Data replacement using addition

[vimwitch]

Right now we implement the replacement field in the unirep data by using two fields:

• replacement field
• metadata field (timestamp)

In the user state transition we compare the timestamps for each epoch key and take the largest, non-zero value as the new field value.

We can implement replacement logic in simple addition fields by overflowing the field. For example, say that in a field data[0] i have the following value: 0xa29fa929a911. An attester can replace this value with a new value 0xaa by attesting to the following, in any order:

• data[0] += FIELD_MAX - 0xa29fa929a911
• data[0] += 0xaa

The first attestation sets data[0] to 0 by overflowing the field element. The second attestation adds the new value to data[0]. Using an approach like this reduces the number of constraints needed in the user state transition proof. It also allows any field to operate as a replacement field.

We can implement this in ZK by revealing an epoch key and a specific data index. In the smart contract this proof can be taken and the FIELD_MAX - data value can be calculated and given as an attestation.