Quantum Exposure Index (IEQ).

What the IEQ measures

The Quantum Exposure Index (IEQ) quantifies, per organization, exposure to harvest-now-decrypt-later (HNDL) risk: encrypted traffic captured today can be archived and decrypted in the future, once a cryptographically relevant quantum computer exists. The IEQ doesn't measure whether your cipher is breakable today — it measures how urgently you need to migrate, given your data's lifetime and your sector's threat horizon.

The score combines three publicly observable dimensions: Vulnerability (V), the share of your cryptographic attack surface using algorithms breakable by Shor's algorithm (RSA, ECDH, ECDSA, DSA); Exposure (E), how reachable that surface is to an external adversary; and Timeline (H), the temporal urgency given the threat horizon and your data's lifetime.

Why multiply, not add

Additive scoring frameworks assume full compensability: a deficit in one dimension is offset by a surplus in another. Under the HNDL model, vulnerability and exposure are complements, not substitutes. An organization with fully vulnerable cryptography but zero external exposure cannot be harvested. An organization with full exposure but no quantum-vulnerable cryptography has nothing worth decrypting. Both conditions must hold simultaneously.

We formalize this as a contest between two constant-rate processes — the attacker trying to exploit harvested data before the defender renders it worthless (through data expiry, key rotation, or cryptographic remediation). The resulting compromise probability takes the form

P_HNDL = H · (VᵃEᵇ) / (VᵃEᵇ + θ),

where θ is the ratio between defense and attack intensity. We formally prove that no additive or ordinal score can reproduce this interaction structure: the cross-partial ∂²(ln P)/∂(ln V)∂(ln E) is strictly negative for any finite, positive V, E, θ — the elasticities of V and E depend on where the organization sits in the vulnerability–exposure plane, not on fixed global weights.

Regime duality

When VᵃEᵇ is much smaller than θ (defense dominates), the elasticities approach their maximum values: the score is maximally sensitive to changes in both V and E. When VᵃEᵇ is much larger than θ (attack dominates), both elasticities approach zero and the score saturates — reducing V or E marginally yields diminishing returns.

The practical consequence is direct: organizations with lower current exposure have the greatest marginal gain from migrating early. Migrating while still in the high-sensitivity regime yields the greatest return per unit of effort.

M — the governance multiplier

The final score applies a multiplier M ≥ 1 over qualitative risk factors not captured by V and E — for example, absence of a cryptographic inventory or regulatory non-compliance. The published IEQ is 100 · min(1, H · Vᵝ · Eᵞ · M), with β and γ the endogenous elasticities derived from the structural form.

What the IEQ is not

The IEQ is an operational prioritization index, derived from a local log-linear approximation to the compromise probability — not a globally calibrated probability, nor a substitute for individual technical audit. No exploit attempts are performed: collection is limited to public signals (DNS, TLS, PQC readiness, HTTP headers, exposed technologies).