5
votes
2answers
1k views

DES — Can I recover the key when I have both ciphertext and the plaintext?

Given a message and DES encrypted form of said message, is it possible to efficiently compute the key used to encrypt the data?
3
votes
1answer
127 views

Combining two hashing functions

I have been working on a Bloom Filter implementation recently and after a discussion with a co-worker about how many hashing functions to use I told him I was limited with using hashing functions that ...
3
votes
2answers
1k views

Advantages of bilinear map

Consider the pairing $e: G_1*G_2 \to G_t$. Why we are mapping element from group $G_1$ and group $G_2$ to an element in $G_t$. How are they used in cryptography? What advantages do they provide?
3
votes
1answer
179 views

Is there a general method to crack this type of fractionating cipher?

I've seen a few puzzles based on a type of cipher in which letters in the plaintext are substituted with groups of characters in the ciphertext. The ciphertext only uses a handful of unique ...
0
votes
1answer
140 views

Generate random in secure message transfer

I’m doing a school assignment about secure communications between a Server and a Client. Basically, messages are exchanged between the clients and the server and these communications must implement ...
3
votes
1answer
378 views

What is the GnuPG process for going from a passphrase to a symmetric key?

Suppose I use GnuPG to symmetrically encrypt a file like so: gpg --no-options -c --cipher-algo AES256 --no-random-seed-file -o my.out my.file I then provide a ...
3
votes
1answer
117 views

Diffie-Hellman for Encryption

In learning how DH works for cryptography applications, I'm coming across some confusing language. For starters, most of the time DH is discussed in the context of "generating" their shared secret. ...
2
votes
0answers
79 views

A confusion about linear span of modified de Bruijn sequence

Recently i'm reading "Cryptographically Strong de Bruijn Sequences with Large Periods" . In the section 2 2.1 Basic Denitions and Properties , property 1 says that ...
1
vote
2answers
522 views

Game traffic encryption AES CTR + HMAC

I was asked to implement traffic encryption between the game server and clients. Currently, the traffic is not encrypted in any way, which lets players use cheats to listen to the traffic in order to ...
1
vote
1answer
121 views

Sample RSA-style signature confusion

I'm trying to study a simplified TLS-style protocol, including verifying RSA signatures. It's an attempt to verify using only a valid certificate. However, I can't manage to get anything to verify ...
4
votes
1answer
112 views

Are IVs and salts the same and usable for each other uses?

I'm new in the crypto world and I've just discovered PBKDF (I used to use typed passeword as symmetric key). When using some crypto mode, you're required to ...
2
votes
1answer
287 views

Bit commitment with 1-out-of-2 oblivious transfer

I know a protocol for bit commitment using regular OT (Bob has 1/2 chance of learning the bit Alice transferred to him) which goes like this: COMMITMENT PHASE Alice chooses a bit $b$ For ...
2
votes
2answers
142 views

What is the impact of different modes on pseudo randomness of AES?

AES in CBC-MAC mode is proven to be good PRF . But is there any study done on other modes ? how good is ECB mode as PRF ?
4
votes
1answer
195 views

Keyed digest function with odds of collision below the birthday bound?

I wonder if it is possible to devise a function $F(K,S,R_S)\mapsto D$ where: $K$ is some key (I have freedom on $K$, it could even be generated by a trusted party); $S$ is in $\{0,1\}^s$, say ...
1
vote
0answers
227 views

Pohlig-Hellman exponentiation block cipher

Let $G$ be a group of order $n$ and let $e,d$ be integers such that $ed\equiv 1 \pmod{n}$. Then the exponentiation maps $x \mapsto x^e$ and $y \mapsto y^d$ are inverse maps on $G$. These maps give us ...
2
votes
2answers
177 views

Can anyone tell me the name of this cipher please?

Can anyone tell me the name of this cipher please? I know it's a simple substitution cipher, I just don't know the name of it. Cipher Key: help Cipher alphabet: ...
6
votes
2answers
1k views

Can the XOR of two non-collision-resistant hashes be collision resistant?

Suppose I have two hash functions, of which neither (or only one) is collision resistant, and I want to create a new hash function by taking the bitwise exclusive or (XOR) of the results of those two ...
8
votes
1answer
465 views

Did NIST verify “post-quantum” claims in the SHA3 proposal papers?

I have been reading Bernstein’s “Quantum attacks against Blue Midnight Wish, ECHO, Fugue, Grøstl, Hamsi, JH, Keccak, Shabal, SHAvite-3, SIMD, and Skein” paper from 2010… This document disproves ...
0
votes
1answer
3k views

ECC key size and signature size

My understanding of ECDSA signature length is that it depends on the key size. So for instance, if a "prime256v1" is used, the signature length will be 64 because (n/8)*2 and for "secp384r1" it will ...
-1
votes
1answer
136 views

How do I cryptanalyze a password field?

In a PostgreSQL database, I can see the following fields in a table (for a specific record): User = administrator Password = "ceMv9Me6go" Now the following are true: The password length is not ...
1
vote
1answer
87 views

collision resistant summarizer for long hash values

how we can summarize SHA-256 into shorter values when we for example can't handle more than 128 bits on system? its better just use first 128bit and ignore the rest of bits or split full value and XOR ...
3
votes
1answer
591 views

File Hash of Encrypted File

Simple question. I have a source file and I have encrypted it with a certain public key. If I take this exact same source file, encrypt it with the exact same key a few hours later would the file hash ...
1
vote
2answers
105 views

What is the difference between PPE and SPPE?

Can somebody explain, in simple terms, the difference between Pseudo Random Permutation Ensemble and Super Pseudo Random Permutation Ensemble?
4
votes
2answers
271 views

Do test vectors ensure a cipher is free of backdoors?

If a cipher implementation passes unit tests using test vectors from some trusted source (these for AES, for example), then can we say that it is fully conforming to the specification, and must have ...
3
votes
2answers
189 views

iterated discrete log problem

Consider the following problem: given $g_1 \ldots g_i,h_1 \ldots h_i \in G$, $\forall i$ find $x_i$ such that $g_i^{x_i}=h_i$ For $i=1$ this is the discrete log problem and is assumed to to have ...
6
votes
5answers
2k views

Software implementation of a commutative cipher?

I've got an application (detailed below) that calls for the use of a cipher that is commutative. I've been doing some googling & reading, and there are two algorithms that seem to get mentioned ...
4
votes
3answers
433 views

Is there a public key semantically secure cryptosystem for which one can prove in zero knowledge the equivalence of two plaintexts?

If Alice encrypts two messages $a$ and $b$, such that $x=E(a)$, $y=E(b)$. Can Alice prove (without revealing $a$, $b$ or the private key) that $a = b$? Obviously the proof must not be too long and it ...
7
votes
5answers
955 views

Applications of Group Ciphers

I've been reading a paper [1], and I've ran across something called a "Group Cipher", which is similar to homomorphic encryption, with an important difference. In homomorphic encryption we have an ...
3
votes
1answer
770 views

What's the main difference between Pohlig-Hellman and RSA?

Both Pohlig-Hellman and RSA perform encryption and decryption by exponentiation modulo some integer ($p$ prime for PH, $n$ composite for RSA). They both use a key $e$ as the exponent to encrypt a ...
7
votes
0answers
171 views

Has the distributed project “Number Fields @ Home” project benefited cryptography in any meaningful way?

Is there any new understanding, property, or knowledge that has come from the Number Fields @Home distributed computing project? Has any outcome advanced the study of cryptography, or altered ...
2
votes
2answers
661 views

How does this happen in RSA malleability?

I don't understand how the $E(m)$ turns into $E(mt)$. I mean, I don't know how does that transformation happen and how does the equation occur. $$E(m) \cdot t^e \bmod n = (mt)^e \bmod n = E(mt)$$ ...
6
votes
3answers
488 views

Slow one-way pseudo-random permutation?

I'm looking for a slow one-way pseudo-random permutation; or in other words a block cipher $E_K: P\in\{0,1\}^b\mapsto C\in\{0,1\}^b$ with moderate block size $b\approx 64$ bits, wide key $K$, ...
3
votes
1answer
126 views

Block Cipher Mode Amicable to Fast Key Change/Rotation Like XOR?

I'm working with documents encrypted in long term storage. When a client requests the document, the document should be delivered encrypted under a distinct key (distinct from the long term storage ...
5
votes
3answers
702 views

AES column mixing and S-Box confusion

I am trying to understand the AES encryption algorithm. I know that we process 128 bits at a time for a 128 bit key in a 4x4 octet form, but am confused by the following: How does the substitution ...
5
votes
4answers
2k views

Is it possible to create an open-source SecurID?

As far as I understand it, the hardware key-fob two-factor systems such as RSA SecurID depend on secret algorithms, and employ tamper-resistant hardware to prevent reverse engineering. Is there some ...
3
votes
2answers
1k views

Determine the Key given ciphertexts and plaintexts?

Given two sets of plaintexts and their corresponding ciphertexts, how does one determine the key $K$? The ciphertexts are: $$C_1= (P_1 \oplus K_0) + K_1$$ and $$C_2= (P_2 \oplus K_0) + K_1$$ $K = ...
3
votes
2answers
284 views

differential and linear cryptanalysis

I have been reading about differential and linear cryptanalysis. They were mainly introduced by Adi Shamir and Biham to show weakness of DES. However, many of articles state that they have been ...
6
votes
2answers
3k views

RC2, RC4, RC5 key length

While reading the Wiki page on RC4 I noticed that the key size must be in the range of 40–2,048 bits. So should I understand like this, that I can have a key that has length of, lets say 333 bits? ...
3
votes
3answers
767 views

What are the pros/cons of using symmetric crypto vs. hash in a commitment scheme?

In a commitment scheme, are there any differences on using a symmetric cipher versus using a hash? If at the "opening", I have to reveal $r$ (a random number concatenated with the messaged at the ...
3
votes
2answers
756 views

Encryption Key derivation from numeric PIN?

I want to store data encrypted within an Android application. Keeping an encryption key safe outside the app is not an option so I looked into password derived key algorithms. I found a nice example ...
3
votes
3answers
346 views

Can one use a Cryptographic Accumulator to efficiently store Lamport public keys without the need of a Merkle Tree?

One of the problems of one-time Lamport signatures is that public keys are disposed after use, so you must generate many keys and store them in a Merkle tree. The root is the "real" public key and ...
2
votes
2answers
169 views

Could someone reuse client certificates?

To my understanding, if a server “cert+key” (RSA) is compromised, than the SSL/TLS network traffic can be decrypted. What about a client “cert+key” (RSA)? Could someone reuse client certificates? ...
3
votes
1answer
484 views

How is an AES Encrypted Message Decrypted with Random Padding?

One thing I was surprised to learn about AES is that random padding can be added to the message to make it a multiple of block size. What I can not wrap my head around is how this random padding can ...
2
votes
1answer
373 views

Diffie-Hellman: Does the size of the prime modulus depend on the potential size of the secret?

I'm writing my own cryptosystem that uses One Time Pad and Diffie-Hellman to generate keys for the OTP. I am using Rails on the server side and Javascript for the client side. Please don't tell me ...
2
votes
1answer
187 views

How is a PRNG used in the PGP?

I'm currently reading up on the components of PGP and I am wondering what exactly the PRNG is used for. PGP uses ANSI X9.17 and entropy from a users keystrokes to generate a temporary key. So, is ...
1
vote
0answers
105 views

Is it possible to modify SSL handshakes to enable PFS while still using RSA during the handshake?

I know we could possibly be using DH key exchange, that would enable PFS. However, what if we make modifications on our SSL 3.0 handshake such that the only cipher we use is RSA for the achieving PFS? ...
5
votes
1answer
1k views

What does Maj and Ch mean in SHA-256 algorithm?

I'm guessing they're some kind of standard function but what do they do and what do the names mean? A little explaination or link me to an article would be great.
1
vote
1answer
527 views

How should I manage Diffie-Hellman parameters on a Web Server?

Is there a best practice for Diffie-Hellman parameters (p, g, and q) used on a web server? The server is using ephemeral keys, but should the parameters be rotated on a regular basis? Is it OK for ...
1
vote
0answers
123 views

Can you provide an example in relation to Hidden Field Equations Multivariate?

I'm reading about Hidden Field Equations Multivariate scheme. My lecture states that the central map is a univariate polynomial $$P(X)=\sum_{i=0}^{r-1}\sum_{j=0}^{r-1}p_{ij}x^{q^i+q^j} \in K[X]$$ ...
1
vote
0answers
226 views

Where can I find examples of ECC implemented in VHDL?

I am looking into implementing ECDSA signature and ECDH key agreement in a Xilinx FPGA. All the examples I have found of VHDL implementations skip over how to construct the low level ECC primitives ...

15 30 50 per page