14 questions linked to/from GMAC vs HMAC in message forgery and bandwidth
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 quantify this security? My question ...
• 213
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What are the rules for using AES-GCM correctly?

When using AES-GCM I know that I am supposed to use a new initialization vector every time I call the AES-GCM algorithm with the same key. What are other rules must be followed to use AES-GCM ...
• 631
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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 ...
• 45.9k
640 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: ...
• 2,917
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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 ...
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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 ...
• 457
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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.
• 2,837
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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 \...
1 vote
948 views

Does the AES-GCM specification require the tag-size to be bound to the GCM context?

Does GCM spec (NIST SP 800-38D) require tag-size to be fixed for an instance of GCM algorithm (which usually fixes a specific GCM key)? Asked differently, is the following a legitimate adherence to ...
677 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 ...
• 141
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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 ...
368 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.)
1 vote