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*From*: Peter Fairbrother <peter AT tsto.co.uk>*Subject*: Re: [Cryptography] A Scheme for Verifiable Lottery*Date*: Sun, 29 Nov 2020 01:40:52 +0000*List-archive*: <https://www.metzdowd.com/pipermail/cryptography>*Sender*: "cryptography" <cryptography-bounces+ben=bentasker.co.uk AT metzdowd.com>*To*: cryptography AT metzdowd.com

On 25/11/2020 03:31, Yunxiang Li wrote: [...]

Post the following info: A Lottery name, this needs to be unique each time MAC tag of a chosen "lucky number" The number of winners When participants sign-up, they are given some sort of proof for joining, "Lottery name + username" signed with the organizer's keypair (for example) Calculate participants' score from their unique username score = min(hash(1, <lottery name>, <username>), hash(2, <lottery name>, <username>), ..., hash(<lucky number>, <lottery name>, <username>

Winners are the participants with the lowest score. Announce the winner, the lucky number with the MAC key used to generate the tag The rationale for the repeated hashing is that since the randomness are picked by the organizer, there's no way to stop them from favoring someone by trying possible lucky numbers. Therefore with this scheme, they would need to give everyone else at least the same number of tries, making picking favorites impossible.

But the scheme is broken anyway. Here is one attack:

Peter Fairbrother

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