# Tag Info

3

This is vulnerable to a length extension attack. Given a valid nonce/MAC, the nonce can be extended to forge a new valid nonce/MAC value. This is because $m_4$ is appended to the end inside the outer hash. How this affects you will depend on how you validate your nonce. But in general, this is not a secure construction. There's probably more things wrong ...

2

Designing such a challenge is Impossible. If we assume that having a connection is equal to being able to exchange any piece of knowledge at any given time then the proof of impossibility of such challenge is as follows: Proof. First assume that there is such a challenge and Alice is capable of querying such a challenge to correctly determine whether the ...

2

Yes! Here is one such scheme. Let $s=\mathcal S(x)$ be what that the question's tiny device produces for 64-bit input $x$, and $\mathcal V(s,x)$ the public verification function for that, which outputs $1$ if $s$ matches $x$, $0$ otherwise. I'll assume this resists existential forgery under adaptive chosen message attack, and we want to extend it to ...

1

In real communications, you often don't reliably know each others' public keys. X.509 certificates provide a way to verify a public key (as the certificate is signed by another key, and that key can be in a certificate signed by a third key, and so on, until we reach a public key that the other person does trust). Certificates are unnecessary when both ...

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