Verified Certificate Checking For Counting Votes

We introduce a new framework for verifying electronic vote counting results that are based on the Single Transferable Vote scheme (STV). Our approach frames electronic vote counting as certified computation where each execution of the counting algorithm is accompanied by a certificate that witnesses the correctness of the output. These certificates are then checked for correctness independently of how they are produced. We advocate verification of the verifier rather than the software used to produce the result. We use the theorem prover HOL4 to formalise the STV vote counting scheme, and obtain a fully verified certificate checker. By connecting HOL4 to the verified CakeML compiler, we then extract an executable that is guaranteed to behave correctly with respect to the formal specification of the protocol down to machine level. We demonstrate that our verifier can check certificates of real-size elections efficiently. Our encoding is modular, so repeating the same process for another different STV scheme would require a minimal amount of additional work.