All Questions

2k views

RSA example-calculation: Public Key = Private Key (e = d)

I am a bit confused. I just calculated manually the single steps of RSA for an implementation with small numbers and suddenly $d$ was equal $e$. Please help me understand what I am doing wrong. Here’...
117 views

Back up an information as $n$ pieces and require exactly $n-1$ to recover it

Let $X\in \Bbb F_2^p$ be some information. How do I create $Y_1,\dots,Y_n \in \Bbb F_2^q$ so that having less than $n-1$ of the $Y_i$s gives you no information on $X$ but having $n-1$ of them allows ...
286 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 ...
690 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 understood,...
643 views

For a one-time pad, which MAC method is information-theoretically secure?

In the the main post about MAC methods it mentions a few methods: Authenticate And Encrypt: The sender computes a MAC of the plaintext, encrypts the plaintext, and then appends the MAC to the ...
331 views

Cryptographic system with double keys with reversible order

While reading Shamir, Rivest and Adleman's paper on "Mental Poker", I've met a mention of system such that $E_a(E_b(x)) = E_b(E_a(x))$, without however disclosing details on it, with $E_a(x)$ being “...
83 views

What does it mean that $BW_N$ is a permutation over the squares mod N?

Let $BW_N$ be a function such that $BW_N:\mathbb{QR}_{N} \mapsto \mathbb{QR}_{N}$ and let if be defined as follow: $BW_N(x) = x^2 \pmod N$ where $N=pq$ and p and q are primes and $p=q=3 \pmod 4$. I am ...
157 views

post-quantum threshold secret sharing

I am looking for a references for post-quantum threshold secret sharing schemes. I am especially interested in knowing whether any one based on one-way compressor functions or cryptographic hashes ...
251 views

When would one prefer a proof of knowledge instead of a zero-knowledge proof?

I've just realized I find it hard to distinguish between these two terms (proof of knowledge, and zero-knowledge proof), specially where only the latter seems to be used in many cryptographic ...
769 views

Will rehashing an SHA256 hash continually, eventually produce every possible value?

So let's say you had infinite time and energy. You have a hashed string of some sort. Because you have infinite time and energy, you can produce a collision(or the original value) easily enough. But, ...
333 views

hardware and software optimizations for Salsa20

I'm using the Crypto++ implementation of Salsa20 for a software project we are working on and I'd like to know if there are any SSE or software optimizations that will increase the speed of the CSPNG?
2k views

Why elgamal is still discussed

As I understand, if two users have a shared key, it is possible to encrypt a message using symmetric key encryption. So when a secret key is shared through Diffie-Hellman asymmetric key exchange, it ...
204 views

Has it become easier to pre-compute tables after MD5 collisions?

Apparently it is not possible to compute all MD5 values from 0x00000000000000000000000000000000 to ...
605 views

Can you break a multi language code using Frequency analysis?

Let say that I wrote a 26 letter alphabet, each letter of my alphabet represent a letter from the latin alphabet. I'm writing in 3 languages, only I know which languages. Grammar is the one from my ...
626 views

Proofs of security methodologies

I'm looking for course material on the subject of proofs, reductions, and games, as used to prove cryptographic schemes secure. What are the methodologies? What are the preferred ones? In what cases ...
440 views

Security of tokenization of plain text conversations - cryptanalysis

I came across a marketing video here. They claim to perform AES encryption and tokenization of sensitive data, at the corporate gateway, before it leaves the company firewall destined for the public ...
694 views

Security for IV in AES-GCM mode

I'd like to use AES-GCM instead of HMAC for authentication and privacy in a token used in a web application, but I'm unsure how to securely tag each token with an IV. Is it safe for me to prepend the ...
222 views

Why work in a subgroup QR(n) of an RSA group $Z^*_n$?

I sometimes read in papers that a (sub-)group generator $g$ is taken from $\mathrm{QR}(n)$ instead of $\mathbb{Z}^*_n$, where $n = p \cdot q$ and $p$ and $q$ are prime. Is there a reason for this? ...
603 views

DES key expansion: why are these two descriptions different?

I would like someone to explain me how to generate DES subkeys, because in the documentation that I saw, sometimes they say: "We rotate the initial key according to the number of iterations given ...
738 views

Efficient Symmetric Mutual Entity Authentication Protocol

I am looking for an Authentication protocol for access control. Let's say both the client and server would share a symmetric secret (256 bit key) and the client wants to access the server. Access ...
651 views

Is there a practical zero-knowledge proof for this special discrete log equation?

We have a multiplicative cyclic group $G$ with generators $g$ and $h$, as in El Gamal. Assume $G$ is a subgroup of $(\mathbb{Z}/n\mathbb{Z})^*$. There are two parties, Alice and Bob: Alice knows: ...
412 views

Is my password entropy calculation correct?

I have a password with a prescribed format: 111 + “2 Random lowercase letters” + “random digit” As you can see, it starts with a fixed value of ...
223 views

How long does it take to extract a key from a FIPS-140 Level 2 device?

How long does it take to extract a key from a FIPS-140 Level 2 device? What records are there of successful extraction? How much did the first extraction from a given device cost? How much would a ...
2k views

How do I calculate CRC32 mathematically? [closed]

I want to calcuate the CRC32 algorithm using polynomials directly but I don't know how. I found the generating polynomial listed here https://en.wikipedia.org/wiki/Cyclic_redundancy_check. This ...
557 views

What kind of multiparty computation is this?

The classic multiparty computation protocols are defined around " Untrusted parties trying to compute something together". But is there a cryptographic abstraction for, "Trusted parties trying to ...
895 views

are CFB and OFB really meant for streaming?

CFB, OFB and other modes are meant for streaming and don't require padding. Are there still limitations such as the text needs to be greater than key length?
1k views

How to account for moore's law in estimating time-to-crack?

It seems to be common practice (at least in some communities) to tack on the phrase "with current computing power" when estimating the absurdly long time it would take to, for example, brute-force an ...
375 views

Why are the random exponents so much bigger in the Socialist Millionaire protocol versus Diffie-Hellman key exchange?

Section 8, Security considerations, of RFC3526, which defines groups used for Diffie-Hellman has a table recommending some random exponent sizes. In particular, it says: The strength of a key ...
1k views

111 views

Transforming SHA-512 into SHA-512/256?

Based on my skimming over FIPS 180-4 it looks like the only difference between SHA-512 and SHA-512/256 is the initial hash value and the fact that the final output is truncated to 256 bits. My ...
339 views

For a given plaintext-ciphertext pair, how many valid AES keys are there?

For $PT\in \mathbb{M}$ and $CT\in \mathbb{C}$, let $\mathbb{F}=\{f|f:\mathbb{M}\rightarrow\mathbb{C}\}$ be the collection of all functions from $\mathbb{M}$ to $\mathbb{C}$. Then AES encryption under ...
235 views

Should Increment functions be near-constant time?

When operating a block cipher in CTR or GCM mode, should the function used to increment the counter or initialization vector be near-constant time? Here, I'm wondering about the "addition with carry" ...
539 views

How to generate unique (random) numbers that can be traced back to a unique source number?

I need to generate unique (random) numbers that I can trace back to a unique source number. For example: source number A can generate numbers A1, A1, A3 .... Am source number B can generate numbers ...
111 views

Hitting a counter example in homomorphic encryption over the integers

The paper Fully Homomorphic Encryption over the Integers talks about a super simple symmetric key implementation on page 1 and 2. It says that to generate a key, you pick a random odd number between \$...
2k views

What are the differences between collision attack and birthday attack?

From my understanding both types of attack, collision and birthday, are based on the principle of two randomly/pseudo-randomly chosen plaintext to hash to the same value. I don't want to launch any ...
934 views

what are the advatanges and disavantages of the digital signature algorithms for JWT?

I am using a Java JWT library jose4j. There is a list of digital signatures algorithms which can be used. HMAC using SHA-2 RSASSA-PKCS1-V1_5 Digital Signatures with with SHA-2 Elliptic Curve Digital ...