Tag Archives: hashes

New Cryptanalysis of the Fiat-Shamir Protocol

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2025/09/new-cryptanalysis-of-the-fiat-shamir-protocol.html

A couple of months ago, a new paper demonstrated some new attacks against the Fiat-Shamir transformation. Quanta published a good article that explains the results.

This is a pretty exciting paper from a theoretical perspective, but I don’t see it leading to any practical real-world cryptanalysis. The fact that there are some weird circumstances that result in Fiat-Shamir insecurities isn’t new—many dozens of papers have been published about it since 1986. What this new result does is extend this known problem to slightly less weird (but still highly contrived) situations. But it’s a completely different matter to extend these sorts of attacks to “natural” situations.

What this result does, though, is make it impossible to provide general proofs of security for Fiat-Shamir. It is the most interesting result in this research area, and demonstrates that we are still far away from fully understanding what is the exact security guarantee provided by the Fiat-Shamir transform.

Good Essay on the History of Bad Password Policies

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2024/11/good-essay-on-the-history-of-bad-password-policies.html

Stuart Schechter makes some good points on the history of bad password policies:

Morris and Thompson’s work brought much-needed data to highlight a problem that lots of people suspected was bad, but that had not been studied scientifically. Their work was a big step forward, if not for two mistakes that would impede future progress in improving passwords for decades.

First, was Morris and Thompson’s confidence that their solution, a password policy, would fix the underlying problem of weak passwords. They incorrectly assumed that if they prevented the specific categories of weakness that they had noted, that the result would be something strong. After implementing a requirement that password have multiple characters sets or more total characters, they wrote:

These improvements make it exceedingly difficult to find any individual password. The user is warned of the risks and if he cooperates, he is very safe indeed.

As should be obvious now, a user who chooses “p@ssword” to comply with policies such as those proposed by Morris and Thompson is not very safe indeed. Morris and Thompson assumed their intervention would be effective without testing its efficacy, considering its unintended consequences, or even defining a metric of success to test against. Not only did their hunch turn out to be wrong, but their second mistake prevented anyone from proving them wrong.

That second mistake was convincing sysadmins to hash passwords, so there was no way to evaluate how secure anyone’s password actually was. And it wasn’t until hackers started stealing and publishing large troves of actual passwords that we got the data: people are terrible at generating secure passwords, even with rules.

More on Apple’s iPhone Backdoor

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2021/08/more-on-apples-iphone-backdoor.html

In this post, I’ll collect links on Apple’s iPhone backdoor for scanning CSAM images. Previous links are here and here.

Apple says that hash collisions in its CSAM detection system were expected, and not a concern. I’m not convinced that this secondary system was originally part of the design, since it wasn’t discussed in the original specification.

Good op-ed from a group of Princeton researchers who developed a similar system:

Our system could be easily repurposed for surveillance and censorship. The design wasn’t restricted to a specific category of content; a service could simply swap in any content-matching database, and the person using that service would be none the wiser.

EDITED TO ADD (8/30): Good essays by Matthew Green and Alex Stamos, Ross Anderson, Edward Snowden, and Susan Landau. And also Kurt Opsahl.

Apple’s NeuralHash Algorithm Has Been Reverse-Engineered

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2021/08/apples-neuralhash-algorithm-has-been-reverse-engineered.html

Apple’s NeuralHash algorithm — the one it’s using for client-side scanning on the iPhone — has been reverse-engineered.

Turns out it was already in iOS 14.3, and someone noticed:

Early tests show that it can tolerate image resizing and compression, but not cropping or rotations.

We also have the first collision: two images that hash to the same value.

The next step is to generate innocuous images that NeuralHash classifies as prohibited content.

This was a bad idea from the start, and Apple never seemed to consider the adversarial context of the system as a whole, and not just the cryptography.

Brexit Deal Mandates Old Insecure Crypto Algorithms

Post Syndicated from Bruce Schneier original https://www.schneier.com/blog/archives/2020/12/brexit-deal-mandates-old-insecure-crypto-algorithms.html

In what is surely an unthinking cut-and-paste issue, page 921 of the Brexit deal mandates the use of SHA-1 and 1024-bit RSA:

The open standard s/MIME as extension to de facto e-mail standard SMTP will be deployed to encrypt messages containing DNA profile information. The protocol s/MIME (V3) allows signed receipts, security labels, and secure mailing lists… The underlying certificate used by s/MIME mechanism has to be in compliance with X.509 standard…. The processing rules for s/MIME encryption operations… are as follows:

  1. the sequence of the operations is: first encryption and then signing,
  2. the encryption algorithm AES (Advanced Encryption Standard) with 256 bit key length and RSA with 1,024 bit key length shall be applied for symmetric and asymmetric encryption respectively,
  3. the hash algorithm SHA-1 shall be applied.
  4. s/MIME functionality is built into the vast majority of modern e-mail software packages including Outlook, Mozilla Mail as well as Netscape Communicator 4.x and inter-operates among all major e-mail software packages.

And s/MIME? Bleah.