2
votes
2answers
2k views

Can a shift cipher attain perfect secrecy?

On a practice question for my intro cryptography exam, it asks the following: Assuming that keys are chosen with equal likelihood, the shift cipher provides:    A) computational security ...
2
votes
1answer
890 views

Learning cryptography using a FPGA

I have a FPGA and am curious what cryptographic applications I can use with it. I'm just a hobbyist / security researcher and not a professional cryptographer. Since I'm creating my own curriculum, ...
2
votes
1answer
2k views

How can I use eulers totient and the chinese remainder theorem for modular exponentiation?

I'm trying to implement modular exponentiation in Java using Lagrange and the Chinese remainder theorem. The example we've been given is: Let $N = 55 = 5 · 11$ and suppose we want to compute ...
2
votes
2answers
125 views

When is each key used when encrypting an email using OpenPGP?

When you send an email using PGP to encrypt emails, is the recipients public key used to encrypt the email, or is your private key used? Are they both used? At what points do each of the four keys ...
2
votes
1answer
2k views

Simulation-based security?

I've been reading Introduction to Modern Cryptography by Katz and Lindell as an introduction to cryptography. The book seems to use the term 'simulator' when it talks about a game like, for instance, ...
2
votes
1answer
527 views

Can I combine two of SHA-3 candidates cryptography hash functions and obtain more secure Algorithm?

For example, Is possible to combine (Concatenate or Chain or XOR) Skein SHA-3 candidate with Grostl SHA-3 candidate to increase security? Note: I just want more secure output and CPU cycles does not ...
2
votes
1answer
287 views

Aes encryption -The relevance of static matrix in mixcolumns operation

Can someone explain to me the relevance of the static matrix used for the mixcolumns operation in aes encryption.i.e the relevance of why the byte is multiplied by 2 + next byte multiplied by 3 + next ...
2
votes
1answer
208 views

What are the dangers of predictable (repeated) plaintext structures?

When using a "good", modern cipher (specifically one that provides ciphertext indistinguishability), is it a problem at all if there is some well-known structure in all plaintexts? For example, ...
2
votes
1answer
2k views

AES GCM implementation in c#

I am implementing an AES cipher in GCM mode in c# and have a few questions. My code is based on the code found here for reference. I'll copy in the relevant portions. ...
2
votes
1answer
495 views

Poor man's SSL - is this method as safe as SSL/TLS?

I need to send data between two applications. I've got requirement that says that data should be transmitted using secure protocol such as SSL/TLS. Data is sent using TCP sockets and I don't have ...
2
votes
1answer
317 views

Using compression to test encryption

Is the uncompressibility of encrypted data a necessary property for a good cryptographic algorithm? To make a crude experiment, I encrypted an 8K file with all 'A' and then compressed both the ...
2
votes
1answer
6k views

How does one implement the Inverse of AES' MixColumns

I got an answer in the related question about Mixcolumn for encryption, but how about decryption? what will I do? Because it said that I will used this: During decryption the Mix Column the ...
2
votes
2answers
394 views

How do I demonstrate that a PRNG not designed for cryptography is not suitable for generating passwords?

This is a replication of this question on Stack Overflow. There's class Random in .NET runtime which is designed for use as a cheap fast source of pseudo-random ...
2
votes
2answers
220 views

Can the premaster secret generated by SRP be used as a secure private key?

It seems like the pre-master secret generated during the SRP protocol would make a good source to generate a shared private key using a secure hash to compress it down into a 128/256 symmetric key. ...
2
votes
1answer
277 views

Cryptographically strong pseudo-random seq. generators

If a pseudo-random sequence generator is built so that it uses an n-bit seed and outputs a string that is of length x. Let's say one wants to generate a bit string of length y and uses the previous ...
2
votes
2answers
425 views

How to generate successive stream-cipher keys?

I've identified a weakness in a distributed simulation system I'm looking at, and I'm looking for some advice on how to fix it. Clients initially negotiate an authentication token with a login server ...
2
votes
2answers
288 views

Seeking special-use fingerprinting/hashing algorithm

For a project I wonder if there exists some kind of fixed-size checksumming/fingerprinting function in which based on this fingerprint given data block 1, it is easy to generate more data blocks that ...
2
votes
1answer
1k views

Finding CRC collisions for specific divisor

My current textbook (Information Security: Principles and Practice by Mark Stamp) discusses how to determine the CRC of data via long-division, using XOR instead of subtraction to determine the ...
2
votes
2answers
159 views

How can I repeatedly prove I have data another has seen without sending the data and without the other storing the data?

I would like to know if this is theoretically possible, or impossible, and if possible, if there is any algorithm/protocol to accomplish this... I want another entity, lets call them the Auditor, to ...
2
votes
1answer
523 views

What algorithm does PGP use to encrypt email?

I know it uses RSA/DSA to create keys, but does it use that same algorithm for the actual cipher?
2
votes
1answer
145 views

How to collect, process, and transmit data securely?

In my question "Authenticating data generated by a particular build of an open source program", Dave Cary requested that I post a question stating my real problem on a high level rather than the ...
2
votes
1answer
54 views

How to find collisions in a hash function knowing its coefficent?

Say I have a simple hash function where $x$ is a sequence of integers: $$h(x)=(a_1\cdot x_1+...+ a_n\cdot x_n) \bmod N=\sum_i a_i\cdot x_i \bmod N$$ where $a_1,a_2,a_3,...,a_n$ is the coefficient ...
2
votes
2answers
78 views

How to make the range of modulo $p$ to $[-\frac{p}{2};\frac{p}{2}]$?

I am using NTL (A Library for doing Number Theory) to implement a FHE (Fully Homomorphic Encryption) scheme. In general, the range of modulo p is $[-\frac{p}{2};\frac{p}{2}]$ in FHE scheme. However, ...
2
votes
1answer
72 views

Trusted Third Party Cryptanalysis Labs?

There's no doubt that third party cryptanalysis is an essential part of the crypto ecosystem. It's how we know that there's some degree of practical "goodness" to a new algorithm or protocol. Unlike ...
2
votes
1answer
47 views

Does XChaCha20 exist with similar properties to XSalsa20?

Does XChaCha20 (built from ChaCha20 just like XSalsa20 is built from Salsa20) exist and have XSalsa20's desirable properties?
2
votes
1answer
75 views

Can the key-complement property of DES be used against 3DES?

The comments in this recent question about the security of 3DES made me wonder if the key complement property of DES (which reduces its security by ~1 bit) can be used in the meet-in-the-middle attack ...
2
votes
1answer
118 views

Are there any successful preimage attacks?

Are there any successful preimage attacks on any serious cryptographic hash algorithm or at least on serious-looking non-cryptographic hashes (like MurmurHash)?
2
votes
1answer
40 views

Winternitz Signature in standard model

I am studying the Winternitz signature and I describe its algorithms in the next W-OTS Key Generation. Select the parameter $w\geq 2$ that is the bit size of the partitions of the message to be ...
2
votes
1answer
135 views

What is the best current way to cryptographically secure databases?

I'm currently working on a proposal for some secure complete system and one of the things currently lacking is the database security. However I don't want to roll my own crypto there and thereby need ...
2
votes
1answer
134 views

Theoretical decryption of RSA

I am currently looking into RSA and have some questions. I am interested in theoretical answers, assuming there is infinite time for decryption. Is it necessary to work out the private-key exponent ...
2
votes
1answer
105 views

Would SHA-1 be safe for certificates with short validity?

There are articles floating around the topic of SHA-1 deprecation. The gist of it is that in (mostly) developing countries, people are still using mobile/computing devices that are 5 years old (or ...
2
votes
2answers
175 views

Given a pseudorandom function F, is $(F_k(r) \; | \; F_k(r + 1))$ also a PRF?

Given: F is a pseudorandom function, G is a pseudorandom generator with $l(n) = n+1$. The following schemes should be classified as being insecure, IND-COA secure, IND-CPA secure. To encrypt $m \in ...
2
votes
3answers
96 views

2 Part Encryption

Can anyone tell me if an encryption / decryption scheme like the following exists? Encrypted1 = SomeCryptoFunction(Data, Key1) ...
2
votes
1answer
85 views

Problem reproducing AES results

First of all, I just implemented AES because I wanted to understand how it works and as a challenge to see if I could do it and I won't use to encrypt anything important, ever. The (German) Wikipedia ...
2
votes
1answer
88 views

Why can you reverse a modulo function when knowing its primes

We are dealing with cryptography in school right now and superficially went over the Rabin cryptosystem with the (apparently usual) example of p=7 and q=11 etc (we didn't do RSA). I understand that ...
2
votes
2answers
115 views

Given PRNG $G(s)$, why is $G(s) || G(s+1)$ not a PRNG?

I have given a pseudorandom Generator $G(s)$, $|s|=n$ whose expansion $l(n)$ is $>2n$. I also know that $G^\prime(s) := G(s_1,\ldots,s_{\lfloor\frac{n}{2}\rfloor})$ is a pseudorandom generator. Now ...
2
votes
2answers
92 views

Securely generating auth tokens

(Disclaimer: I'm an experienced server-side Java programmer, but with little crypto experience.) Objective: I've got a login server which, given a login+password pair, should issue a unique auth ...
2
votes
1answer
204 views

RSA vs ECDSA/ECDH

So we've been using RSA for a while now. I wonder, is it still secure? I'm wondering whether surveillance agencies can at this point read traffic encrypted with say, TLS_ECDH_RSA_WITH_AES_128_GCM? ...
2
votes
3answers
429 views

Prevent replay attack without storage

I am designing a set of low-power sensing motes. Each mote will communicate to a central base over RF. I want the mote to be able to authenticate itself with the base, and send its data without being ...
2
votes
1answer
85 views

What is the difference between “securely realizes” and “securely implements”?

In some security proofs it is stated that "a protocol securely realizes an ideal functionality" while in some others "a protocol securely implements an ideal functionality". Is there a meaningful ...
2
votes
1answer
138 views

Single bit error in CBC

Why does a single bit error when using the CBC block cipher mode of operation affect the decryption of a block (Oj) with a probabilty of 50%? And in the following block, why is the one bit error in ...
2
votes
2answers
46 views

What is the correct way to notate elliptic curve properties

When looking through elliptic curve and modulus posts i can see many examples of people referencing $p$ and $n$, in upper and lower case, and sometimes $Q$ is written instead of $p$ or $P$, and I ...
2
votes
2answers
111 views

Inverting RSA function

I am in high school and I am writing a paper on RSA. I want to show that low values of the public key exponent can make it easy to 'invert' the function so that the encrypted message can be recovered. ...
2
votes
2answers
70 views

Diffie-Hellman secret key choices

I'm trying to understand why the secret keys used by Alice and Bob in Diffie-Hellman are $a,b ∈ ℤ∗p$. Say $a$ is $p - 1$, wouldn't the transmitted $g^a$ be 1 mod $p$ by $FLT$? Now anytime a ...
2
votes
2answers
71 views

Is it safe to use single-block AES for mutual authentication?

I read the question Symmetric mutual authentication with client using a derived secret and its answer which, if I'm not mistaken, assert that it should be safe to use the proposed protocol for mutual ...
2
votes
2answers
91 views

Using related pair of cryptographic keys to open/seal plaintext

I am using curve25519xsalsa20poly1305/fn.seal from sodiumoxide crate written in Rust language which is a wrapper over libsodium ...
2
votes
2answers
223 views

Authenticating code securely and generating a certificate

I'm working on a product with strong security. Essentially there is an App, downloaded from the iTunes App Store. When the App starts up, it generates a strong RSA keypair and stores it in the ...
2
votes
1answer
433 views

Encryption algorithm used in WPA/WPA2

I want to know how EXACTLY the WPA password gets encrypted I have been searching through and I was able to catch: The actual password is converted to some hash Salting is applied to the hash The ...
2
votes
2answers
114 views

Digital Signature Attack

In a typical digital signature scheme in which in order to sign we compute: hash of the message: $h(m)$ resulting hash signed with the private key $[h(m)](private key)$ If there were a collision ...
2
votes
1answer
221 views

Any reason to use Shamir given faster XOR threshold secret sharing algos?

TL;DR: Is the Kurihara algorithm really what it purports to be (dramatically faster but equally secure replacement for Shamir Secret Sharing)? ...

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