Securely store Proofs of Prior Knowledge using KPOP

KPOP is short for Proof of Prior Knowledge

< Insert a cool logo of the word KPOP in big bold letters here, but there's a mirror that in the reflection reads POPK which is short for Proof of Public Knowledge >

As a journalists, whistleblower or activist you want to prove the fact that you knew of something at a specific date. A common way to do this is to share the hash of your publication on a public platform like Twitter but this requires you to trust these platforms to not alter content or actively censor you.

Since 15 July 2020, Twitter has blocked people of posting content that looks like hashes, to limit the fallout of the 2020 Twitter bitcoin scam, making the platform demonstrably unreliable for these purposes. This inspired us to come up with a more reliably secure way to solve this problem.

Ironically, Bitcoin is a rather useful piece of technology in this regard. People can put arbitrary data in blocks, that once mined are immutable. This information is stored in a Merkle tree for efficient retrieval and verification. This allows you to claim knowledge of certain information at a specific time, and record that fact securely.

One downside of Bitcoin is that you will have to pay a hefty transaction fee in order to store information. KPOP also stores its hashes in a distributed Merkle tree, but one that is not associated with cryptocurrencies. We add your hashes in certificates that we issue every few hours through Letsencrypt and publish those certificatres to Certificate Transparency, a distributed Merkle tree of issued certificates.

Once you have published the hash on our platform, you will get a shareable link that you can use to prove the hash was published at a specific date and time. Once you reveal your document, people can check at the trustworthiness of the document by looking up its entry in Certificate Transparency.

the SANs will point to the CT log entry in question; making it easy to shortlink to the proof of knowledge.

Specification

• Certificates are published in batches. Each batch containing several SANs with hashes. This is done to accomodate rate-limits
• Every batch's CN is <YYYY>-<DD>-<MM>-<hh>.kpop.sh
• The batch's first certificate Not-Before date is between hh:00:00 and hh:59:00
• The SAN list is in the form of [0-9a-f]{64}.kpop.sh denoting the sha256sum
• Amount of batches per day is at the discretion of kpop.sh to accomodate changes in the ToS of Letsencrypt
• https://<hash>.crt.sh will stay reachable and will have its certificate renewed using cerbot
• however it will serve a 301 redirect to the crt.sh page of the first certificate issued for that SANs batch
• The user clicking on https://<hash>.kpop.sh can look at the Not-Before date of the first certificate in crt.sh to obtain the proof that the person submitting the hash knew it at Not-Before

Example

Related work

This is basically the same as https://wiki.mozilla.org/Security/Binary_Transparency