u/pascalschaerli

Internxt is a post-quantum secure encrypted cloud storage provider which is open-source and has passed multiple independent audits.

I reviewed their code and found that post-quantum security should have been the least of their problems. Clicking a link in your browser could trigger remote code execution on the desktop app or leak your long-term encryption keys to an attacker-chosen URL. Their cryptographic architecture stands on shaky grounds with public keys never being verified, in some cases man-in-the-middled by design, a flat key hierarchy and a KDF with just 3 iterations of MD5.

We need PQC and we need it now, but adding a (self-rolled) PQC hybrid on top of a weak protocol does not make it more secure.

> LLM Disclosure: I used an LLM to help me write this post along with many hours of work, some of the findings (i.e. the RCE) were found with a local Qwen 3.5 27B model inside opencode (thanks r/LocalLLaMA ). It is impossible to share a single prompt since it was an iterative process, but I'm happy to elaborate on my methods if anyone cares.

I'm a paid Proton subscriber who recently reviewed some of their open source code. I was a bit disappointed:

Some desktop clients auto-update without user interaction or signature check on the update binary. A compromised backend could silently push malware as a supply-chain attack.
ProtonPass does not properly ship Key Transparency, contradicting what they claimed in the security model. Following my report they weakened the security model but without notice to their users.
The overall Key Transparency implementation has more fundamental flaws such that MITM attacks by a malicious backend would still be possible.

I will continue to pay for ProtonMail and ProtonVPN but avoid the other apps. Following my responsible disclosure I have now published my findings here: https://schaerli.org/weblog/5-proton/

Post-Quantum Crypto Won't Fix Your Architecture

Proton's crypto is not Transparent and not OPAQUE