Advanced Privacy Notions in E-Voting: Receipt Freeness, Coercion Resistance, and Beyond

As electronic voting (e-voting) systems become more widely adopted, so do the expectations for stronger privacy guarantees. Basic ballot secrecy—ensuring that a voter’s choice remains confidential—is only the beginning. In high-stakes elections and adversarial environments, more robust properties like receipt freeness and coercion resistance are crucial to preserve democratic integrity. This article explores the advanced privacy notions in e-voting and how state-of-the-art cryptographic designs aim to enforce them.

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Beyond Ballot Secrecy: Why Advanced Privacy Matters

Standard privacy in e-voting ensures that no one can learn how a voter voted. However, in many real-world scenarios, voters may face pressure, manipulation, or coercion. This is where advanced privacy requirements become essential. These requirements aim to:

• Prevent vote buying
• Deter coercive practices
• Enable voters to vote freely without external influence
• Withstand long-term threats (such as future decryption of archived data)

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1. Receipt Freeness

Definition: A voter cannot obtain any proof of how they voted—even if they want to.

Purpose:

• Prevent vote selling or vote trading
• Ensure attackers can’t verify if coercion succeeded
• Protect voter anonymity even under post-election scrutiny

Example Technique:

• Deniable vote updating: Voters can recast their vote, and the system masks this activity by adding dummy ballots. This hides whether a change occurred, making it impossible for a coercer to know if the final vote reflects their demands.

Challenges:

• No universally accepted definition of receipt freeness exists
• Some cryptographic models may be too strong or too weak to capture all realistic threats

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2. Coercion Resistance

Definition: A voter can still vote as they intend—even when being watched, threatened, or manipulated by an attacker during the election.

Why It’s Important:

• Models stronger threat environments where voters are under direct control or influence
• Defends against simulation (fake voting) and abstention attacks (forced non-voting)

Key Points:

• Coercion resistance assumes that at some stage (e.g., registration), the voter is not controlled by the coercer
• It typically involves complex interactions, such as using fake credentials or re-voting under plausible deniability

Real-World Example:

• Civitas: One of the most well-known coercion-resistant voting systems, designed to counter strong adversaries with a mix of cryptographic techniques and voter behavior models.

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3. Other Advanced Privacy Models

Coercion Evident Voting:

• Not necessarily coercion-resistant, but detects and quantifies coercion during the election
• Can help election observers identify irregular patterns or targeted voter suppression

Everlasting Privacy:

• Ensures that a voter’s choice remains secret for decades, even if cryptographic schemes are broken in the future
• Relevant for protecting data from quantum computing threats or historical decryption

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4. Current Research and Limitations

Despite growing interest, both receipt freeness and coercion resistance lack universally agreed-upon definitions in cryptographic literature. Existing schemes vary based on:

• The threat model (type of attacker)
• Trust assumptions (e.g., honest registration authorities or untampered bulletin boards)
• Usability trade-offs (some privacy models complicate user experience)

Current innovations focus on improving these models to be more practical, scalable, and compatible with user behavior.

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Conclusion

Advanced privacy in e-voting isn’t just theoretical—it’s essential for safeguarding modern democracies. With threats ranging from organized vote manipulation to subtle social coercion, properties like receipt freeness, coercion resistance, and everlasting privacy provide a robust foundation for future-proof elections. Systems like Helios and Civitas are already exploring these capabilities, and ongoing research continues to refine these concepts to make digital voting both secure and free.