Latest Crypto related questions

What uses are there (if any) for asymmetric encryption of a message, instead of hybrid encryption? Specifically, I'm not considering asymmetric encryption of a symmetric key (eg old TLS RSA key exchange) as asymmetric encryption of a message, nor am I considering KEMs or (EC)DH as asymmetric encryption of a message, since all these methods are used to set up symmetric encryption.

I'm interested i ...

The formatting function in Appendix A of NIST-SP-800-38C is a monster enabling CCM to support variable-length nonce from 7-13 bytes, variable-length encoding of the length of the payload. Also, the tag length is encoded in the formatting function making naive truncating of the MAC tag potentially incompatible with ciphertext instances with specific parameters.

The GCM mode and ChaCha20-Poly1305 are much  ...

How can I solve this problem: I have a directed graph of nodes that can be malicious and all of them have a private value.

• Consider a node "B" with private value "BPrivateValue = b"
• B's ancestor is called "A" and A's private value is "APrivateValue = a".
• B's descendant is called "C" and C's private value is "CPrivateValue = c".

I want every node in this graph to be able to do the following(here we  ...

Rather than storing user data when interacting with an app, I am storing the SHA3-256 of the data. This is because data storage in this particular environment is very limited.

The data can be several variables, e.g., a, b, and, c, but instead of saving them individually, I save the hash of the concatenation: SHA3(a,b,c).

When the user wants to interact with the system, they should send the variables ...

A program I reverse engineered is using $f(x) = Cx \oplus Dx$ where C = 0x20ef138e415 and D = 0xd3eafc3af14600 as a hash function. Given a byte array, the hash is is obtained by repeatedly applying $f$ to the current hash xor next byte.

Java code:

    public static long f(long x) {
        return (0x20ef138e415L * x) ^ (0xd3eafc3af14600L * x);
    }

    public static long hash(byte[] bytes) {
        l ...

I'm implementing IDEA encryption and decryption and I'm trying to implement it in different variants(e.g if we didn't do the output transformation or if we don't do full 8-round encryption).

I got the encryption and decryption right for all variants when there is an output transformation, but I'm having trouble in decryption when there is no output transformation.

I know that decryption in IDEA is b ...

I know that ECB-mode of block cipher is not recommended to use as it is not secure and is not even EAV-secure. However, I still don't get how to prove that it is not EAV-secure and what adversary is associated with this insecurity? Any help would be appreciated. Thanks in advance. Edited: I found a similar article here: Proving a scheme's EAV-insecurity, and also figured it out. Thank you for all your ...

I am a researcher in cryptography. Most of the time I generally do theoretical/Mathematical work only and not doing the implementation part.

I am not able to get the feel about the time complexity of algorithms theoretically. We can get the time complexity of algorithms by doing proper implementation. I want to implement algorithms/schemes to find out the time complexity and other aspects of algo ...

Facts

1. Blockciphers are used to build encryption algorithms and compression functions for hash algorithms.

2. (Unkeyed) Permutations are also complementing blockciphers in encryption and hashing.

3. Large block sizes are preferred.

4. But There are embedded/IoT/lightweight systems.

5. There are still mode of operations defined for 64-bit blocks AND there are still 64-bit blockciphers with key lengths that're ...

Let $BC$ be a block cipher with similar security as AES (in ECB mode). This $BC$ is applied to a message $m$ of same bit size. The result is a cipher $c$.

e.g.;

$$BC(key_A,m) = c_A$$ $$BC(key_B,m) = c_B$$

I'm looking for a $BC$ with: $$BC(key_A,c_B) = c_{BA}$$ $$BC(key_B,c_A) = c_{AB}$$ where $$c_{AB}=c_{BA}$$ but for the majority: $$c_{A}\not=c_{B}$$

Is there any way to construct keys $key_A, k ...

As I understand from this post, mapping from plaintext space to ciphertext space is the fundamental point of all order-preserving encryption. So the only way that we let someone encrypt an arbitrary plaintext is to give him/her this mapping. But, on the other hand, if we give someone this mapping, the encryption breaks because anyone who has access to it can easily decrypt any ciphertext since this mapp ...

I'm curious on how computational indistinguishably is proved.

For instance, would the following be computational indistinguishable? If it is, how do we prove it?

Let $P_a$ be a probabilistic machine which knows a secret $a$ and generates a sequence of $n$ tuples: $(x_1,{x_1}^a),...,(x_n,{x_n}^a)$ where the $x_i$ for each tuple is randomly sampled from a prime order cyclic group. Similarly, let a PPT ...

I know this sounds crazy, but hear me out. As a follow-up to Can the AES-GCM authentication tag be used as a key derivation function?, I'm also considering more creative solutions to derive unique (symmetric) device keys from a securely stored root key.

(The background is that we have a lot of clients (=devices) that can't use asymmetric cryptography (mostly because of hardware constraints). To e ...

I'm trying to figure something out, but it is difficult for me. I need to generate a fully random string in Python. My current function is attached below. I just want to know whether this is secure and good for the project that I'm working on, a cryptocurrency type of website.

import random
import string

def get_random_string(length):
    result_str = ''.join(random.choice(string.ascii_letters) fo ...