Purpose of EnsureRank

[terenceyak]

Currently, if you try and to run alice on an example in the docs, it will already fail when calling NewDKG with error large rank.

The example states:

Let threshold = 3, and participants = 4. Assume that the corresponding rank of each shareholder are 0, 1, 1, 2. Then authorised sets in this setting are:

- 0, 1, 1
- 0, 1, 2
- 0, 1, 1, 2

The other combinations of shares can not recover the secret (e.g. 1, 1, 2).

This is due to ErrLargeRank of EnsureRank in crypto/utils/utils.go, . The related code is linked here.

If you think about it for the participant with rank of 2, then rank + 1 will be 3 which is equal the threshold in the example, and will fail.

There can either be 1 of the 2 possibilities:

1. the restriction rank+1 >= threshold is too strict
2. the example in the docs is invalid

Should EnsureRank be less strict or should the example in the docs be changed?

Thanks.

[cychuang0924]

Hi terenceyak,

Yes, your comment is correct. We maybe offer two options to users:

1. Current version.
2. You can set any condition as long as we check that there is any way to recover the private key. Then we will accept it.

As for document, we will add some comments.

Thanks for your suggestion!