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Leg day: Fri → Fri (19d) Recess

Root Architectural Concern

Credential Integrity

What stops a registration authority from minting unlimited voter credentials?

The Problem: Rogue Credential Minting

Blind Schnorr signatures provide a powerful privacy property: the registration authority (issuer) cannot link a credential to a specific voter after issuance. This is essential for ballot secrecy. However, the same property that makes credentials unlinkable also makes it impossible to audit how many credentials a single authority issued.

The attack:

  1. A rogue registration authority holds the issuer private key (sk_I).
  2. They generate fake voter keypairs and run the blind Schnorr ceremony with themselves.
  3. Each fake credential produces a unique nullifier (since nullifier = SHA-256(voter_pk || election_id)).
  4. They hand N credentials to a bad actor — that actor casts N valid ballots.
  5. The blinding property makes these credentials indistinguishable from legitimate ones.

This is a fundamental tension in cryptographic voting: privacy vs. auditability at the issuance layer. Blind signatures were originally designed by Chaum (1983) for e-cash, where the bank is economically incentivized not to over-mint. In voting, the "bank" (registration authority) may have political incentive to do exactly the opposite.

Why existing defenses are insufficient

Mechanism Does it help? Why not?
Nullifier dedup Prevents one credential from voting twice Does NOT limit how many credentials exist
VCL event logging Logs each ballot cast Cannot log credential issuance without breaking unlinkability
Bulletin board Proves ballots exist Cannot distinguish legitimate vs. rogue ballots
Threshold decryption Protects ballot secrecy Irrelevant to credential count

The Solution: Two-Layer Defense

VoteChain addresses rogue credential minting with two complementary mechanisms that together transform the trust model from "one honest authority" to "threshold collusion resistance with public auditability."

1 Threshold Credential Issuance

Instead of a single registration authority, credentials require blind Schnorr signatures from t-of-n independent issuers. Each issuer runs an independent blind Schnorr ceremony — no single issuer sees the final credential, and no single issuer can forge one alone.

# POC configuration: 2-of-3 independent issuers

issuer_threshold: { t: 2, n: 3 }

# Each issuer independently signs via blind Schnorr:

Issuer_1: blindSign(sk_1, voter_pk) → (R'_1, s'_1)

Issuer_2: blindSign(sk_2, voter_pk) → (R'_2, s'_2)

Issuer_3: blindSign(sk_3, voter_pk) → (R'_3, s'_3)

# Verification requires ≥2 valid signatures:

verify(pk_1, voter_pk, R'_1, s'_1) &&

verify(pk_2, voter_pk, R'_2, s'_2) → VALID

Why this works: A rogue issuer controlling one private key can produce one blind signature per ceremony, but cannot forge the other issuers' signatures. To mint a fraudulent credential, an attacker needs to compromise t independent authorities simultaneously — each operated by different organizations, in different jurisdictions, with independent key ceremonies.

2 Voter Roll Commitment

Before credential issuance begins, the election authority publishes a voter roll commitment in the election manifest: a Merkle root of all eligible voters and the total count. This is anchored on VoteChain before any credentials are issued.

# Published in the election manifest:

voter_roll_commitment: {

merkle_root: "SHA-256 root of eligible voter entries"

total_eligible: 50000

}

# Each credential issuance is logged on VCL:

event: credential_issued {

issuance_sequence: 1

voter_roll_ceiling: 50000

}

# Monitors verify: issuance_count ≤ total_eligible

Why this works: The VCL issuance counter is publicly visible. If it exceeds the voter roll commitment, monitors (academic institutions, NGOs, media) detect the anomaly immediately. The credential issuance event is privacy-preserving — it logs a sequence number, not voter identity — while still enabling public auditability of the total count.

Combined Defense Properties

Collusion Resistance

Forging a credential requires compromising t independent registration authorities simultaneously. Each operates under different governance, different jurisdictions, different key ceremonies.

Public Ceiling

The voter roll commitment is published before issuance begins. Any over-issuance is detectable by comparing VCL issuance events against the committed total.

Privacy Preserved

Blind Schnorr unlinkability is maintained. The VCL logs issuance sequence numbers, not voter identities. The Merkle root commits to the roll without exposing it.

Independent Verification

The Trust Portal verifies: (1) credential signatures meet threshold, (2) issuance count ≤ voter roll ceiling, (3) VCL issuance events match the counter.

Residual risks

  • Threshold collusion: If t issuers collude, they can forge credentials. Mitigation: select issuers from independent organizations (e.g., bipartisan board members, different government agencies, independent auditors).
  • Voter roll inflation: If the voter roll commitment itself is fraudulently inflated (e.g., registering phantom voters), the ceiling is too high. Mitigation: voter roll is independently auditable and published before issuance begins.
  • Post-hoc detection only for ceiling breach: The voter roll commitment prevents undetectable over-issuance, but detection is post-hoc. Threshold issuance provides the preventive layer.

Verify It Yourself

The POC Trust Portal includes a live credential issuance integrity check. After registering a credential in the voting client, visit the Trust Portal to verify:

  • All 3 independent issuer public keys are published in the manifest
  • Your credential carries blind Schnorr signatures from all 3 issuers
  • The VCL records a credential_issued event with the correct sequence number
  • Total issuance count does not exceed the voter roll commitment ceiling
Trust Portal Voting Client