# Tagged Questions

A pseudo-random function is an efficiently-computable function which emulates a random oracle.

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### How to prove a function is pseudorandom function?

I am currently enrolled in a cryptography course, which uses the book by Katz and Lindell. I am using a function having $F_{k}$ = $A_{k}(B_{k}(r,m),r)$, the random number is generated using the ...
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### Proving the existence of a pseudorandom function

I've been reading the Introduction to Modern Cryptography book by Katz and Lindell as part of my own learning and have come across this exercise which I am not sure how to approach. The exercise is: (...
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### Is F' a pseudorandom function when F is composed with G, a pseudorandom generator?

If $F$ is a pseudorandom function, is $F'$ also a pseudorandom function in the following: $$F'_k(x)=F_k(G(x)) \space \space \text,$$ where $G$ is $a$ pseudorandom generator? Also, does the other ...
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### pseudo-random-function

Please, can you help me about this question... Design a blockcipher E : {0, 1}k Ć {0, 1}128 ā {0, 1}128 that is secure (up to a large number of queries) against non-adaptive adversaries, but is ...
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### Best differential characteristic for this PRF

Consider the following construction of a PRF $F: \{0,1\}^{128} \times \{0,1\}^{64} \to \{0,1\}^{64}$ on 64-bit messages and 128-bit keys. Define $f:\{0,1\}^3 \to \{0,1\}$ by f(c,x,y) = \begin{...
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### How to prove that a function F isn't a pseudo random function

Let $F$ be a length-preserving pseudorandom function. For the following constructions of a keyed function $F' : \{0, 1\}^n \times \{0, 1\}^{nā1} \to \{0, 1\}^{2n}$, state whether $F'$ is a ...
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### Pollard's Rho - Restricting the random function to the exponents

Pollard's Rho is usually constructed using a function $f:G \rightarrow G$ which behaves 'random enough' in order to detect a collision with Floyd's cycle detection trick. It is easy enough to observe, ...
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### Implementing a pseudo random function in practice

Can anyone point me to a C++ crypto library with an implementation of a pseudo random function (PRF)? I don't have much background in crypto theory, but I am in the middle of a graduate course ...
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### TLS 1.2 PRF Cipher Suite Specification

I am upgrading our assembler SSL/TLS implementation to support TLS 1.2 and am reviewing RFC5246 specification for TLS 1.2. It states that the PRF is now part of the cipher suite specification but ...
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### NIST SP800-108 KDF modes comparison

According to NIST SP800-108, there are three modes that can build KDF from PRF: counter mode, feedback mode and double-pipeline iteration mode. Assume that the same PRF and input distribution are used,...
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### Proof that this is not a secure pseudorandom function

$p$ is a large prime number. Consider the following function $F:\mathbb Z^*_p \times \mathbb D\rightarrow\mathbb Z^*_p$ where $\mathbb D=2,....,p-1$. $F_k(x)=x^k \bmod p$ Proof that it's not ...
Let $F_1$ and $F_2$ be two random functions taking as input $N$ bits and returning $N$ bits. Let $C_1$ and $C_2$ two constants of $N$ bits. Is there a random function $P$ taking as input $2N$ bits and ...
Is it possible that we take a $PRG$ $G(k)$ of stretch $n\cdot2^n$, and read its output as the table of a $PRF$ $F(K)$ with input and output size of $n$? Intuitively it sounds possible however I read ...