Linked Questions

9
votes
2answers
3k views

How many trials does it take to break HMAC-MD5?

I know that you can find collision in MD5 with 2^64 trials using Birthday paradox. Now everyone is saying that HMAC-MD5 is significantly more secure. How can I ...
14
votes
2answers
600 views

Does GCM (or GHASH) only provide 64-bit security against forgeries?

In a recent comment a doubt was voiced about my answer, which claims GCM to requires $2^{128}$ for a successful forgery. The doubt was that the square root needs to be taken meaning the security would ...
10
votes
1answer
339 views

how does BearSSL's GCM modular reduction work?

BearSSL (in src/hash/ghash_ctmul.c) seems to be doing a modular reduction that I don't completely understand. Here's the code: ...
6
votes
2answers
635 views

Proof that MAC and hash composition is insecure

Let $F$ be a secure PRF and $H$ a universal hash function. How can I exhibit a pair $(F,H)$ whose composition $$S'((k_1, k_2), m) = F(k_2, H(k_1,m))$$ is an insecure MAC (or an insecure PRF, since a ...
7
votes
1answer
1k views

What are the constraints on using GCM with a tag size of 96 and 128 bits?

Say we want to use AES (or any other secure 128 bit block cipher) with GCM and a tag size of 96 or 128 bits. I'm assuming an AES key size of 128 bits and an IV size of 96 bits (the default). NIST SP ...
7
votes
2answers
554 views

OCB and GCM security

Is OCB as secure as GCM or CCM ? Since OCB design is quite different from GCM and CCM, I was wondering if the security properties of these latters are satisfied by OCB, as well.
4
votes
3answers
515 views

What is the function of the secret key “r” in Poly1305?

Poly1305-AES uses two per-connection keys $k$ (for AES) and $r$ (for Poly1305's compression function) and a per-message nonce n to execute. I've read the original paper on the implementation, and I ...
2
votes
2answers
286 views

Difference between strongly universal and $\delta$ univeral hash functions

Define a family of strongly universal hash functions as: $$\forall x_1, x_2 \in \{0,1\}^n, \forall y_1, y_2 \in \{0,1\}^m, ~ x_1\ne x_2: ~~ \Pr_{h\in H} [h(x_1) = y_1 ~\text{and}~ h(x_2) = y_2] \le \...
2
votes
1answer
189 views

How does Gilbert, MacWilliams, and Sloane's solution to “perfect integrity” work?

Gilbert, MacWilliams, and Sloane defined a solution to "perfect integrity" in Codes Which Detect Deception, unconditionally secure message authentication, not vulnerable to any cryptanalysis on its ...
1
vote
2answers
137 views

Is this a better GCM?

Using the keystream from the block cipher for both parts of the GMAC key (initialization and encrypting the auth tag) seems to produce a better mode of operation than GCM, as successful forgeries and ...
2
votes
1answer
111 views

Does an information-theoretically secure hash function exist?

Does an information theoretically secure hash function exist? (By exist I mean is discovered/invented and implemented, not whether it could exist.)
0
votes
1answer
66 views

What's the proper way of fitting hash digest to encryption scheme?

I would like to know what is the proper way of fitting the hash digest to the prime in which the encryption scheme operates. regardless if the bits of the hash digest is larger or smaller than the ...