In a blog post on the 6th, Cloudflare introduced a next-generation anonymous authentication method that can protect user privacy even in the era of quantum computers. The method, being researched with partners including Mozilla, is called "MOLE," and the post was written by Bas Westerbaan.
The background is Privacy Pass, which Cloudflare has run for about 10 years. The protocol lets users present an issued token to access a site, limiting the number of requests while making it impossible to link a token to whoever it was issued to. However, it currently relies on conventional cryptography such as RSA and elliptic curves, so a quantum attacker could forge tokens and bypass the limits.
In MOLE, the issuing party, called an anchor, provides an attestation, and a mediator verifies it. The user proves only that one of an agreed set of anchors issued it, without revealing which one. The mediator then issues a credential that governs use of the site.
The biggest obstacle is signature size. The post-quantum signature ML-DSA-44 is 2,420 bytes, far larger than the 64 bytes of the ECDSA-P256 used today. The researchers present a way to reduce this by combining the zero-knowledge proof method VOLE-in-the-Head with the multivariate signature UOV. The blind signature scheme using UOV is reportedly about 4.3 KB at presentation with a 46.6 KB public key, and the computation takes a few milliseconds.
Cloudflare said the security analysis of multivariate cryptography is not yet mature enough. It explained that even if the optimization were broken, it would only make forgery possible while user privacy would remain intact. Going forward, it plans to build a proof showing that if UOV is secure, the protocol is secure, and to research ways to spread a single authentication across multiple requests. The implementation has been published on GitHub.
Source: Cloudflare Blog
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