Cryptography Stack Exchange Newsletter – Thursday, January 5, 2023

Top new questions this week:

Quantum Computing Used to Break RSA by “fixing” Schnorr's Recent Factorization Claim?

There is a claim by Chinese researchers making the rounds (Schneier’s blog here) that RSA can be broken by Quantum Computers. The paper is on arXiv. Wading through the discussion in Schneier’s blog, …

rsa factoring quantum-cryptanalysis

asked by kodlu Score of 10 answered by Daniel S Score of 5

Is RSA longer supported in TLS 1.3 and are RSA and DH fundamentally different?

I’m watching “SSL, TLS, HTTPS Explained” by ByteByteGo on YouTube and at the end, starting around 5:15, there are two points that are interesting: in TLS 1.3 RSA is no longer supported as …

rsa public-key diffie-hellman tls

asked by c_idle Score of 6 answered by DannyNiu Score of 6

Must RSA exponent and modulus be odd

I’m working on some RSA code that uses Toms Fast Math (TFM for short), and I’m trying to understand why the functions fp_exptmod (for modular exponentiation …

rsa modular-arithmetic

asked by ubiquibacon Score of 6 answered by forest Score of 20

How many qubits can break NIST P-521 ECC?

NIST P-521 has the longest key size for standardised ECC, which has 521 bits instead of 512. If a quantum computer is available, how many qubits can break P-521?

elliptic-curves quantum-computing

asked by Flan1335 Score of 6 answered by Daniel S Score of 10

Proof of Knowledge & Rewinding Lemma

I’m somewhat confused about how the definition of a proof of knowledge relates to the Theorem 19.1 in Boneh-Shoup (toc.cryptobook.us/book.pdf), particularly in relation to Schnorr’s protocol …

zero-knowledge-proofs schnorr-identification interactive-proofs

asked by George Herbert Score of 4

256-bit security with Sponges

Cryptographic sponge functions are a hot topic right now with LWC and all. They solve the symmetric crypto problem with simplicity and efficiency. However, none of them seem to achieve 256-bit …

sponge

asked by luiz Score of 4 answered by bk2204 Score of 3

Deriving private keys from a signature/HSM

I have access to an HSM that can sign messages but doesn’t have a way to expose the private key. The signatures are deterministic (RFC 6979). Another application only works with ed25519 signatures …

key-derivation ed25519

asked by telescope Score of 4 answered by Maarten Bodewes Score of 1

Greatest hits from previous weeks:

Creating your own encryption and decryption algorithm

I’m very uneducated when it comes to cryptography. I have tried to find an answer to my question, but what I’ve read doesn’t quite cover what I’m asking. I have thought up my own encryption algorithm …

encryption classical-cipher vigenere

asked by user1575550 Score of 18 answered by NuminousName Score of 15

HMAC-SHA1 vs HMAC-SHA256

I have three questions: Would you use HMAC-SHA1 or HMAC-SHA256 for message authentication? How much HMAC-SHA256 is slower than HMAC-SHA1? Are the security improvements of SHA256 (over SHA1) enough to …

hash hmac sha-256 sha-1 sha-2

asked by Mario Score of 34

Why do web-services tend to use preshared secret keys for client authentication instead of public keys?

Every API that I have ever signed up with gives you a secret key that you can then paste into your application. They know the key, and so do you. (The one exception may be VAPID for Web Push). Why is …

public-key symmetric

asked by Gregory Magarshak Score of 15 answered by JackW Score of 12

Does the SHA hash function always generate a fixed length hash?

I’m using the SHA1/2 family of algorithms for a particular project. I was wondering if all the SHA algorithms return a fixed length hash regardless of the length of the data.

sha-1 sha-2

asked by Robin Rodricks Score of 19 answered by fgrieu Score of 17

How is XOR used for encryption?

I am a programmer, so when I hear XOR, I think about the bitwise operator (e.g. 0110 ^ 1110 = 1000). The mention of “XOR” comes …

encryption xor

asked by IQAndreas Score of 29 answered by Nova Score of 28

Should I use ECB or CBC encryption mode for my block cipher?

Can someone tell me which mode out of ECB and CBC is better, and how to decide which mode to use? Are there any other modes which are better?

encryption block-cipher modes-of-operation cbc ecb

asked by midhunhk Score of 82 answered by Thomas Pornin Score of 22

Why does my SSH private key still work after changing some bytes in the file?

I (for a test) just randomly altered a private RSA key by opening it up in Vim and changing a few bytes. It is the private part of an SSH key pair used for logging in on a remote system. Puzzlingly, …

rsa public-key authentication ssh

asked by PhilPotter1987 Score of 143 answered by yyyyyyy Score of 201

Can you answer these questions?

Difference between Non-Concurrent Composition and Concurrent Composition

You can get access to this document via institutional login at www.researchgate.net/publication/220556089_Security_and_composition_of_cryptographic_protocols_A_tutorial So in Ran Canettis …

universal-composability

asked by killertoge Score of 2

High density SIS and Low density SIS

I am searching for the exact definition of High density SIS and Low density SIS, but there is something unclear about it. SIS problem is to find $x\in \mathbb{Z}^m$ such that for random $A\in\mathbb{Z}…

lattice-crypto sis

asked by Lee Seungwoo Score of 2

They key size of the Schnorr Signature Algorithm

Based on the Schnorr signature below: What is the suitable size of lamda to generate a secure key?

public-key discrete-logarithm key-size key-generation schnorr-signature

asked by Shi Li Score of 1