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Epoch Key

Epoch keys are public, temporary, identifiers for users.

Unique epoch keys are generated for each user in each epoch.
They are the users' temporary personas, allowing anonymous interaction with others.
Epoch keys receive attestations to change user data.
An epoch key is computed by:

const field = 
    attesterId + 
    (epoch << 160) + 
    (nonce << 208) + 
    (chainId << 216)
poseidon2([identitySecret, field])

The field looks like:

chain id	nonce	epoch	attester id
36 bits	8 bits	48 bits	160 bits

where nonce can be any value between 0 and numEpochKeyNoncePerEpoch - 1, so that a user can have numEpochKeyNoncePerEpoch epoch keys per epoch.

Only a user knows their identitySecret, so only they know if they have received an attestation; others see an attestation given to a random value.
In the epoch key proof circuit, a user can prove that they own an epochKey, and so are able to receive and process attestations given to that epochKey.

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