0
votes
1answer
58 views

RSA signature attack check on lowest 16 bytes - Implementation

I am referring to solution given in this question I need help to implement Poncho hint in this comment: As the first question i have an RSA signature of 1024 bit applied on an hash (MD5) of the ...
8
votes
1answer
1k views

Why do crypto libs use table lookups when they're vulnerable to timing attacks?

AESEngine.java, from BouncyCastle, uses table lookups as does aes_x86core.c, in OpenSSL. But per Cache-timing attacks on AES table lookups like what OpenSSL and BouncyCastle are doing are vulnerable ...
1
vote
0answers
28 views

FFX Encryption over alphabet of arbitrary radix

I understand how standard numeric FPE using a halved string in cycled Feistel networks is constructed and operates. I then read the paper regarding FFX encryption which should allow for any non-binary ...
-3
votes
1answer
41 views

RSA problem - how to find $d$ [duplicate]

For example, let $p = 3$ and $q= 11$, choose $e = 3$. What computation I have to apply in order to find the corresponding $d$? I know it's 7, but I want to know the exact process to be applied to find ...
0
votes
0answers
24 views

Enterprise message bus encryption [migrated]

I'm contemplating using Amazon's Simple Queue System as the basis of an enterprise message bus. According to Amazon, the contents are not encrypted, which means the application(s) will have to handle ...
0
votes
0answers
39 views

RSA Encryption: What happens if n is a factor of the message M? [duplicate]

From what I have learned about RSA encryption, the message M and the modulo n must be coprime because Euler's theorem only holds for coprime numbers? for example, what happens if I choose p = 3, q = ...
2
votes
1answer
56 views

Can string lookups be vulnerable to timing attacks like array lookups are?

Per “Can non-assembly crypto libraries truly be secure against timing attacks?”, array lookups are vulnerable to timing attacks. Like, if you do cresult[map[i]] and ...
4
votes
2answers
171 views

Customizable Crypto Algorithms in Hardware

Several companies offer customized cryptographic algorithms, for example Crypto AG (www.crypto.ch/en/company): The individually created customer algorithms are unknown to anyone else and are not ...
2
votes
0answers
29 views

Public SRP verifiers or public hash chain “public keys” when secret is low entropy password

I want to set up the following: An untrusted server should host user sessions and authenticate users without knowing their passwords or being capable of creating a fake user session in the user's ...
0
votes
1answer
35 views

Key exchange resilience to DoS attack

I am designing a user registration protocol for a system where I control both clients and servers. The aim is to establish a symmetric key for subsequent authentication, while minimizing the work ...
2
votes
2answers
71 views

Is Checksum to file back decoding possible?

So, I was having this weird thought. If the checksums(MD5 or SHA...) are unique to any file and are modified if any part of the file is edited, why cant the checksums be decoded back to get the ...
2
votes
1answer
66 views

Is Cramer-Shoup backdoorable?

We recently had the question whether it's possible to have multiple private keys with one public key for the cramer-shoup cryptosystem. There it was stated that finding such "secondary" private keys ...
2
votes
1answer
52 views

CP-ABE for threshold cryptography

I would like to know if there is any research work using CP-ABE for threshold cryptography? I want to know also if applying CP-ABE for threshold cryptography would overcome other techniques.
2
votes
1answer
52 views

Extend OTP on random data?

If Alice and Bob both start with a shared OTP $P_0$, which is 256-bytes long, and Alice wants to send a 512-byte message, would it be secure to send the first 256 bytes with standard OTP (...
7
votes
1answer
115 views

Can non-assembly crypto libraries truly be secure against timing attacks?

http://cr.yp.to/antiforgery/cachetiming-20050414.pdf states the following: So what went wrong? Answer: NIST failed to recognize that table lookups do not take constant time. “Table lookup: not ...
2
votes
1answer
57 views

In SSL and TLS, why is there a separate Change Cipher Spec Protocol rather than including a change_cipher_spec message in the Handshake Protocol?

In SSL and TLS, why is there a separate Change Cipher Spec Protocol rather than including a change_cipher_spec message in the Handshake Protocol?
1
vote
1answer
37 views

ssh-keygen DH Primality Testing

I'm pretty familiar with using ssh-keygen to create groups that go in the /etc/ssh/moduli file for the Diffie-Hellman Group Exchange in openssh. Reading over the man page, it says "By default, each ...
5
votes
2answers
85 views

More efficient and just as secure to sign message hash using Ed25519?

According to the Ed25519 paper, the (potentially long) input message is hashed twice (see Section 4 page 12 steps 1 and 3). This webpage has a nice diagram toward the bottom that illustrates this, ...
4
votes
2answers
119 views

Implementing CBC Encryption Using Decryption

I've implemented this algorithm, which, working from the end of the message backwards, creates a valid CBC ciphertext from any plaintext, using the block cipher's decryption operation instead of the ...
0
votes
1answer
36 views

can cramer-shoup public key have two difefrent private key?

since I am new to Cramer Shoup algorithm I want to know is it possible to have 2 different private key for a public key due the key pair generation algorithm that Cramer Shoup use? I know that in ...
3
votes
0answers
27 views

Is modular Barrett reduction usable for Ed25519?

I currently try to implement Ed25519 following this draft. I have implemented every field operation except the reduction by the group order $q$ (in chapter 5). I looked for a reduction algorithm and ...
1
vote
1answer
41 views

Signature Verification: High level

I'm trying to understand how certificate signing and verification works conceptually. I know the classic example with asymmetric cryptography- the sender encrypts data with public key of receiver, ...
0
votes
0answers
7 views

How to correctly convert TAG value to the right format so that to Verify HMAC? [migrated]

I'm working on HMAC generation and verifying to check data integrity. I can correctly generate the MAC value but when sending it through socket to another program for verification, I faced with ...
0
votes
1answer
34 views

Is constructing IND-CCA2 public key encryption schemes particularly easy with the KEM/DEM approach?

After the introduction of McBits, I was interested what security notions are neccessary for IND-CCA2 security of integrated encryption schemes (IES, following the key encapsulation mechanism / data ...
1
vote
1answer
86 views

Is the GOST block cipher broken?

Is the GOST cipher broken or known to be weak and insecure? Please try to simplify your answer since I am a programmer and not a cryptographer and please consider this web page and tell me if the ...
1
vote
1answer
45 views

Encryption algorithms larger than 256 Bit for “big data” encryption?

I'm somewhat new to encryption. When looking at encryption programs for big data, I frequently see a maximum of 256 bits. Why do we generally restrict our (symmetric) keys to 256 bits? Can more ...
2
votes
1answer
65 views

One time pad ciphers in emails

In order to achieve very high security for privacy, would it be cryptographically secure to use one time pad ciphers in emails? The distribution of the keyword would pose no problems since I would ...
1
vote
0answers
13 views

Are there any paper/human strong crypto systems? [duplicate]

Are there any paper/human executed crypto systems that have reasonable strength? I am aware of a few candidates so far Solitare from Cryptonomicon https://en.wikipedia.org/wiki/Solitaire_(cipher) ...
0
votes
1answer
31 views

ElGamal in $Z^*_{p^n}$

If $p$ is an odd prime and $n$ natural,it is known that the group $Z^*_{p^n}$ is cyclic.Explain why the selection-choice of the group $Z^*_{{3^{1000}}}$ for the construction of a cryptosystem ElGamal ...
5
votes
3answers
266 views

What are the practical implications of ciphertext distinguishability?

Commonly there are four ways to "break" a secrecy-focused cryptosystem: Recover the secret key Recover the message Distinguish an encryption from random noise Distinguish the encryption of two ...
1
vote
1answer
46 views

How AES treat string input to encrypt data and what will happen if the cipher key is less than that expected?

The official docs of AES have several example of how hexadecimal or byte input can be encrypted. I have some problem about the input text and cipher key. My question is: How can encrypt string input ...
2
votes
2answers
63 views

What is the advantage of AEAD ciphers?

What is the advantage of AEAD ciphers? Why is the TLS working group pushing for them? I thought modern cipher suites require SHA256 for authentication. What advantage is there to including Poly1305? ...
0
votes
0answers
15 views

Cryptanalysis on two time pad ciphers? [duplicate]

One time pad ciphers are unbreakable if the message is encrypted correctly and if only used once. However if used more than once, it becomes decipherable. How come? Also, would it be smart to use one ...
1
vote
2answers
44 views

Let $F$ be a PRF, is the following $G(x)=F_{0…0}(x)∥F_{0…1}(x)|| … ||F_{1…1}(x)$ a PRG?

$F$ is a PRF with $n$ bit key, input, and output length. I know the answer is "NO it isn't!", but I can't understand why. Is that because on any input $x$, the keys for every $F$ in the concatenation ...
0
votes
0answers
33 views

How to verify perfect secrecy of a cryptosystem? [closed]

I’m playing with a self-made encryption algorithm: “a new approach to data encryption” by Suhail Yazji (video available at youtube.com/watch?v=h1kABn5dfXc). How can I prove my algorithm to be ...
2
votes
2answers
82 views

Present an attack for the combination of OTP and textbook RSA

Let $N=p \times q$ where $p$ and $q$ are two primes. Compute $\Phi (N) = (p-1)(q-1)$. Choose $e$ s.t. $1 < e < \Phi(N)$ and $gcd(e,\Phi(N))=1$. Compute $d = e^{-1}(mod \ \Phi(N))$. ...
1
vote
0answers
34 views

Prove that certain amount of data was stored

I'm looking for a way to prove that a certain amount of data was stored, through some easily verifiable piece of information. Similarly to how proof-of-work can prove through a hash that a certain ...
2
votes
2answers
69 views

Can we solve the Hidden Number Problem in $GF(2^n)$?

Is it possible to solve the Hidden Number Problem in extension fields? In particular in $GF(2^n)$? Let's suppose an attacker knows some least/most significant bits of $r_i = a_i \times k$ in a given ...
1
vote
1answer
49 views

Knowing that $G(s)$ is a PRG, is the following construction $G'(s) = G(s||0)$ a PRG?

I know there already is a very similar question. However, I can't understand why $G'(s)$ is not a PRG. If $G(s)$ is a PRG, then why $G(s||0)$ can't also be a PRG? How can the distribution over ...
1
vote
1answer
31 views

For $p=23$, $g=2$, $q=11$ you are given the signature $(18,3)$ in the message $m=2$. Construct a signature in the message $m'=3$

Alice uses an ElGamal signature in group $(Z/pZ)^{*}$ without using hash function. To sign the message $m \in (Z/pZ)^*$ calculates the signature $(r,s)$ as follows: choose random $k \in \{0, 1, ...
4
votes
0answers
80 views

Attacks on elliptic-curve based cryptosystems through solving the Decisional Diffie-Hellman Problem with the Weil Pairing

Are there any examples of practical attacks on cryptosystems set over elliptic curves which utilize the easiness of DDH for certain choices of curves $E(\textbf{F}_q)$, and as such their lack of ...
0
votes
0answers
16 views

Light weight crypto library implementations [migrated]

Are there any open source crypto libraries that can be used in embedded systems or in other memory constrained places like a boot rom? I am looking for libraries that I can compile only the algorithms ...
1
vote
0answers
32 views

What consequences do the plaintext space size has on the performances in the BGV scheme?

In the BGV paper [1], the authors say in §5.4 that you can have $\mathbb{Z}_p$ as plaintext size with a large $p$. What is the impact of the size of $p$ on the ciphertext size and computational work ...
3
votes
2answers
113 views

Is there any such thing as “proof of location”?

I'm looking for a way for someone to prove to me their geographical location without a trusted third-party. Imagine wanting to prove to someone you're actually physically in a specific place. I'd ...
1
vote
0answers
51 views

ElGamal signatures systems

Let $p$ prime number $q/p-1$ prime and $g\in (Z/pZ)^*$ element of order $q$.Also $a\in \{1,...,q-1\}$ the private key and $y\equiv g^a\pmod p$ the correspoding public key.For each of the following ...
2
votes
3answers
90 views

Attack on Double Encryption Scheme

In this protocol: A → B : { <[A, {K}pk(B)]> }pk(B) B → A : { <[B, {K}pk(A)]> }pk(A) I'd like to determine why this is insecure. How can the attacker learn the key K if A and B are both ...
-2
votes
1answer
36 views

In which step does AES use the key to encrypt data?

AES has the following steps in encryption: SubBytes, ShiftRows, MixColumns, and AddRoundKey and the following steps in decryption: InvSubBytes, InvShiftRows, InvMixColumns, and Inverse of AddRoundKey. ...
4
votes
2answers
153 views

Signature and hash function properties

I understand the need for the hash function to be collision resistant and second pre-image resistant. For what reason, exactly, does a hash function need to be pre-image resistant? If this property ...
1
vote
1answer
54 views

Playfair Cipher + Vigenere Cipher?

I'm researching cryptography for a school project, came across the above two ciphers, and something occurred to me. Would combining these two schemes give a considerably stronger encryption than ...
1
vote
0answers
61 views

Library to find an addition chain for a large number? [closed]

I need a short addition chain for a number $n>2^{100}$ in order to implement a fast exponentiation. The memory footprint is not important. Finding an optimal chain for a large number $n$ is ...

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