Sigvex
ZKRemediation3 min read

Unused and Dead Signals Remediation

How to fix unused public inputs, unassigned signals, and dead signals in ZK circuits.

zkunder-constrained

Unused and Dead Signals Remediation

Overview

Related Detector: Unused and Dead Signals

Three patterns fall under this detector: unused public inputs (declared but not appearing in any constraint), unassigned signals (declared but never given a value), and dead signals (assigned but never referenced in constraints). Each weakens the circuit in different ways. The fix depends on the specific pattern.

Unused Public Input (Before / After)

// Before: Public input not constrained
template UnsafeVerifier() {
    signal input timestamp;  // Public input, never constrained
    signal input data;
    signal output hash;

    component h = Poseidon(1);
    h.inputs[0] <== data;
    hash <== h.out;
    // timestamp has no effect on proof validity
}

// After: Include public input in constraints
template SafeVerifier() {
    signal input timestamp;
    signal input data;
    signal output hash;

    component h = Poseidon(2);
    h.inputs[0] <== data;
    h.inputs[1] <== timestamp;  // Now part of the hash computation
    hash <== h.out;
}

Unassigned Signal (Before / After)

// Before: Signal declared but never assigned
template Broken() {
    signal input x;
    signal intermediate;  // Never assigned -- defaults to 0 or undefined
    signal output y;
    y <== x * 2;
}

// After: Remove or assign the signal
template Fixed() {
    signal input x;
    signal output y;
    y <== x * 2;  // intermediate removed -- it was dead code
}

Dead Signal (Before / After)

// Before: Signal assigned but never used in constraints
template Wasteful() {
    signal input a, b;
    signal output sum;
    signal product;

    sum <== a + b;
    product <== a * b;  // Computed but never used
}

// After: Remove dead signal or use it
template Clean() {
    signal input a, b;
    signal output sum;

    sum <== a + b;  // product removed
}

Alternative Mitigations

1. Connect Unused Inputs to a Commitment

If a public input must exist in the interface but is not directly used in the main computation, include it in a commitment hash:

// Hash all public inputs together to bind them to the proof
component inputHash = Poseidon(numInputs);
inputHash.inputs[0] <== input1;
inputHash.inputs[1] <== input2;
// ...
inputCommitment === inputHash.out;

2. Document Intentionally Unused Signals

If a signal is reserved for future use, add a trivial constraint and a comment:

// Reserved for protocol v2 -- constrain to zero for now
signal input reserved;
reserved === 0;

Common Mistakes

Removing public inputs without updating the verifier: If you remove a public input from the circuit, the verifier contract and all proof-generation code must be updated to match the new interface.

Assuming dead signals are harmless: Dead signals waste constraint space, but more importantly, they often indicate incomplete circuit logic. A signal that was intended to be constrained but is not represents a missing security check.

Confusing unused component outputs with dead signals: A component like Num2Bits constrains its input through internal bit decomposition. Even if you never read the bit outputs, the constraint on the input is still active. This is different from a dead signal, which has no constraint effect.

References