The cryptography behind e-voting (and why paper ballots are still essential)

Electronic voting must preserve ballot secrecy, prevent duplicate or altered votes, and let the public trust that the tally reflects voters’ intent. Internet voting is especially difficult because voters may lack cryptographic identities, while clients and servers can be compromised. A software flaw could affect many machines and jurisdictions at once, so secure code alone is not enough: election systems need “software independence,” meaning errors or attacks on software can be detected through evidence independent of that software. Paper ballots provide physical records and safeguards that make large-scale, covert manipulation harder. A National Academies report concluded in 2018 that no known technology guarantees the secrecy, security, and verifiability of ballots sent over the Internet.

Cryptographic systems can strengthen elections without removing the need for paper. Helios, intended for lower-stakes elections where coercion is not a concern, and ElectionGuard, designed as a verifiability layer for existing election infrastructure, illustrate the approach. Threshold encryption divides decryption power among election officials, while homomorphic encryption lets servers tally encrypted votes without exposing individual choices; zero-knowledge proofs can establish that ballots contain valid selections. Benaloh’s challenge method lets voters test whether a client recorded and encrypted their choices correctly, and a public bulletin board lets them check that their ballot was included. These safeguards have limits: challenge-based detection depends on enough voters using the checks, and cryptography cannot address every compromise. The central case is for combining cryptographic verification with paper ballots, not replacing paper in general elections.