13 questions linked to/from Why does HMAC use two different keys?
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### Is $H(k \operatorname\| H(k \operatorname\| m))$​ with double length $k$ like NMAC? [duplicate]

I know that NMAC is mathematically proved with two different keys with length $L$. But, if I choose to implement a MAC this way: $$H(k \operatorname\| H(k \operatorname\| msg))$$ with a key $k$ of ...
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### Cycles in SHA-256

Let's say I start with a particular 256 bit value. Call this $v$. I then hash that value, and get another 256 bit value. Call this $\text{SHA256}(v)$. I take this value and get another 256 bit value. ...
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### Using the same secret key for encryption and authentication in a Encrypt-then-MAC scheme

Is it a weakness to use a single shared secret for protecting messages using a Encrypt-then-MAC scheme? Assuming a system is using AES-256-CBC and a SHA1-HMAC and the same secret key for both ...
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### "Weaknesses" in SHA-256d?

According to this answer, "SHA-256d" was proposed in one of the Ferguson/Schneier books like so: SHA-256d(x) = SHA-256(SHA-256(x)) Apparently, the motivation for ...
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 ...
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### Keys in HMAC and NMAC

I'm quite new to this topic and have several questions concerning HMAC and NMAC: Why does NMAC need two keys? How can it be attacked if we just used some sort of initialization vector instead of the ...
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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 ...
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### Is there any good attack model of HMAC?

Are there any good attack models of HMAC? Like how it can be attacked? If there are any possible attacks of HMAC I would be happy to know about it.
1 vote
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### is MAC double-hashing enough to prevent length extension attacks?

I know that $$mac=\operatorname{SHA1}(secret\mathbin\|message)$$ is prone to length extension attacks, but what about: $$mac=\operatorname{SHA1}(\operatorname{SHA1}(secret\mathbin\|message))?$$ In ...
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### Adding parameters to sponge's capacity

Is it safe to XOR parameters like domain, length of the message or block counter into sponge's capacity or that gives attacker control over capacity? For example NORX XORs domain into capacity. Does ...
162 views

### Under which conditions would it be OK to use “H(pass||data)”?

I understand that the main motivation for HMAC is extension attacks. If data always has content-length either implicit in the app or even explicitly put into the hash, I get the impression that HMAC ...
1 vote