1
vote
1answer
188 views

randomly generated key reused with XOR

If I have a long randomly generated key (k), XORed with a message (m), but since the message is longer than the key, there is a chance of reusing the keystream with the same message, but the key is ...
0
votes
2answers
204 views

In Elgamal, the generator g is always quadratic non-residue modulo p where p is a safe prime and inverse of g can be also generator?

In Elgamal, the generator $g$ is always quadratic non-residue modulo $p$, where $p$ is a safe prime and the inverse of $g$ can also be generator? Can I prove it? I can't come up with it at all.
1
vote
0answers
178 views

Factoring large numbers

I am trying to factor few integers that are each between 115 and 135 digits long. I have just, little over a month ago, began my study of Cryptography. I was wondering if anyone knew of any efficient ...
1
vote
3answers
910 views

How to use HMAC-SHA1, HMAC-SHA2 in conjunction with AES

I understand how to compute HMAC-SHA1, HMAC-SHA2 etcetera. Please explain why/how is the resulting HMAC value is used in conjunction with AES for encryption/decryption and signing/verifying. In ...
1
vote
1answer
80 views

secret sharing of product of 2 numbers

Does evaluating the products of shares $a_jb_j$ give a secret sharing of $ab$? I think it's a no, but I am not sure on how to prove it.
0
votes
1answer
85 views

What is the difference between exhaustive search and factorization in relationship to determining a key?

It seems to me an exhaustive search would simply try to use all the possible bit combinations of a key, while factorization is some mathematical formula for determining the key? When discussing the ...
1
vote
1answer
102 views

Is there a developer's resource that ties encryption methods and strengths to the appropriate usage scenarios?

I'm not looking necessarily for implementations in any language (although it would help to list those resources as well), but what I am most interested in is a discussion of what advantages and ...
3
votes
2answers
1k views

Using a Hash as a secure PRNG

I was just looking at some NIST PRNG recommendations, specifically at Hash_DRBG. I read briefly through the algorithm, and even though it is not overly complex, it still seems unnecessary to me. I ...
7
votes
3answers
297 views

Is there an algorithm or hardware that can sign/verify natural time?

PGP/GPG can used to sign text, others use public key to verify them. So one could say, that these cryptographic algorithms deal with space. Are there any algorithms that can deal with time? E.g. I ...
1
vote
1answer
167 views

What are the security effects of reusing a public RSA key to encrypt large data by blocks? [duplicate]

I'm trying to figure out a way to do public key encryption on a very large piece of data. For this, I of course want one side to do the encrypting but is unable to decrypt the data, and the other ...
2
votes
1answer
105 views

Verify Messages to Embedded Device

I'm building an embedded device that I plan on distributing. Periodically the device will poll my server to check for updates and commands. I'd like the device to verify that any messages(JSON strings ...
2
votes
2answers
204 views

Alice's forgetful banking

Alice has a bank account number, but has forgotten which bank it is for. There are 4 banks, run by Bob, Carlos, David, and Eve. She could find out by going to all of the banks and asking if they have ...
7
votes
2answers
1k views

How will Cryptography be changed by Quantum Computing?

I realise this isn't a 'yes or no' question, and I apologise for asking something that could be seen as a discussion thread, but I had to ask. I'm currently doing an EPQ in CS (specifically how QC ...
2
votes
2answers
217 views

plaintext and its encrypted version known [duplicate]

I have a (long) text in two versions, encrypted and plaintext. I think it was encrypted using a substitution cipher method (I'm pretty sure, indeed). I'm not good in this matter, I know little about ...
2
votes
3answers
273 views

How can I accomplish Key Derivation in JavaScript?

I need to take a potentially weak user password and derive a strong key for AES encryption in JavaScript. How can I do this? I expect the hardest problem is generating a good random salt - perhaps an ...
3
votes
1answer
396 views

Are there any Javascript CSPRNGs? [closed]

I'm writing some software that requires the use of a CSPRNG with javascript for use in the browser and I'm wondering what libraries are available. Crypto-js doesn't seem to have a CSPRNG I can ...
3
votes
1answer
276 views

Serpent cipher : Osvik S-Boxes confusion and test vectors

I'm having hard time with the implementation of the S-Boxes by Osvik found in this paper: Speeding up Serpent. At the end of the paper, all the s-boxes are given and then, I just implement them. ...
0
votes
2answers
620 views

HMAC vs Encrypted Hash

What is best to use (for message integrity) between… AES(Data) + HMAC(Data) AES(Data + HMAC(Data)) AES(Data + Hash(Data)) …and why?
4
votes
1answer
121 views

Why does CBC-MAC need prefix-free inputs to be a good PRF?

In the FFX spec, there is a note about using CMAC as the round function. Security notes. The round function F is constructed in such a way that the set of inputs on which the CBC-MAC is ...
6
votes
4answers
2k views

What is the recommended replacement for MD5?

Since MD5 is broken for purposes of security, what hash should I be using now for secure applications?
0
votes
2answers
540 views

If I have the unencrypted text and the encrypted text, can I calculate the key?

If I have the unencrypted text and the encrypted text, can I calculate the key? It seems logical based on my limited knowledge of encryption that I should be able to... but it also seems like too ...
4
votes
1answer
98 views

Question about the definition of a perfect cipher

I need to prove that the following encryption scheme is a perfect cipher: Let $p$ be a prime. The secret key is a pair $(a,b)$ sampled uniformly at random from $\mathbb{Z}_p^* \times \mathbb{Z}_p$. ...
1
vote
2answers
97 views

Password as it relates to various encryption schemes

Suppose you have 2 drives you want to encrypt using different encryption algorithms, as an example: AES256 and Twofish. I am aware that using the same password for both could potentially weaken ...
1
vote
1answer
105 views

Public Key encryption /Assymetric encryption

For a start, I 've got a question regarding the Public key encryption. I know the public key encryption overcomes the Key distribution issue with the symmetric encryption and that in the case of ...
3
votes
1answer
86 views

How would you encrypt-then-MAC when using pen-and-paper and a Caesar cipher?

I'll probably get shot for asking this, but I've got some kids (aged 8-10) in my neighbourhood that I've been showing/teaching the simple pen-and-paper Caesar cipher and they're successfully playing ...
3
votes
1answer
154 views

Derive a public EC key from two public EC keys

Alice has two EC key pairs: $a_1$, $a_2$ are private keys (integers), $A_1$, $A_2$ are the corresponding public keys (points). Alice and Bob want to create a new public key $C$. Alice must prove that ...
0
votes
1answer
60 views

Hash of multiset of values, which lets me compute the hash of the union

Cryptographic hash functions normally take as input a bitstring. I am looking for a hash function that takes as input a finite multiset of values. In other words, given $S \subset \{0,1\}^*$, I want ...
4
votes
2answers
178 views

How to Distribute the Shares using Secret Sharing for arbitrary Monotone Access Structures

Consider 4 people, $A$,$B$,$C$,$D$, & a secret $s\in\{0,1\}^k$. Construct a scheme which enables the following subsets of people to retrieve the secret $\{A,B\}$, $\{A,C\}$, $\{B,C,D\}$. I know ...
3
votes
2answers
537 views

Attacking 2DES efficiently

Meet in the middle on 2DES uses $2^{56}$ memory. Given the fact that the attacker has only $2^{45}$ memory. How can the attacker adjust the attack so even with this memory limit, it will still be more ...
11
votes
1answer
1k views

Duration for attacking Two-Key Triple-DES Encryption using all RAM ever built?

I am considering attacks on Two-Key Triple-DES Encryption assuming $2^{32}$ known plaintext/ciphertext pairs (that's a mere 32 Giga Bytes of ciphertext) by the method devised by Paul C. van Oorschot ...
2
votes
0answers
80 views

How does use of HMAC affect hash function combiners?

In the comments of this question, Ricky Demer proposes a function $HMAX$ to combine two $HMAC$s based on different hash functions: $HMAX_{H0,H1}(⟨k_0,k_1⟩,m) = HMAC_{H0}(k_0,m) \oplus ...
5
votes
1answer
218 views

HMAC construction based on the combination of two hash functions

I would like an $\operatorname{HMAC}$ that's based on two different hash functions ($H_1$ and $H_2$), so that a break of the combined $\operatorname{HMAC}$ would imply a break of ...
4
votes
3answers
316 views

Endless PRNG from PRNG with specific stretch?

Every definition of pseudorandom generator that I've seen says it takes n-bit strings and gives poly(n)-bit strings. Is there a proof that if there is such a pseudorandom generator then there is a ...
3
votes
2answers
258 views

Definition of a CSPRNG

I am interested in what conditions are necessary and sufficient to define a cryptographically secure pseudo-random number generator (CSPRNG). Wikipedia lists two defining characteristics: It ...
2
votes
1answer
131 views

Feasibility of using a base 26 LFSR for cryptography by hand

I have been playing with base 26 LFSRs (i.e. using the alphabet) and noticed that the XOR operation for base 26 is just the tabula recta and so can be done very quickly. This made me wonder whether a ...
1
vote
0answers
112 views

How to develop a public key cryptosystem based on a hard problem?

Recently, I found a function that is performed on a sequence to return another sequence. All known algorithms for finding the input, given the output are of exponential complexity. I want to propose ...
1
vote
0answers
47 views

Can you help me understand this strange implication direction in security reduction for OAEP 3-round?

I'm reading the paper OAEP 3-round, where they introduce a 3-round version of the OAEP (which originally used only two rounds). However, in their security statement for this construction (Theorem 4 ...
1
vote
1answer
234 views

XOR secret sharing scheme

I was wondering how we can get a secret sharing of $a+b$ from the sharing of secrets $a$ and $b$ themselves by using an XOR secret sharing scheme, where $a_1,...,a_n$ and $b_1,...,b_n$ are the shares ...
3
votes
0answers
84 views

Why does the new encryption scheme proposed by authors stop an adversary from guessing the subspace of the secret key?

In this paper, the authors construct an encryption scheme that is supposed to be resilient to tampering and leaking (as opposed to just leaking). Specifically this scheme: If you look at the ...
1
vote
2answers
163 views

Can you get two unpredictable hashes from the same message with, e.g. SHA1?

I'd like to create a database of questions and answers so that an attacker with access to the database can't obtain either the questions or the answers. If H0 and ...
45
votes
7answers
4k views

Guarding against cryptanalytic breakthroughs: combining multiple hash functions

Assume I want to design a protocol (or data format or similar) including some cryptographic hash, and want it to be as future-proof as possible, i.e. I want to avoid that breakthroughs in cryptography ...
3
votes
1answer
202 views

How does the IV or initial counter increase internally for each block in AES CTR mode?

I am working on Intel AES-NI library and since the AES function is coded using low-level assembly instructions. I could not see how the IV or initial counter increases in CTR mode. In general, how ...
2
votes
2answers
560 views

Estimating bits of entropy

Best ways to estimate average bits of entropy in a byte stream. I know there are different statistical tests out there (NIST, Dieharder, etc), which all do different ways of analyzing entropy. What ...
1
vote
1answer
64 views

Why do authors say conflicting things regarding leak-free stages in their paper?

In this paper the last sentence of the first paragraph in the abstract states: We assume a (necessary) “leak-free” preprocessing stage. But then later on in section 1.4 they say Another ...
4
votes
2answers
361 views

What is the definition of “security beyond the birthday paradox”?

I'm reading a paper about MACs and I would like to be sure about the meaning of a security beyond the birthday paradox. Is there a definition?
2
votes
0answers
62 views

Any functioning system for interactive proof?

My problem is to outsource an array of data and ask the prover to sort the data. I am wonder if there is any working system out there that support interactive proof for the above computation? Or if ...
7
votes
5answers
2k views

Are there any simple and yet secure encryption algorithms?

Being very new to C++ and cryptography, I finally managed to implement a version of the Vinegere algorithm. I would like to try something a bit more complicated. I have looked at AES and DES and ...
1
vote
1answer
100 views

Are there any practical examples of breaking a system using linear attacks?

I've read some papers about linear cryptanalysis. But since I am going to give a lecture about it I am looking for an example of breaking a system using this attack. Is there such an example ...
2
votes
1answer
189 views

How vulnerable is RSA when using it to encode ~1000s of datasets with 500bytes each? How easy can the private key passphrase be hacked?

I know that this is not the best use case for RSA, but for a customer of mine I have to encode user data on the server in a way so that it can be decoded/downloaded by the customer through a "secure" ...
2
votes
1answer
207 views

Constructing secure key exchange protocol

I have $\Pi=(Gen,Enc,Dec)$ and let it be semantically secure public-key encryption scheme. Security parameter is $n$, then the message space of plaintext is always $\lbrace 0, 1 \rbrace^n$. By using ...

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