# All Questions

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### Can a cryptographic hash be used as a cryptographic RNG?

Say I roll a six sided die 200 times (6^200 = ~2^517) and string all the results together into a long string of numbers and hash then result with SHA-512. Can I use that result for cryptographic ...
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### Is this transposition cipher information-theoretically secure for 1 message?

Given: $m = \{0,1\}^{n}$; a plaintext message of length $n$ encoded in binary $k = randomshuffle([0, 1, ..., 2n-1])$; A secret key consisting out of unique numbers between 0 and $2n-1$ in a true ...
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### RSA: Why must e be relative prime to phi(n) [duplicate]

I've got a question concerning fields used in RSA: Let's use the following symbols for my example: {p,q} = primes {e,n} = public key {d,n] = private key I learnt that an inverse element exists if a ...
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### How to securely map an element from an smaller domain to the other element in a large domain

Problem: I have a small sized domain, say s-bit. It's clear that the probability for an adversary to guess an element is $\frac{1}{2^s}$. I need to make the probability negligible. However, I need to ...
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### Has human-generated entropy ever been a real problem?

I know the topic of human brains generating random numbers has been discussed here and in Cognitive Sciences before, but I am interested in a particular part of it: In an introductory book to ...
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‎ ‎‎please help me what is number of invertible matrix $‎m*m$‎‎‎ on Group $\mathbb{z}_n$ ?‎‎‎, assuming we know‎ this number in $\mathbb{Z}_p \quad$ (‎p is prime‎‎) is $‎(p^{n}-1)(p^{n}-p) \... 1answer 78 views ### Which one these alternatives using authentication and encryption will solve this multiple-user database problem? Which one these alternatives using authentication and encryption will solve this multiple-user database problem ? I'm trying to understand the field of cryptography, so I've started reading a ... 2answers 2k views ### Why does second pre-image resistance imply pre-image resistance I am studying hash functions. I can understand why collision resistance implies second preimage resistance, but I don't get why second preimage resistance should imply first preimage resistance. ... 2answers 3k views ### Multiple private keys for single public key I am currently working in security in mobile ad-hoc networks. I have several clusters, and I want to send some data encrypted with its public key, from the cluster head to the cluster members. I ... 1answer 41 views ### What is the signifcance of inside threat in “How to Construct Multicast Cryptosystems … Secure Against CCA2” In How to Construct Multicast Cryptosystems Provably Secure Against Adaptive Chosen Ciphertext Attack, the paper states under preliminaries: We consider the scenario where a single party, called ... 1answer 83 views ### Swap$x$and$k$in GGM construction I'm studying the Goldreich-Goldwasser-Micali construction from pseudo-random generators to pseudo-random functions. In this specific construction, assume$G:\{0,1\}^n\rightarrow \{0,1\}^{2n}$is a PRG,... 1answer 163 views ### Linear Recurrence, keystreams, and periods I am trying to understand the following problem: Consider the following linear recurrence over$Z_2$of degree four:$z_{i+4} = (z_{i+3} + z_{i+2} + z_{i+1} + z_{i}) \bmod 2$i >= 0. ... 0answers 51 views ### Generating interactive, secure multiple ECC key pairs deterministically In Elliptic Curve Cryptography (ECC) assuming user A has a private public key pair of$S$,$P$accordingly with generator point$G$which as we know is: $$P=S*G$$ Assuming that user A wants to ... 1answer 66 views ### How are onetimepads distributed? OTP encryption is unbreakable as it seems. I have read some stuff (wiki, blogpost, etc.) but i would not find any information how they are used. If i wanted to create a messenger service, how would i ... 0answers 76 views ### Is the reverse of the “discrete logarithm problem” equally dificult? [duplicate] It is not easy to understand why this becomes a hard problem. The discrete logarithm problem as defined here: “any integer k that solves$b^k = \{g\mod{n}\}$is termed a discrete logarithm” i.e.: ... 1answer 332 views ### 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: (... 1answer 106 views ### How to generate the APA SBox? I am trying to generate the S-Box provided in the Affine Power Affine paper (PDF) but I'm not sure I understand it fully. The essence of the paper is this: ... 1answer 60 views ### Deterministic outputs based on non-connected inputs I am looking for a one way function that can generate outputs deterministically. However, revealing any of the inputs dont allow the person to generate other inputs. E.g: lets say we have a list of ... 1answer 275 views ### Multiple AES/ECB encryptions of different data with same key all end with same pattern We have used "AES/ECB/PKCS5Padding" with a 256 bit key. We encrypt different streams of data (blockwise) with the same key. Each encrypted stream ends with same pattern. What is the reason for this? 0answers 57 views ### Determine encryption method with input and output A website stores username and password in a session cookie. In the cookie the password is "encrypted" or modified in some way. Given that the "encrypted" version is always the same, I do not believe ... 1answer 78 views ### Key distribution and computation for homomorphic encryption How can a system where the party performing a computation also possess the private key and still not know the answer of computation be designed ? Also the other party who does not have the private key ... 1answer 80 views ### SSL certificate / SSL configuration - FIPS-197 compliance What does it mean when we talk about FIPS-197 compliance in the context of SSL certificates, their deployment and usage ? I am almost certain that the FIPS-197 compliance in the context of SSL ... 2answers 657 views ### How does a certificate authority issue a digital certificate? I am new to cryptography and I want to know the details of how a Certificate Authority issues a digital certificate. From what I know (please correct me if I'm wrong at any parts of my explanation): ... 2answers 13k views ### How is XOR used for encryption? I am a programmer, so when I hear XOR, I think about the bitwise operator (e.g. 0110 ^ 1110 = 1000). The mention of "XOR" comes ... 2answers 413 views ### Why isn't CTR mode (counter mode) used more often? For the CTR mode, the design is good for parallelization, yes, it seems the benchmark of the program downloaded from crypto++ proves that on an Intel I7 CPU. My question is that as most of CPU on ... 1answer 121 views ### Verification of Pinocchio (verifiable computation) I am reading the Pinocchio paper. The calculated result$y$is a part of the verification input, but it seems to me, the verification procedure does not utilize the result$y$. Can anyone can help me ... 1answer 151 views ### Beginner question about secure communication with one user using DES I am currently learning about encryption and I was asked a question: If a customer only had a DES system how could I secure messages as DES is not secure. I was researching this and I came across a ... 1answer 278 views ### Super-simple encryption of short strings I want to anonymise the initials of experiment participants by giving each a pseudonymic set of initials, by encrypting their true initials. Their initials are only two or three letters in length. ... 0answers 104 views ### side channel attacks against TDES (compared to AES) We all know AES has some pretty strong side channel attacks, some where (with a couple of hundred know plain- and ciphertext combinations) a key can be derived in a matter of minutes. Others may take ... 1answer 232 views ### Elliptic curve cryptography G*G I understand how ECC works for the point multiplications and stuff. All we normally do is to multiply a scalar number (lets say d as a private key) with the base point generator ... 1answer 148 views ### Known plaintext attack on 3 round SIMON block cipher I am attempting to write a program to perform a linear approximation attack on a reduced round version of the SIMON block cipher, but I am stuck on how to actually apply the linear approximation to ... 0answers 52 views ### Cache Attacks on Cloud Infrastructures I read something about cache attacks like this: Enter KSM (Kernel SamePage Merging). KSM enables the kernel to examine two or more already running programs and compare their memory. If any memory ... 1answer 2k views ### How does ECDH arrive on a shared secret? I read a brilliant, three part article on Elliptic Curve cryptography (one, two, three). It was able to explain Elliptic Curves to me in a way that didn't require a math degree to understand. The ... 2answers 367 views ### Is SSL getting faster because it's getting less secure? There has been some discussion about it being more practical to use SSL due to advances in hardware. From my understanding, stronger public-key encryption means that both encrypting/decrypting and ... 1answer 219 views ### what are the NTRU keysize and application in industry ? I'm a student currently working on a research project on the NTRU cryptosystem, I've read some papers about this cryptosystem, and in those papers they are talking about keysize, is it the size of the ... 1answer 61 views ### Why rogued certificate from hash collision is harmful I cannot understand why rogued certificate is thought harmful though It can be used to impersonate some website. The attacker can craft a rogued certificate with different identities and public key ... 1answer 57 views ### Different keys for clients but 1 key on server Is it possible to have different keys on alot of (embedded) devices and just have 1 key on a server (to witch all these devices connect)? The main point here being when 1 device is compromised this ... 1answer 146 views ### What is SHA-256 in Conjunctive Normal Form? What is the conjunctive normal form (CNF) representation of$\text{SHA-256}(m)=h$, where$m$is of fixed (or bounded) size and$h$is 256 bit? 1answer 82 views ### Is there any patent free EC point compression available? If not, should I GPL/LGPL my code? I'm currently using the BouncyCastle implementation in uncompressed mode, and considering releasing an optimized version of EC code that uses point compression. It appears the only thing limiting me ... 2answers 92 views ### Integrity compromised - HASH modified As we are all aware that HASHing is user to verify integrity in cryptography. I have a query about hashing. Suppose a man in the middle attacker manipulates the data and creates new HASH (by running ... 2answers 3k views ### phi(P*Q) = (P-1) * (Q-1) I was trying to understand RSA when I encountered the Euler Function. I do understand this:$\phi(P)$, where$P$is a prime is$P-1$. However it seems that for a number$N$such at$N=P\cdot Q$where ... 2answers 207 views ### Are there drawbacks to encrypting data locally before being encrypted in cloud backup (multiple encryption)? I am planning on backing up my hard drive using a cloud backup service but have become skeptical of the industry based on incidents like these: http://dbmsmusings.blogspot.com/2011/02/why-i-no-longer-... 0answers 122 views ### Under what conditions did a Bletchley bombe stop? I am trying to understand the conditions necessary for one of the Bletchley Park bombes to stop. Let me give an example. I have been experimenting with Enigma machine and bombe simulators to try to ... 1answer 80 views ### DHKE choice of private keys In some literature it is written that the private key should be chosen random from {2,3, ..., p-1} and in some it says {2,3, ..., p-2} I guess p-1 is a weak private key, but why is that so? The ... 0answers 118 views ### Rabin cryptosystem with 1 mod 4 We have p and q which are distinct primes congruent to 1 mod 4. Then we have n = p*q. Do you know any algorith for calculating square roots for the decryption in this case? I'm asking for some ... 2answers 266 views ### correct usage of keys and nonce I've being studying up on AES, GCM, CBC, HMAC and a lot of other primitives and am somewhat ( a little bit, perhaps) familiar with them however I am still a bit weary on the use of the keys and nonce. ... 2answers 73 views ### Length of prime number used in Pedersen Commitment I am writing a program using a Pedersen commitment scheme and all I'm missing is an appropriate length for my prime$p$. I have heard that a length of$2^{80}$is ok, is that correct? 1answer 106 views ### Plaintext block chaining, bad idea why? Ahead of the question, little warning. Question is not about what are the better options. Question is why it is bad idea? Ok, suppose we have plaintext$p$, which divided into blocks:$p_1,p_2,p_3,......
Given two 56-bit keys, $k_1$ and $k_2$, why does $E_{k_1}(E_{k_2}(M))$ only give 57 bits of security? So basically I'm unsure why it only gives 57 bits of security; I understand that one key will ...
We assume that there is at least one PRG .Now prove there is a PRG like $G:\{0,1\}^{n} \rightarrow \{0,1\}^{l(n)}$ such that it is not necessarily one-to-one.