What is blindly.cash and this magic blind signature?

Blindly.cash is a coin mix service based on blind signatures.

The concept of a blind signature can be illustrated using the classic carbon paper-lined envelope analogy: writing a signature on the outside of such an envelope leaves a carbon copy of the signature on a slip of paper within the envelope.

Imagine you are a voter in an anonymous election, where each paper ballot must be signed by a trustee to make it valid. To stay anonymous, you can put your paper ballot in the aforementioned carbon paper-lined envelope, then mail it to the trustee to get it signed on the outside of the envelope. The ballot within also gets a carbon copy of the signature. After you get the mail back, you can remove the envelope, and now you have a signed ballot. On election day, you can mail the ballot to the trustee to have it counted. Since the trustee signed the envelope without knowing the content within, they can make no connection between the envelopes they signed and the ballots made public. Thus, the trustee cannot determine how anyone voted.

When it comes to Blindly.cash, the steps include:

• Buy a ticket from Blindly.Cash. The purchase transaction is sent with a blinded redeem note as the calldata. Only you know the original redeem note and the secret factor used to blind it. This purchase corresponds to putting the ballot into a carbon-lined envelope.
• Get a signature of the blinded note from Blindly.cash, then unblind the signature. Now you have the signature of the secret note that you generated in step 1. This corresponds to the step where you mailed your ballot to get signed on the envelope and then, after it was mailed back, you removed the envelope to get the carbon-signed ballot.
• Redeem the ticket with the secret redeem note and its signature. Since Blindly.cash only saw the blinded note, no direct connections can be made between the purchase and redemption. All it can do is verify if the signature used for redemption is valid and redeem it if it is true.

Note that step 2 is done automatically, so for the user, there are only two steps: buy the ticket and redeem it.

OK, but why not Tornado.Cash?

Tornado.Cash is expensive for everyday privacy. A complete round of deposit and withdrawal requires at least 1.5M gas units. At a gas price of 1 gwei, this translates to 0.0015 ETH, which is 3.75 USD when ETH price is $2500.

For a coin-mixing service, is it possible to achieve decentralization, privacy, and affordability simultaneously? At least for now, it seems we must make some tradeoffs. Tornado.Cash prioritizes the former two—decentralization and privacy—while Blindly.Cash opts for the latter two—privacy and affordability.

Blindly.Cash offers the same level of privacy as Tornado.Cash, but at a fraction of the cost, only 3%. Under the same conditions mentioned earlier, this translates to 0.125 USD, making privacy much more affordable. However, there's a tradeoff: Blindly.Cash is a centralized service.

com'on, why trust a centralized service?

Although Blindly.Cash is a centralized service, the process is designed to be transparent. Your secret redeem note is essentially an RSA-based signature from us, making it easy to prove any wrongdoing on our part if we refuse to redeem a valid signature. In such cases, whistleblowing would expose the dishonesty.

We’ve chosen to start with a small face-value ticket (0.01 ETH). While larger denominations will be introduced in the future, beginning with 0.01 ETH reduces anxiety for newcomers, making it easier for them to try out the service.

Profitability is essential for sustainability. For each redemption, we charge a 1% service fee. With incentives aligned towards honest behavior, it’s a rational choice to maintain integrity.

Finally, we encourage you to watch us with a skeptical eye and let time be the judge of our integrity.