Applicability to end-to-end verifiability and risk-limiting audits
The methods described in this specification can be used to enable either end-to-end (E2E) verifiability or enhanced risk-limiting audits (RLAs). In both cases, the ballots are individually encrypted and proofs are provided to allow observers to verify that the set of encrypted ballots is consistent with the announced tallies in an election. In the case of E2E-verifiability, voters are given tracking codes to enable them to confirm that their individual ballots are correctly recorded amongst the set of encrypted ballots. In the case of RLAs, encrypted ballots are randomly selected and compared against physical ballots to obtain confidence that the physical records match the electronic records.
To support enhanced risk-limiting audits (RLAs), it may be desirable to encrypt the master nonce of each ballot with a simple administrative key rather than the “heavyweight” election encryption key. This streamlines the process for decrypting an encrypted ballot that has been selected for audit. It should be noted that the privacy risks of revealing decrypted ballots are substantially reduced in the RLA case since voters are not given tracking codes that could be used to associate them with individual ballots. The primary risk is a coercion threat (e.g., via pattern voting) that only manifests if the full set of ballots were to be decrypted.
While the administratively encrypted nonce can be stored in an electronic record alongside each encrypted ballot, one appealing RLA instantiation is for the administrative encryption of a ballot’s nonce to be printed directly onto the physical ballot. This allows an RLA to proceed by randomly selecting an encrypted ballot, fetching the associated physical ballot, extracting the nonce from its encryption on the physical ballot, using the nonce to decrypt the electronic record, and then comparing the physical ballot contents with those of the electronic record. A malicious actor with an administrative decryption key would need to go to each individual physical ballot to obtain the nonces necessary to decrypt all of the encrypted ballots, and the access to do so would enable this malicious actor to obtain all of the open ballots without necessitating the administrative decryption key.
If E2E-verifiability and enhanced RLAs are both provided in the same election, there must be separate ballot encryptions (ideally, but not necessary, using separate election encryption keys) of each ballot. The E2E-verifiable data set must be distinguished from the enhanced RLA data set. Using the same data set for both applications would compromise voter privacy for voters whose ballots are selected for auditing.