# All Questions

145 views

### Pick faster private exponent

I recently tried to send 1536-bit modulus CSR to COMODO. They refused to sign the certificate. I later found out that it's because NIST mandated 2048-bit modulus on the SSL certificate. I think it's ...
484 views

### How (in)secure is a signature based on DSA with DSS1 (SHA1) in reality?

As I found out the OpenSSL implementation provided by PHP (openssl_sign) only allows DSS1 as the hashing algorithm for DSA signatures (see also my related question). I’d prefer DSA over RSA in my ...
128 views

### Three different numbers with x³=x mod p

p is a prime greater than 2 and $a \in \mathbb{Z}_p$. Why are there exactly three solutions for a³ = a mod p? Obviously 0 and 1 are both in $\mathbb{Z}$ and valid solutions, but that still means, ...
175 views

### Modulo properties of two prime numbers

I am supposed to prove that x = y mod (p*q) <=> x = y mod p and x = y mod q with p and q are prime numbers. It somewhat sounds reasonable to me, but unfortunately I don't have any clue how to prove ...
57 views

### server aid secure equality test

The scenario is like this: $P_1$ has $m_1$, $P_2$ has $m_2$, they want to compare whether $m_1=m_2$ via the server $S$. After the protocol, only $S$ knows the result, and there is no interaction ...
591 views

### Can I create a fixed length output from a fixed length input?

I use AES256 for various purposes in my app. However, the purpose I am now trying to achieve is the following: Input is for example AU00000001 and I would like a ...
180 views

### Is there an oblivious decryption scheme?

Alice has $K$; Bob has $E(K, m)$; Is there such a scheme that enables Alice decrypts $E(K, m)$ without knowing $m$, and Bob gets $m$ ?
203 views

### Universal Circuits in Indistinguishability Obfuscation Candidate Construction

I am currently working on an implementation of the candidate construction for an indistinguishability obfuscator that was recently proposed by Garg et al. The relevant paper can be found here. ...
199 views

### Padding to increase entropy and size of (otherwise) small JSON objects

Maybe this better fits, the security stackexchange... I'm building a web API. We want to enable some relatively simple encryption, instead of going full-blown OAuth2 (which frankly, IMO, sucks). So I ...
141 views

### Proxy re-encryption mod operations

I managed to implement the proxy re-encryption scheme from http://eprint.iacr.org/2009/189.pdf in Python 2.7, however I am having performance issues. As it is, I can run the algorithm for key sizes up ...
185 views

### Comparing two values without revealing them

Suppose Alice wants to prove to Bob that t1 is less than t2. t1 and ...
135 views

47 views

### P = NP security implications [duplicate]

If someone finds an efficient way (ridiculous or not) to turn NP into P (such as creating multiple universes, saving the one with the correct answer), would we have any good algorithms for encryption, ...
194 views

### Is CTS necessary on a AES-CBC DRBG?

As I understand it, CTS pads the last block and swaps it with the second last to compensate for a partial block of data. I have written a DRBG (Deterministic Random Bit Generator) using AES-CBC that ...
151 views

### Twisted curves in protocol

I've come to understand that twisted curves, as for instance defined in the Brainpool specifications, are $F(p)$-isomorphic to their regular $F(p)$ equivalents. So brainpoolP256r1 is isomorphic to ...
196 views

### Hamiltonicity proof of knowledge

I'm learning the POK notion and definitions and as a self exercise I wante to prove the statement that the Hamiltonicity protocol is a POK system with knowledge error $1/2$. So the question will be ...
75 views

### Theory for pre-paid debit card: card-to-card transfer [closed]

In Sweden in the late 90's they deployed a electronic prepaid cash system system, and if I remember correctly you could transfer money between cards without online access. Wikipedia states that the ...
199 views

### Number of different substitution alphabet ciphers possible with given conditions?

If you consider a 26 letter alphabet, and a cipher where 24 of the letters are sent to themselves, and only 2 letters switch, how many different substitution alphabet ciphers are there, and what ...
114 views

### How can I process data within a cloud securely?

I'm new to programming and I'm getting ready to design a proof of concept for an app that I'm thinking of building. Problem is I'm a complete noob to cryptography and need a little nudge to point me ...
410 views

### Security of a security protocol for key exchange, using symmetric-key cryptography

This is an exam question: $A \to B$: Alice, $nonce_A$ $B \to A$: $nonce_B$, $E_{k_{AB}}(nonce_A \| k_1)$ $A \to B$: $E_{k_{AB}}(nonce_B \| k_2)$ Alice and Bob both compute $k_s$ = $k_1$ xor $k_2$ ...
458 views

### Precomputation attacks on RSA

Are precomputation attacks - such as outlined in RFC 3610 chapter 5 - possible on RSA PKCS#1 v1.5 signature generation? If yes, are such attacks taken into account when calculating the cryptographic ...
102 views

### Use additional keys to thwart key compromise?

Is it good or bad practice to design crypto protocols for key compromise by using additional keys? Argument for bad practice would be: When you have a key you should trust it and not throw more keys ...
284 views

### Measure ECC key size

I have implemented a ECC key generation scheme successfully. Now I need to find ECC key sizes of each generating key pairs. I assumed that ECC key size is the size of the ECC private Key. So I would ...
845 views

I would like to know what (if any) are the advantages of using Montgomery Power ladder over the Double-and-Add-Always algorithm. I think that firstly, Monty would be slightly faster than DoubleAndAdd. ...
380 views

### Is it a requirement to understand mathematics when implementing (or breaking) cryptography?

Related: What is the lowest level of mathematics required in order to understand how encryption algorithms work? and Recommended skills for a job in cryptology In the context of putting crypto into ...
357 views

### How Does Progressive Hashing Work?

Example: http://jsfiddle.net/KGtUE/ https://code.google.com/p/crypto-js/#Progressive_Hashing sha256.update("abc"); sha256.update("def"); sha256.update("ghi"); ...
411 views

### How to prove identity without revealing identity

Let us say Alice publishes a book under the name of Claire. The book becomes wildly popular and now Bob comes along, claiming to be Claire, to reap all the success. How does Alice prove that she wrote ...
240 views

### Choice of MAC and handling it correctly

I have recently learned that Encrypt-then-MAC is recommended when using symmetric encryption. I have two closely related questions related to that, which is why I am posting them as one: Looking ...
1k views

### Encryption of log files

I'd like to encrypt the contents of a log file that my application writes. The log system has the following characteristics: The file format is binary. Log files are left open while the application ...
102 views

### How can I make a message into a blocks for NTRU?

I have more bits than the degree of the NTRU polynomial. For example: $x^{11}-1$ message: Hello (I am using ascii, 8 bits for 1 symbol) got: ...
145 views

### Parallel hash construction

I'm wondering why hash constructions aren't parallel like CTR mode for encryption? For example: We have ideal block cipher E, where we encrypt message block with counter and xor output block with ...
99 views

### One Time Pad for large changing files

i wondered what the security implications are if I do the following. If I had a large file encrypted with a OTP and want to change only a few bytes in the plaintext. what security vulnerabilities do I ...
130 views

### Correct way to read a given permutation cipher?

In some literature I am reading – where they are explaining the permutation cipher – they have two examples, which encrypt a small block of text. But they seem to contradict each other: First ...
1k views

### Creating a license system based on asymmetric encryption (RSA or ECDSA)

I’ve spent a couple of days researching the topic of creating a license system for my desktop software. While I fully understand that there’s no perfect copy protection, this approach seems to have ...
898 views

### RSA: Fermat's Little Theorem and the multiplicative inverse relationship between mod n and mod phi(n)

I'm learning about the proof of the RSA encryption algorithm, and I'm clearly fudging or missing something, because for me it doesn't add up. When generating keys for RSA encryption, we make sure ...
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134 views

### Encryption time in ECC

In RSA, encryption time is usually much less than decryption time due to having a small public exponent. Can this be achieved in Elliptic Curve Crypto (ECC)?
45 views

### Can anyone explain how the modified r-adding walk works?

I was going through a paper titled “Accelerating Pollard's Rho Algorithm on Finite Fields” by Jung Hee Cheon et al. I understand the table(Ml) creation part of it, but after that I somehow fail to ...
210 views

### How can one parallelize tasks in CTR-AES for maximum performance?

From what I have read it seems like one of the purported benefits of using CTR mode AES, is that it can be parallelized to a greater degree, or maybe more easily than the other block cipher modes, ...