space. ...
- 139k
14
votes
What's the difference between PBKDF2 and HMAC-SHA256 in security?
HMAC is still very efficient. It's used in PBKDF2 not for the lower efficiency (that's handled by iterating it many times) because of the fact that it takes two inputs. That lets the password and ...
- 5,496
13
votes
Authenticating a message with HMAC vs AES-CBC
Given some string s I want to [integrity protect], are following methods are equivalent to produce message with signature, assuming it does not matter whether ...
- 139k
13
votes
Accepted
Why not authenticate full-disk encryption?
With 4096-byte sectors, space is a complete non-issue, less than 1 %
Problem 1: 10GB per TB is not a "complete non-issue" for many people.
Problem 2:
If the checksums are inside of their data ...
- 1,187
13
votes
Accepted
The difference between MACs vs. HMACs vs. PRFs
A PRF or pseudorandom function family is a family of functions $F_k\colon \{0,1\}^n \to \{0,1\}^m$ such that if $k$ is uniformly distributed, then $F_k$ appears to be uniformly distributed among all ...
13
votes
Accepted
How many trials does it take to break HMAC-MD5?
You can find a collision in MD5 at much lower cost than $2^{64}$ evaluations of MD5. You could do the same for HMAC-MD5, if you knew the key, which renders it unfit for unusual applications such as ...
12
votes
Accepted
Keys in HMAC and NMAC
NMAC is really just an "education tool" on the way to HMAC and I don't think anyone intended it to be used. The two keys are needed since the first and second hashes have different purposes. The first ...
- 27.3k
12
votes
Accepted
Is there a null HMAC?
It is well defined.
The hash function has no impact on whether HMAC is defined for a null string text argument. As long as HMAC is defined for a particular hash function, the resulting HMAC of a null ...
- 10.3k
12
votes
Accepted
How many codes can I safely generate using the same HMAC key?
I don't think there is any official limitations when it comes from standardization bodies such as NIST. However, there do seem to be some papers such as New Generic Attacks Against Hash-based MACs.
...
- 89.2k
11
votes
Is HMAC needed for a SHA-3 based MAC?
KMAC has now been specified in NIST SP 800-185, chapter 4. It is based on cSHAKE128 and cSHAKE256, which both are based on the same Keccak sponge that SHA-3 is. It doesn't use any additional methods ...
- 89.2k
11
votes
Accepted
Is HMAC-MD5 still secure for commitment or other common uses?
No, message commitment by disclosing its HMAC-MD5 with a key later revealed is no longer any secure, because of the ease with which MD5 collisions can now be found. There's however no compelling ...
- 132k
11
votes
Accepted
Why is Fernet only AES-128-CBC?
This seems to me to be less secure...
Do you have a plausible adversary that can break AES-128? AES-128 is believed (to the best of knowledge) to require $O(2^{...
- 139k
11
votes
Accepted
Which MAC to choose?
As a MAC, HMAC is fine, with any of the SHA-* functions. It's even fine with MD5, even though MD5, as a hash function, is quite broken. One interesting characteristic of HMAC is that it requires no ...
- 85.9k
11
votes
Accepted
Would a HMAC digest make sense in an RSA / ECDSA signature?
If we view HMAC as a message authentication code or a PRF, this doesn't quite make sense: the security property for a MAC or a PRF assumes that the forger doesn't know the key, but you've given them ...
10
votes
HMAC with public-private key
Consider the following system involving a message authentication code like HMAC-SHA256:
Alice generates a key and shares it with Bob, and Bob alone.
Alice authenticates a message with the key.
Bob ...
10
votes
Accepted
Choosing between simple Hash and HKDF to derive the second key used for MAC
The HKDF paper answers this question at length. Section 8 in particular seems to be the most relevant.
But perhaps you may find that the explanations are very technical. To help you out a little ...
- 14.3k
10
votes
Accepted
Signing dynamic data on an embedded system
Signature algorithms of the ECDSA family are amenable to precomputations. Indeed, when you want to sign message m with ECDSA, the process goes thus:
We work in a curve (or a subgroup of a curve) of ...
- 85.9k
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