Using Monero as proof of funds

Question

I'm writing an article about the future of Healthcare. I wondered if it was possible to use Monero to make a proof of funds. For example: the hospital wants to know that I can pay for the medical care I'm going to receive, I sign some kind of message that proves that I own at least x amount of XMR, they are convinced and I get treated with no delay.

Is that possible? I understand that range proofs do precisely that, the question then becomes: is it possible for an owner of XMR to create this proof without deanonymizing his account? Note that I do not want to make an on-chain transaction, it should just be some kind of signed message. If the answer is yes, then I will mention Monero in the article.

Answer 1

It is partially possible and will probably be implemented in the future. An interesting post was created about this and a member of the Monero Research Lab stated the following 8 hours ago:

[...]

Currently, the only way to prove your balance to an auditor is to give them your view secret key along with the set of signed key images (i.e. ingredients needed for cold signing using watch-only wallet). This approach has a serious drawback that the auditor knows (and continues to know in the future) about all the funds you received and when those funds (whose key images are disclosed) get spent. If the auditor (i.e. a government) does this for a lot of users, they will acquire a considerable amount of information about which output belongs to who and which is the real spend in each ring signature. This is clearly detrimental to everyone's privacy.

A new feature called "tx proof" (https://github.com/monero-project/monero/pull/2487) was designed to partially address the above issue; instead of disclosing your view key to the auditor, for a given transaction where you received some funds, you can generate a cryptographic proof that proves your having received funds in that tx, and then you can give the proof to the auditor. Importantly, this can be done on a per-tx basis, so the auditor will have no way of knowing the rest of your transactions.

An idea that occurred to me just now inspired by this post, is to augment the above tx proof with the signed key image, so that the auditor can also check if the fund received in the given tx is spent or not. This way, you can easily prove to the auditor that you have at least X amount of funds in your wallet. The major advantage compared to the existing method (giving out the entire key image set) is that you don't need to reveal the entirety of your balance to the auditor.

-/u/stoffu