# All Questions

2answers
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### 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. ...
1answer
523 views

### Blind signature with openssl

I'm trying to understand (to reproduce in practice) how does blinded tokens work. Currently i'm lacking examples. According to Wiki the blinded signature protocol is the same as ordinary signature ...
1answer
669 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 ...
1answer
48 views

### Could a strong round function be immune to slide attacks

An excerpt from the wikipedia article on slide attacks states: ...The only requirements for a slide attack to work on a cipher is that it can be broken down into multiple rounds of an identical F ...
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329 views

### For RSA keys, is the safety of a given key-length different for signing vs encryption?

I'm wondering if considerations of key-length are different when thinking about signing vs encryption. If for some reason, it was safter to use smaller keys for signing that would be interesting.
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517 views

### Making my steganography code more hard to detect and crack

I'm doing a college project about digital image steganography on MATLAB. So far i've been able to get the help i needed from cool guys on stackoverflow but i now need to make my algorithm more hard to ...
1answer
203 views

### RSA time function

I am doing a paper on textbook RSA (highschool-level). I am explaining and proving the significance of the public key exponent in time and security. So far, I have encrypted a constant message with a ...
1answer
308 views

### Protocol for Randomized Oblivious Transfer?

If we define Oblivious Transfer as following: Alice inputs $(x_0,x_1) \in F^2$, where $F$ is a field, and Bob inputs $b\in\{0,1\}$, then Alice gets a dummy output(for which she knows nothing about b),...
1answer
581 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): ...
2answers
212 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 ...
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 $s=32$;...
3answers
326 views

### Cryptographic data structure: sparse array without membership test

I would like a data structure supporting two operations: $\mathsf{set}(k,v)$ and $\mathsf{get}(k)$, with the following properties: $k$ can be any number between $0$ and something impressively large, ...
2answers
131 views

### Hash functions vs Stream Ciphers in terms of Speed

We know that it's possible to create a hash using a stream cipher (for instance, RC4 or AES in Counter Mode). As stated here: "A Pseudo Random Generator (PRG) and a hash function are both Pseudo ...
3answers
207 views

### paradox on fully homomorphic equality checking

Imagine, a client encrypts a corpus of data (say documents of text) with the public key of a Fully Homomorphic Encryption scheme (FHE) and outsources the data to an untrusted server.Now the client ...
2answers
135 views

### Is there an asymmetric encryption algorithm where the public key cannot be derived from the private key

Is there an encryption algorithm that will produce a public key and a private key such that the public key cannot be generated using the private key. For example, if the private key is compromised ...
2answers
211 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 ...
1answer
705 views

### True Random Number Generator by milliseconds per keystroke (TRNG-Kms)

The simplest way to generate truly random numbers for OTP keys is to measure the time in milliseconds between each keystroke on a keyboard. The randomness depends on the user typing in various speeds. ...
1answer
121 views

### What is Deterministic Authenticated Encryption?

I came across something known as deterministic authenticated encryption in my studies and a lot of people were associating it with Synthetic IV mode. I am having trouble what exactly DAE is because I ...
1answer
104 views

### Using HMAC as a nonce with AES-CTR encrypt-and-MAC

Suppose I use the following encrypt-and-MAC construction: $E(k_1, k_2, n, m) = E_\text{AES256-CTR}(k_1, n, m) \| \text{HMAC-SHA256}(k_2, m)$, where: $k_1$ and $k_2$ are 256-bit keys $n$ ...
1answer
95 views

### Changing encryption key without revealing the original key

Say that Alice encrypts a $\text{plaintext}$ with $\text{key}_{m}$ and gives the $\text{ciphertext}_m$ to Bob. Alice wants to send several gigabytes of $\text{plaintext}$ to Eve but she is on a mobile ...
2answers
536 views

### Can I construct a feasible stream cipher out of HMAC and a secure hash algorithm?

I have constructed a stream cipher from a secure hash algorithm and a HMAC. Here is a brief description of the algorithm: Let: (Actually Objective-C styled pseudo code) ...
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1k views

1answer
88 views

### Can I store the initialisation vector (IV) in the filename?

I'm encrypting a file and I'm using an initialisation vector. Given that the IV doesn't need to be a secret, is it secure if I add it to the encrypted file's name?
2answers
146 views

### Encrypt-then-HMAC implementation - am I doing it right?

I want to implement authenticated encryption using C#. There is a dot net class called encryptAndAuthenticate, but it is only supported on windows 8 or later, and I need the code to also work on ...
1answer
124 views

### Are there comprehensive alternatives to RSA?

If we wished to, is there a comprehensive alternative to RSA? I say comprehensive as I wonder if there is one which does both encryption and digital signature like RSA? If not, simply what ...
1answer
244 views

### Will hashing over and over eventually give the same hash?

There are many cases in maths where doing the same thing over and over again will bring you closer to a value. Derivation of polynomials is the striking example: no matter what you start with, you end ...

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