3
votes
2answers
606 views

How does blocksize affect security?

I have recently read about how blockciphers and how stream ciphers work, and although I think I finally got everything right, I'm still wondering how increasing blocksize increases security.
3
votes
2answers
117 views

Cryptography survey as an undergrad

Is cryptography so specialized that taking a survey class is a waste of time? I'm not the type of person who can do logs in my head or has a deep mathematical aptitude, but cryptography is certainly ...
3
votes
1answer
163 views

Unable to understand hash function representations

I have some hash function representations, which I find it hard to understand. I have searched a lot but didn't found anything helpful. Please help me understand the hash function representations ...
3
votes
2answers
199 views

Is it possible to choose which point will have the public key of a given Elliptic Curve?

I am wondering if there is a feasible way that, given a specific elliptic curve (such as secp256r1), I could create a keypair where the public key has a given $x$-coordinate. If it is not possible, is ...
3
votes
2answers
370 views

Is there a way to systematically calculate the public exponent $e$ in RSA?

I'm learning RSA in one of my classes and we were given a problem: $p = 5$, $q = 11$ I have done the following steps: $n = 5 \cdot 11 = 55$ $\phi = (5-1)\cdot(11-1) = 40$ I know that to ...
3
votes
3answers
696 views

How to perform frequency analysis of a substitution cipher using a Base64 alphabet

Let's imagine a cipher that works like the following: Plaintext is encoded to Base64. The characters in the encoded plaintext are substituted with a randomly shuffled character set(A-z, 0-9, -, _, =)...
3
votes
2answers
175 views

Does a stream cipher require a KDF when it's key is variable length?

RC4 has a variable length key. Is it still necessary to use a key derivation function, such as PBKDF2, to initialize it? Would using a single character password weaken it's output when compared to a ...
3
votes
1answer
840 views

Theoretical attack on RSA

The high security of RSA is granted, because it is very hard to factorize $$ N = p * q $$ Nevertheless, there is actually no need of factorizing $N$, in order to generate the $Private$ $Key$, but the ...
3
votes
2answers
226 views

Diffie-Hellman insecure on addition modulo $n$

Assume that the group $G$ is the set $\mathbb{Z}_{n} = \{0,\ldots, n-1\}$ for a 1024 bit integer and $+$ is addition modulo $n$. Then why would Diffie-Hellman key exchange in this group be insecure?
3
votes
2answers
603 views

RSA performance

I was wondering what is the performance bottleneck in RSA algorithm? Is it the size of message, calculating the modular inverse, exponentiation? Say I have a fixed value of n to encrypt and decrypt ...
3
votes
3answers
818 views

Is there a cryptographic method to add noise to a plaintext instead of actually encrypting it?

I'm looking for a cryptographic technique that instead of permuting a cleartext, would add noise to the cleartext to obfuscate the true text among a bunch of garbage. For example, a list of 10 ...
3
votes
2answers
3k views

What happens when a root CA has its private key compromised?

What happens when a root CA has its private key compromised? Then all children in the tree are compromised too? And then all certificates are compromised? What needs to happen then? Related: - How ...
3
votes
3answers
979 views

random access stream cipher

Some time ago I was working in a pet project for a type of database engine. And I thought about securing the data in disk with encryption.. and since the main code was already developed.. I had a few ...
3
votes
1answer
11k views

Difference between linear cryptanalysis and differential cryptanalysis

What is the main difference between linear cryptanalysis and differential cryptanalysis? For example in relation to DES?
3
votes
2answers
538 views

Finite fields and ECC

I understand modular arithmetic(or at least I think I do!) and I've tried to read and learn about how the Math in RSA works(and I think it went pretty well). I've been reading up on ECC and it looks ...
3
votes
2answers
515 views

Map Bytes to Number

Using node.js crypto library I can generate cryptographically strong bytes. How can I use this to generate a random number between 0 and 9999 (both inclusive)?
3
votes
4answers
3k views

Why is RSA encryption significantly faster than decryption?

I am designing an implementation of RSA . I recorded computation times in Java by using System.currentTimeMillis(). It returned an encryption time of 0.05 ms and ...
3
votes
3answers
234 views

Can the encryption exponent e be greater than ϕ(N)?

So I was just wondering in RSA, can the encryption exponent e be greater than ϕ(N)?? For an examples sake, lets just say N = 707, so p = 101 & q = 7. So, we have ϕ(707) = 600. Can I have e = 707?...
3
votes
2answers
755 views

Calculating cycles per byte

Cycles per byte seems to be a critical concern in designing and choosing stream ciphers. For example, from Wikipedia, RC4 has 7 cycles/byte on original Pentium chips. How is this calculated? Do they ...
3
votes
2answers
3k views

How to perform file encryption using 128-Bit AES?

I am confused, how can I encrypt a file using 128 Bit Advanced Encryption Standard? Do I need only to encrypt the file name and it's content or is there something that I need to do to encrypt it? Is ...
3
votes
2answers
2k views

Why do all hash functions use big-endian data?

I've looked at several hash function specifications, and they all emphasize the need for big-endian byte ordering. Is there a reason for this that has to do with security, or is it simply convention?
3
votes
2answers
697 views

Are there public slow-but-strong algorithms out there that resist brute-force attacks better?

I'm reading that AES uses 4x4 bytes (4*4*8 = 256 bits key) matrix for performances matters (since it's a requirement for common standard encryption algorithms), but are there implementations with ...
3
votes
2answers
64 views

Why DES s-box non-linear? Why it makes the cracking of the cypher more difficult?

I know that if we have a cypher which makes only linear transformations (let's say a bunch of $XOR$'s), we break it simply writing a system of equations with $\oplus$ operation starting from the ...
3
votes
1answer
68 views

How to meter and limit number of files encrypted?

First my apologies if this isn't the right forum for this but here's my problem : I have a black box that takes an input file and produces an output file. I want to sell this black box to a customer ...
3
votes
1answer
67 views

Insecure third party connection

My organization processes some PII data and shares it with an outbound connection (outflow only) that is a third party. The data transfer transactions occur without human intervention. That outbound ...
3
votes
2answers
195 views

Should I use XTS or GCM to encrypt my hard drives?

I want to start encrypting all of my hard drives, but I don't know whether to choose XTS or GCM mode. Why is it that XTS is recommended (since the most websites I visit use GCM in their HTTPS ...
3
votes
2answers
96 views

GCM: use the same key:IV pair to encrypt file and filename

Using Java, I wrote a piece of code that encrypts a given file with AES in GCM mode of operation. So far nothing uncommon. Now I would also like to encrypt the filename, in the following way: ...
3
votes
2answers
308 views

RSA: Recover e given factorization of N and plaintext/ciphertext pair

Is there a way to recover the public exponent e (assume in this case it is in fact not public) used in an encryption if I know the following: Factorization of <...
3
votes
2answers
703 views

Is AES solvable by reducing to SAT?

Consider a known plaintext attack on AES — just so we have an actual system of equalities that we can feed to a SAT solver. Is AES solvable in this way? In other words, will the algorithm eventually ...
3
votes
2answers
238 views

Constant time multiplicative inverse within a word

I was playing with an algorithm which at one step, calculated $f(x) = x^{-1} \mod p$ for $0 < x < p = 2^{64}-59$ (note $p$ is a prime). I used Knuth's Vol 2 Algorithm X algorithm for calculating ...
3
votes
1answer
100 views

How can there be AES-256-GCM, when GCM is defined for 128-sized blocks?

In both node.js and OpenSSL, I am seeing cipher "aes-256-gcm". However, in NIST Special Publication 800-38D, that defines GCM, I am reading: The underlying block cipher shall be approved, the ...
3
votes
3answers
216 views

Does complicating a flawed algorithm make it secure?

It's a basic question I guess, but I don't know the answer and I don't think that anyone asked it. Suppose I built a system, it relies on MD5 for security, suddenly I read am article on how easy are ...
3
votes
3answers
697 views

Is it usual for a signed message to contain the public key of the signer?

In protocols/systems which use ECDSA, is it common for the message to contain the public key of the message signer? If not, how do recipients know that the message has been signed by the correct ...
3
votes
2answers
422 views

Are all possible EC private keys valid?

I usually generate a key pair using openssl or Bouncy Castle. I'm using curve secp256k1. The 256bit private keys look fairly random. Do all values of "private ...
3
votes
2answers
513 views

Is it safe to reuse a symmetric key when using asymmetric encryption?

I've written a C# Library that I hope will be used for security purposes in a commercial application. The application will communicate through the internet and not on an internal network. I am using ...
3
votes
2answers
160 views

Combine two sha512 hashes to a single hash

Assume I have two data fragments: $Frag_1$ and $Frag_2$. I can build sha512 hashes the usual way with $SHA512(Frag_1)$ and $SHA512(Frag_2)$ and then hash the two fragments appended to each other: $$...
3
votes
2answers
128 views

Is it OK to use a hash of the key as nonce for AES GCM?

I don't see an immediate risk of using the hash of the key as the nonce for AES GCM. Is there something I could be overlooking?
3
votes
1answer
253 views

How is information disclosed by modular multiplication?

Consider the case that $c = a \cdot b \mod p$ where $p$ is a known prime and $0 < a < p$ and $0 < b < p$ are unknown integers numbers. Furthermore, some bits on the value of $c$ are ...
3
votes
3answers
409 views

Fast PKI for embedded device

We are creating a device with a small microcontroller (20 MHz CPU 16 KiB RAM). We need some way to securely send signed files to device (only signature, no encryption necessary). An external company ...
3
votes
2answers
221 views

What, cryptographically speaking, is a “long message”?

I have read about long message attacks on some cryptographic hash functions. However, I don't quite understand what is being referred to as a "long message". Also, do long message attacks only apply ...
3
votes
3answers
259 views

SRP-6 vulnerabilities when N is small

I'm one of the developers of an application which uses SRP-6 as the authentication mechanism. The authentication part of the code is very old and uses N with only 256 bits (all arithmetic is done in ...
3
votes
2answers
393 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 ...
3
votes
5answers
390 views

Effective security of block cipher – equal the key size, or half the key size?

The Wikipedia “Key Size” article states: The security of an algorithm cannot exceed its key length (since any algorithm can be cracked by brute force), but it can be smaller. … … … Most symmetric-...
3
votes
2answers
2k views

What curve and key length to use in ECDSA with BouncyCastle

I'm developing a client/server system in Java which is not interacting with third party software, so I don't have to worry about compatibility. At a certain point, I need the client and server to ...
3
votes
3answers
201 views

Do sites store login password with hash? If so, can people can use hash collision to log in?

I was researching about hash, and I thought, If sites store passwords with hash algorithms, then can't this happen: User A has the password 'hello' User B finds out the hash code of the password of ...
3
votes
2answers
417 views

Avoiding overflow when encrypting with RSA

When encrypting with RSA one calculates $ m^e \pmod n $ by doing the following: m^e % n Where $m$ is what we encrypt. Often $e$ is a very big number to make it ...
3
votes
2answers
266 views

Can you really insert the text you want in one-time pad?

The Wikipedia article "One-time pad ~ Authentication" says : For example, an attacker who knows that the message contains "meet jane and me tomorrow at three thirty pm" at a particular point can ...
3
votes
4answers
1k views

random number generator 10-side dice alternative

A lot of sites (e.g. Dirk Rijmenants') refer to 10-side dice to generate random number for one-time-pad. I was thinking about new ways to generate random numbers, letters and passwords. here what I ...
3
votes
1answer
991 views

CBC-MAC , fixed length, all blocks returned

CBC-MAC, with fixed length message. Is it safe to return all ciphered blocks instead of the last? My intuition says it is less secure, since is gives an attacker more information. But how could one ...
3
votes
1answer
1k views

Can we replace the XOR operation in DES with some other operation?

Can we replace the XOR operation in the DES algorithm with some other operation? If so, does it work for both encryption and decryption?

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