# Questions tagged [cryptography]

Questions about the construction and analysis of protocols and algorithms for secure computation and communication (including authentication, integrity, and privacy aspects).

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### Weaknesses of the alternative RSA-like algorithm

Disclaimer: it is a purely out-of-curiosity question. We discussed this implementation with a friend and could not find any breaches, but I have a gut feeling that there might be some. Consider a ...
• 942
1 vote
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### Applications of a SAT Solver Oracle for Determining the Uniqueness of Solutions

I am exploring two kinds of model $π_{π,π,k}$ and $S_{m,n,k}$ within the realm of satisfiability problems (SAT). Formal construction of $π_{π,π,k}$ To construct the $π_{π,π,k}$ model in ...
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### Can you unwind a cryptographic hash function's last round?

Given a cryptographic hash, $\text{hash}(A || B || C)$, and the last block added to the hash, $C$, can you determine $\text{hash}(A || B)$? In other words, can you roll back the last round of a ...
• 145
1 vote
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• 327
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### Not CPA secure for CBC mode if some code are changed that is if we replaced a function $\pi(k,x)=k\oplus x$

Consider the CBC mode where the block cipher is replaced by a function $\pi$ where $\pi(k,x)=k\oplus x$. Show that the mode is not CPA secure. I am not got how to proceed to solve this question. ...
1 vote
369 views

### What algorithm is prefered to do a x b mod P with big numbers (256 bits)

I'm trying to implement multiple precision arithmetic operations modulo P, with P < 2^256. More specifically, ...
• 113
1k views

### How are extremely large integers stored and implemented in programming languages?

We have data types like int and long etc. which can store just a few bytes of integers. But when we are implementing ...
• 139
67 views

### Factor $n$ in RSA if we know $Ο(n)$

If we know that $n = 1363$ and $Ο(n) = 1288$, how can we factor $n$?
• 23
1 vote
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### Is it possible to have a zero knowledge proof with a P Prover?

In the literature, when reading about zero knowledge proofs, the prover (prover/verifier) is always given an unlimited computational power or just capacity to solve NP. Is it necessary for the prover ...
• 21
135 views

### Zero knowledge proof - case of graph 3-coloring (not understanding the concept)

In zero knowledge, in the context of graph 3-coloring, I do not understand how the prover is actually showing that he solved the problem. The context: both know the graph (the prover and the verifier)...
• 21
341 views

### How to prove that existence of one-way functions implies Pβ NP?

Wikipedia: The existence of such one-way functions... would prove that the complexity classes P and NP are not equal. How is this proved?
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### Why do public key systems involve private keys?

I read up the definition of a public key cryptosystem. It mentions there is a public key and a private key. That's confusing. Why is it called a "public key" system if it involves a private ...
• 145
1k views

### What are the advantages of using PRNG over TRNG?

True random number generators use an unpredictable physical means to generate numbers, whereas pseudo-random numbers utilize mathematical formulas to produce a certain sequence of numbers that will ...
• 118
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### How 'Avalanche Effect' got its name?

I wonder how or why Avalanche Effect got its name. Avalanche Effect is a desirable property of cryptographic algorithms, wherein if an input is changed slightly (...
1 vote
74 views

### NSA Recommended Cryptography Algorithms changes from a single paper

I know that the NSA has recommended cryptography algorithms, such as their A/B suite algorithms and their Commercial National Security Algorithms Suite. I also know that they change their ...
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233 views

### Hardcore bit of a one way function

Let $b:\{0,1\}^*\mapsto \{0,1\}$, be a hardcore bit of a one way function $f$. I want to show that $\vert Pr_{x\in U_n}[b(x)=1]-Pr_{x\in U_n}[b(x)=0]\vert=n^{-\omega(1)}$. Intuitively, it looks ...
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• 163
1 vote