# All Questions

791 views

### Is this a correctly formatted PGP session key packet?

RFC 4880 may be full of information, but it can be incredibly vague at times, so im looking for someone who actually knows the answer to this. Given this public key: ...
472 views

### Using a derived key for CMAC

Consider the following authenticate-and-encrypt scheme that uses AES-128 in CBC mode for encryption and AES-128 - based CMAC for authentication: Two keys are derived from the master key k (16 byte): ...
201 views

### Addition step in MD5 implementation

In MD5, there are four rounds. After every round, why do we need to add the computed $Q$ values to the initial values and then take this value as input to the next round. For example after the first ...
2k views

### What is the 'Version Incompatibility" Problem in TLS 1.1/1.2 and 1.0?

It's unclear to me if there are multiple problems, or a single one, (and just what that problem is) with servers incorrectly negotiating down from a client offering 1.1 or 1.2 to something they ...
606 views

### Differential cryptanalysis - breaking the last round of FEAL4?

I've been trying to learn cryptanalysis. I've come across this resource which proved very helpful: http://theamazingking.com/crypto-feal.php So far I've been almost successful in breaking FEAL4 ...
401 views

### Provable Encryption

Is it possible to encrypt data in a way that it can be proven that the data is encrypted, without revealing the key? Alice chooses some plaintext, then she encrypts it with a certain scheme. She ...
806 views

### Malleability of ElGamal and Hashed ElGamal

Question: Suppose A encrypts a number $x$ which indicates her bid on a contract, using ElGamal encryption. Say that the encryption of $x$ produces a ciphertext $c$. Explain how E can modify $c$ to ...
1k views

### How does a key wrapping like RFC 3394 secure my cryptographic keys?

So I'm messing around in the BouncyCastle library with the RFC 3394 AES Key Wrap engine and I'm trying to understand the benefit of it. The problem I'm running into is how to store keys securely on a ...
79 views

### How does generating random numbers “remove entropy from your system”?

In a previous question, I quoted the --gen-random entry in the GPG Man Pages. At the end it says: PLEASE, don’t use this command unless you know what you are ...
42 views

### Making Pascal Paillier' output absolute

Can we make subtraction result of cipher texts encrypted by Pascal Paillier absolute. Just like we use method Math.abs() in Java ? For example, if we subtract 0 from 1: 1-0 = 1, it is positive but 0-1 ...
448 views

### Which is/are the strongest known Fully Homomorphic Encryption scheme(s)?

As it is discussed here that the highest security any homomorphic encryption scheme is at most IND-CCA1, Is there any known fully homomorphic encryption scheme that achieves this security level? Out ...
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### How to test a FHE implementation?

I want to implement a FHE scheme based on NTRU, namely the scheme described here https://eprint.iacr.org/2014/039.pdf . How to test the security of my implementation ? Do I have to implement the ...
97 views

### Are there MD5 collisions for inputs of different length?

There are many examples of MD5 collisions (some of them can be found here Are there two known strings which have the same MD5 hash value?). But as far as I know two inputs should have the same length ...
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### How to find the value of a vector modulo a basis in lattice-based cryptography

In Gentry's paper on fully homomorphic encryption using ideal lattices, he finds the values of vectors modulo a certain basis. For instance: $\psi \leftarrow \psi' \mod B$ Taken from page 69 of ...
52 views

### Is there a partially homomorphic quantum secure public key cryptosystem with IND-CCA1 security?

I recentely asked "IND-CCA1 RSA padding?" about whether there is a IND-CCA1 secure variant of RSA. The original version of the question also allowed usage of ECC which would allow usage of ElGamal, ...
97 views

### How can you produce visual cryptography?

I would like to be able to "code" images such as the one seen here on Wikipedia. Is this generally produced with software, or does this require complex programming? Ideally, I would love to be able ...
131 views

### Are all stream-ciphers IND-CPA?

A stream cipher usually generates a stream of pseudo-random bits which get XORed with the plaintext to form the ciphertext. The stream is generated using a given IV / nonce and a secret key. CTR-mode ...
105 views

### Where is SRP-Z from?

The Stanford license for SRP says: Broader use of the SRP authentication technology, such as variants incorporating the use of an explicit server secret (SRP-Z), may require a license; Yet ...
121 views

### Turning a 64 bit block cipher into a 128 bit block cipher

There are quite a few block cipher modes of operation that require 128 bits. There are also modes of operation where a higher block size than 128, e.g. a block size of 256 bit would even be practical. ...
123 views

### Is there a scheme that allows to use RSA based challenge response protocol limited to signing only 8 bytes to create a secure signature of any data?

I have a tiny device that has a challenge-response authentication mechanism where the device signs 8 bytes of any data with it's internal 2048 bit RSA key. I would like to be able to certify any data ...
108 views

### Is it ever unsafe to compress an EC point?

I am working with a library that outputs EC points in uncompressed form. To save space, I'm considering modifying said library to use compressed EC points. Assuming that I keep track of the sign bit ...
255 views

### Decrypt all RSA ciphertexts if you can break 1% of them

I was given a cryptography question during an interview the other day. Imagine you are given an efficient algorithm which for a given RSA public key (n,e) is able to decrypt 1% of the messages ...
444 views

### AES-ECB as an authentication mechanism

ECB is considered to be insecure when used for confidentiality because identical plaintext result with identical ciphertext. But what if we use ECB for authentication? Assume A wants to transmit an ...
134 views

### Partial hash code protocol for security tokens providing signatures

I've read the following two sentences within a beta protocol specification, which references ISO/IEC 7816-8 for PSO:HASH which already contains notions of a "partial hash". The off-card entity is ...
349 views

### Security issues of a MAC-then-Encrypt-then-MAC approach?

Encrypt-then-MAC does provide ciphertext integrity, but no plaintext integrity. With MAC-then-Encrypt it’s the other way around: Plaintext integrity but no ciphertext integrity. What comes to mind is ...
333 views

### How feasible is word-level frequency analysis over English (or any language)?

Say I have some black box which, given any English word, deterministically outputs a token for that word. Assume our black box is implemented using strong cryptography, i.e. the hardness of reversing ...
233 views

### Using PBKDF2 twice with different argument order

I'm pretty sure this is a really bad approach (in theory), but one of my clients is doing this and I was wondering… How bad it is to perform pbkdf-2 in this way (with 2000 iterations)? ...
336 views

### What differences between Menezes–Vanstone ECC and ElGamal ECC?

After researching ECC encryption, I found that we can use ElGamal cryptosystem with elliptic curve and can we use Menezes-Vanstone cryptosystem with elliptic curve. What is the essential difference ...
91 views

### Does $i^n=j^n$ for $i, j \in GF(2^q)$ and $i \neq j$ for some $n<2^q-1$

Let $i, j \in GF(2^q)$ and $i \neq j$ and $i,j\neq0$. Is that possible that $i^n=j^n$ for some $n$ such that $0 < n < 2^q-1$? I am looking for a proof if the answer is no, or for a method to ...
3k views

### Why is Serpent faster than AES in this benchmark?

I have a laptop without the AES-NI CPU instructions but with SSE4.1. I'm using Linux and tried the (in-memory) cryptsetup benchmark to get the rough tendency which returned: ...
357 views

Trying to find related answer for a long time, but not convinced yet. What I am trying is to encrypt using RijndaelManaged. To create Key, I am passing password, ...
252 views

### How to attack this authentication protocol from “Cryptography: An introduction”

Consider the following protocol (from the book "Cryptography: An introduction", by Nigel Smart): ...
465 views

### Two-timing a one-time pad [duplicate]

Is a one-time pad still breakable on a depth of 2+ if the plaintext is completely random? I'm assuming this idea is wrong, mostly because it occurred to me, but let me walk you through my logic so ...
737 views

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?
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2k views

### Using one-way hash functions as the encryption method

Suppose two parties want to communicate securely with each other (Bob and Alice) using a simple messaging system in English. There are approximately 180,000 currently used words in the English ...
229 views

### Does impersonating an SRP server give you enough information for an off-line dictionary attack?

In a comment to an answer I wrote to another question, CodesInChaos wrote that: "Problem with SRP is that an attacker who impersonates a server learns the password hash, enabling offline search." ...
260 views

### Quadratic residuosity problem reduction to integer factorization

How can one show how to reduce the quadratic residuosity problem to an integer factorization?