-1
votes
2answers
40 views

How to make sure the pre-agreed information safe for DH-Key Exchange

The definition of DH key exchange was given as the method let two authenticate each other & exchange the crypto key over an insecure channel. DH-key-exchange was innovated to defence ...
2
votes
0answers
179 views

implementing a cryptographic hash with a block cipher [duplicate]

How can a cryptographic hash function be implemented/created using the block cipher? I was wondering if we can use AES in cipher block chaining mode to implement it.
2
votes
2answers
202 views

PRP, PRF and modular arithmetic

Are there any arithmetic or mathematical functions that can be used as PRPs or PRFs ? Since, Conventional block ciphers like AES are that are proven to be good PRPs are not based on mathematics but ...
-1
votes
1answer
65 views

How do they implement MAC in TLS?

I recently read questions about "MAC then Encrypt/Encrypt then MAC" which made me curious about how they actually use the algorithm. MAC only calculated once either case (according to what I read). ...
6
votes
4answers
2k views

Simply put, what does perfect secrecy means?

I would like to ask for a clear (but maybe not so deep) explanation of what the term "perfect secrecy" means. As far as I have researched and understood, it has to do with probabilities of assuming ...
2
votes
2answers
135 views

Perfect Forward Secrecy in TLS

I read that TLS does PFS using Diffie Hellman. However, DH can be used even without certificates - so how is DHE-RSA better than plain DHE? Is DHE a insecure algorithm, that DHE-RSA is needed?
3
votes
1answer
152 views

Given a message and signature, find a public key that makes the signature valid (ECDSA)

This question is a variant on Given a message and signature, find a public key that makes the signature valid, which discusses the analogous question for RSA. It was suggested to me by this post over ...
6
votes
1answer
238 views

How secure is the knapsack?

Can the knapsack be used in cryptography in a secure sense (described below)? Knapsack problem: Given some number $X$ and a set $W$ of weights $w_1, w_2, ... w_n$, find a subset $S$ of $W$ (if it ...
1
vote
0answers
67 views

How we can said a crypto system have perfect secrecy?

For example: I have 3 plaintexts ($a$, $b$, $c$) and 4 keys ($K_1$, $K_2$, $K_3$, $K_4$), making a table map to the cipher text, key as row and plaintext as column $\begin{matrix} \ \ \ \ \ \ \ \ \ \ ...
4
votes
1answer
158 views

Why are collision attacks important when talking about MAC schemes?

I'm reading Ferguson, Schneier, and Kouno's Cryptography Engineering, and it has a section on MACs. It discusses different types of MACs, but it seems to claim that to acheive 128-bit level security, ...
7
votes
2answers
231 views

RSA: Letting $p$ and $q$ have different bit-size

I am aware that there are concerns if $p$ and $q$ are close i.e. $\Delta=|p-q|$ can't be too small. But I would like to know if there are any known attacks for cases where $p$ and $q$ take on ...
1
vote
1answer
104 views

Disadvantages of one-way accumulators?

One-way accumulators are built upon a (quasi)-commutative one-way function. With quasi-commutativity, I refer to the following property: For $f : X \times Y \to X$, it is true that $f(f(x, y_1), y_2) ...
4
votes
1answer
184 views

Is there an algorithm to check if an elliptic curve is secure?

As I understand it elliptic curves are of the form $y^2 = x^3 + ax + b$ Where $a$ and $b$ are the curve parameters. However not all parameters will give a curve suitable for crypto purposes. Is there ...
2
votes
1answer
150 views

Generate a new public/private key pair for each new message?

When using PGP type public encrypted communication, would it be a good idea to generate a new public for each message and attach that in the new public key in the reply to back to sender mail body? ...
10
votes
2answers
3k views

What is difference between PRG, PRF, and PRP

Until what I have gotten is: A PRG is generator is a part of PRF that produces pseudo-random values for the function. PRF is semantically secure and has no worries of being invertible. Fine, then ...
1
vote
1answer
50 views

Showing that security of a elgamal invariant is insecure

Original Elgamal signature is defined $S(m, \alpha) = (r, s)$, where $$r = g^k \bmod p$$ $$s = (m – r*α)k^{-1} \bmod (p – 1)$$ more information on Elgamal signature can be found here. Variant ...
1
vote
1answer
124 views

Difference between plain AES CTR and “CCM-style” CTR

I am trying to adapt the following function from SJCL, which does "AES-CCM" style CTR encryption, to do "pure CTR" encryption. I am playing around with SIV and would rather use this well-reviewed ...
-1
votes
1answer
165 views

cryptography and use of python programming language [closed]

why is it recommended to write cryptography programs ,for example crypt-analysis projects with python programming language? what is the advantages of python? why not use c or any other language?
-1
votes
1answer
71 views

How to keep phi(n) secret in RSA?

As we know, RSA cryptosystem have both private key(a,p,q) and public key(b,n), by chinese remainder theorem and fermat's little theorem, we know that the importance of keep p and q secret, and from ...
6
votes
1answer
192 views

Why does this square root algorithm work?

I've been doing some elliptic curve cryptography, and a library I'm using has this slightly bizarre algorithm for computing modular square roots: Let $x$ be some quadratic residue modulo $p$ for some ...
1
vote
0answers
76 views

How we transfer 128bits cipher to a 4x4 matrix for AES input?

Why the input cipher for AES encryption, can write in 4x4 matrix with 8 bits entires, and can said is 128bits? if each cell is 8bits in binary, how we said this matrix is represent 128bits?
2
votes
0answers
79 views

Threshold signatures vs. certificate authority+voting verification

Perhaps a silly question, but I am wondering what the advantages are of threshold signatures. Let's consider the following two signature schemes: $(t,n)$-threshold signature with a trusted dealer, ...
2
votes
0answers
110 views

Security of this deterministic encryption scheme

I recently came across a library that promises to do deterministic encryption with the following scheme: AES with 256 bit key in CBC mode with PKCS7 padding and Synthetic Initialization vector ...
1
vote
0answers
44 views

Index of coincidence for completely random text over k alphabets

I'm confused about /Friedman's method 2 (using a table with column of k size of key, and n/k rows, n is the total size for cipher text) \begin{equation} m = ...
2
votes
2answers
390 views

Why does Fortuna (RNG) require entropy sources to specify accumulator pool?

Re-reading my copy of Cryptography Engineering for the holidays, I was struck by the following question about the Fortuna RNG: Why require an entropy source specify which pool a random event should ...
0
votes
2answers
160 views

Why are Fortuna entropy pools reset after a reseed?

I've implemented LibTomCrypt's version of fortuna and am wondering about the pool resetting after a reseed. In fortuna.c in libtomcrypt we have the following (which is that part that adds the entropy ...
7
votes
3answers
2k views

What are some disadvantages of homomorphic encryption schemes?

I'm doing some self-teaching / research for my own benefit in homomorphic cryptography. I've studied both additive and multiplicative schemes (Pallier and RSA respectively), but all I can seem to ...
2
votes
1answer
117 views

Rotation table for 8 round DES

I'm trying to implement DES from scratch using the NIST paper and the Wikipedia article on DES. I got 16 round DES done, but I can't seem to get 8 round DES working. I figure it's because I got the ...
2
votes
0answers
154 views

An example of Knapsack Cryptosystem cracks/attacks?

I have been studying papers on various ways to crack the knapsack cryptosystem, unfortunately the mathematics in these papers involves lattices and LLL which is over my head. The paper "New Attacks ...
1
vote
0answers
77 views

Is this a correct understanding of Universal Hash Functions?

I'm studying universal hash functions and have been reading several papers but now i'm focusing on Wegman and Carters original paper from 1979 (Universal classes of hash functions) and the H1 class. ...
1
vote
1answer
142 views

AES-256 Shift-Rows Offsets

I am currently trying to implement a AES-256 cipher according to this sepcificaciton: http://techheap.packetizer.com/cryptography/encryption/spec.v36.pdf As well as on Wikipedia and in the document ...
1
vote
0answers
55 views

Reliability of a single-pass deniable authentication protocol?

I look for one-pass deniable authentication protocol with a short message payload for my project and find a solution: ...
6
votes
2answers
762 views

Will repeated rounds of SHA-512 provide random numbers?

If I hash a keyword with SHA-512 and then feed the output as the key for the next round ....and keep repeating this process, will I gather a stream of random numbers?
5
votes
2answers
757 views

Analogue encryption algorithms

I have a basic understanding of how strong encryption works on digital data. Bits can be changed so that they appear completely random and map one-to-one back to the original set, given the correct ...
1
vote
1answer
227 views

SHA256-based stream cipher

Can anyone comment if the stream cipher described here is safe? The author claims it to be unbreakable, but does not provide any evidence or proof to support this. For completeness, I have reproduced ...
0
votes
0answers
52 views

How to know private key is computed from specific partial private key?

In a certificate-less cryptography, I have learned that – to resolve the key escrow problem of IBE – the concept of a partial private key is used. Now, consider the following Scenario: After ...
1
vote
1answer
73 views

Is it possible to deduce the IV from CBC ciphered data, without knowing the key?

The question came up here, which left me thinking: Is it possible to deduce the IV from CBC ciphered data, without knowing the key? And if not, why is it considered a bad idea to create an IV by, for ...
2
votes
1answer
112 views

Why do we encrypt-then mac but sign-then-encrypt?

This question was asked in comments on my answer to should we sign then encrypt or encrypt then sign. I think posing the question as a separate entity is best, so: The general advice for applying ...
3
votes
1answer
63 views

Does displaying HMAC hashes of user inputs compromise the HMAC validation?

Here is the scenario: "SECRET" is a secret key that is kept private and only used in this one application. The application allows users to enter in any "VALUE" (within various open-ended limits such ...
7
votes
3answers
2k views

How to attack a classical cipher using known partial plaintext?

I have a ciphertext generated by a classical cipher. I do not know what was cipher used to generate it. I do however have the beginning of the plaintext. What are the cryptanalysis approaches for ...
1
vote
1answer
56 views

Secret Sharing 1 Required

Devise a scheme so that a message M can be shared among X, Y and Z in such a say that the only way of recovering the message is when X is present with either Y or Z. When X isn't present or each is ...
0
votes
2answers
120 views

Convert SpookyHash to semi-secure 192bit hash [closed]

What do you think of converting SpookyHash into semi-secure 192bit hash function by this way: increase security by using 8 * 8byte = 512bit blocks instead of 8 * 12byte blocks, but remaining 8 * ...
2
votes
2answers
262 views

RSA Key Generation Parameters - public exponent, certainty, string-to-key count

I want to know what values are appropriate for the public exponent and certainty when generating an RSA Key (for example using Bouncy Castle RSAKeyGenerationParameters function). What is the ...
0
votes
0answers
37 views

Locally Dedcodable Codes and Private Information Retrieval

Does anyone know about Locally Decodable Codes (LDCs)? It seems the private information retrieval schemes that based on LDCs are computationally efficent (answer a query without processing the whole ...
0
votes
1answer
102 views

IV = Filename XOR CipherKey?

i want to encrypt transparently using Rijndael. So this is, what I thought of and I would like to have an opinion from "Experts" whether this will harm encryption strenght. I am using chunks of 1MB ...
-1
votes
1answer
55 views

What's wrong with this “order-preserving MAC” function?

Please note: this is purely a thought experiment and not intended for any real-world usage! I came up with a simple function $\mathrm{OPF}$ to map the integers $[0,C)$ (where $C$ is the "ChunkSize") ...
0
votes
1answer
57 views

Multi cipher CTR

What is your opinion on this scheme: 1)get IV from CSPRNG 2)set 3 encryption keys k1,k2,k3 (3*key_lenght) (KDF/PBKDF) 3)Use 3 ciphers in CTR mode like this: The initial IV is encrypted using ...
4
votes
2answers
263 views

Is it possible to create a “proof-of-upload” system for BitTorrent ratio tracking?

One issue that private BitTorrent trackers that track users' share ratios often run into is how to keep track of people who are screwing with their upload statistics, something known as ...
4
votes
2answers
448 views

Randomness test question from FIPS 140-1 and comparison with 140-2

In FIPS 140-1 there are 4 statistical random number generator tests (The Monobit Test, The Poker Test, The Runs Test and The Long Runs Test. Then FIPS 140-2 came along and supposedly tightened the ...
5
votes
1answer
254 views

Secure AES Key Generation via Salsa20?

I am not creating my own cryptography or anything, but to learn about the Dos and Donts of cryptography, I am looking into AES encryption and ways to generate keys for that. As far as I've ...

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