A confidence-based classification system for verifiable credentials with time-based trust decay, zero-knowledge proofs, and risk-aware adaptive security.
Trust is not static. A credential that was trustworthy yesterday may not be trustworthy today. The Trust Decay Framework (TDF) provides a standardized approach for assessing and expressing the current trustworthiness of digital credentials over time.
Instead of binary valid/invalid decisions, TDF introduces a four-tier confidence hierarchy where trust naturally diminishes without ongoing evidence of security and identity consistency. Credentials can be refreshed through cryptographic proofs, heartbeat mechanisms, and risk assessment—all while preserving privacy through pairwise identifiers and zero-knowledge proofs.
Built on established standards like W3C Decentralized Identifiers (DIDs) and Verifiable Credentials (VCs), TDF enables issuers to create credentials with an initial trust level, and verifiers to evaluate the current trust level based on age, cryptographic strength, proofs, and risk signals.
| Level | Name | Half-Life | Typical Use Cases |
|---|---|---|---|
| Level 4 | Maximum Trust | 30 days | Government services, critical systems |
| Level 3 | High Trust | 90 days | High-value banking, corporate access |
| Level 2 | Standard Trust | 90 days | Enterprise applications, retail banking |
| Level 1 | Basic Trust | 7 days | Anonymous access, low-risk services |
Half-Life Explained: The period after which confidence decreases by 50% without refreshment. Higher trust levels require more frequent maintenance because they grant access to more sensitive resources.
- Demotion — Trust decreases due to time decay or detected risks
- Promotion — Trust increases when valid heartbeats are provided (limited to original issuance level)
| Mechanism | Description |
|---|---|
| Time-Based Decay | Trust diminishes predictably according to a mathematical formula |
| Zero-Knowledge Proofs | Prove security status without revealing sensitive underlying data |
| Heartbeats | Cryptographic proofs that refresh or restore trust (biometric, proof-of-possession, liveness) |
| Risk Signals | Real-time anomalies that trigger immediate trust reduction |
| Pairwise Identifiers | Unique IDs per relationship prevent correlation across services |
Defines the problem statement, design principles, and usage examples. Trust should not be permanent—credentials require ongoing verification.
Standardized definitions for confidence levels, half-life, heartbeats, risk signals, zero-knowledge proofs, and pairwise identifiers.
Specifies the four-tier hierarchy with half-lives, proof requirements, and demotion/promotion rules.
Extends W3C Verifiable Credentials with confidence level information, assessment policy references, and validity timestamps.
Mathematical formula for computing current trust level:
DecayFactor = 2 ^ (-(EffectiveAge × CryptoMultiplier) / HalfLife)
- EffectiveAge = Actual age minus time offset from valid heartbeats
- CryptoMultiplier = 1 for quantum-resistant algorithms, 5 for quantum-vulnerable
- HalfLife = Based on initial confidence level
Defines required proof types for each trust level and heartbeat mechanisms:
- No High-Risk Activity (24h) — Required for Level 2+
- Device Consistency (7d) — Enables Level 3
- Device Consistency (30d) — Enables Level 4
- Stable Location (72h) — Enables Level 4
Heartbeat types include biometric verification, proof of possession, and liveness checks.
Specifies observable events that may indicate compromised security:
- Device Change — Demote to Level 2
- Geographic Anomaly — Temporary demotion (2 hours)
- Behavioral Anomaly — Require heartbeat (15 min)
- Concurrent Session — Severity-dependent action
- Velocity Anomaly — Severity-dependent action
Severity levels range from Low to Critical, with corresponding actions.
Defines supported algorithms and quantum considerations:
- Ed25519 — REQUIRED (classical)
- ML-DSA-44 — RECOMMENDED (post-quantum)
- ML-DSA-65/87 — OPTIONAL (post-quantum)
- Hybrid Modes — OPTIONAL
Quantum-vulnerable algorithms incur a 5× decay multiplier.
Privacy-preserving mechanisms:
- Pairwise Identifiers — Derived per relationship:
HMAC(RootKey, RelationshipContext || ServiceIdentifier) - Zero-Knowledge Proofs — Reveal only that conditions are satisfied, not the underlying data
- Nullifiers — Prevent replay attacks while maintaining unlinkability
Recommendations for state management, multi-tier caching (L1 memory, L2 local, L3 distributed), policy evaluation, and decision types (Granted, Reauthentication Required, Denied).
Attack mitigations and key rotation protocol (pre-rotation, rotation, post-rotation phases).
Instead of a global identifier, each relationship uses a unique derived identifier. The same holder interacting with different services appears as different entities, preventing correlation without the root key.
- NoHighRisk24h — Proves no high-risk alerts exist without revealing which alerts
- DeviceConsistency — Proves device stability without revealing device fingerprint
- GeoStable — Proves location stability without revealing actual coordinates
Each proof includes a unique nullifier derived from the holder's root key and context, ensuring proofs cannot be replayed or correlated across different contexts.
| Algorithm | Type | Quantum Resistance |
|---|---|---|
| Ed25519 | Classical | No |
| ML-DSA-44 | Post-Quantum | Yes |
| ML-DSA-65 | Post-Quantum | Yes |
| ML-DSA-87 | Post-Quantum | Yes |
| Hybrid Ed25519-ML-DSA | Hybrid | Yes |
Quantum-resistant algorithms receive no decay penalty; quantum-vulnerable algorithms decay 5× faster.
A Level 4 credential (30-day half-life) issued 45 days ago with no heartbeats:
- EffectiveAge = 45 days
- HalfLife = 30 days
- DecayExponent = -(45 × 1) / 30 = -1.5
- DecayFactor = 2^-1.5 = 0.3535
- 0.3535 > 0.25 → Demote two levels
- Level 4 → Level 2
After a valid biometric heartbeat, trust may be restored or decay reset.
The Trust Decay Framework is built on and compatible with:
| Standard | Relevance |
|---|---|
| W3C DID Core 1.0 | Decentralized Identifiers for subjects and issuers |
| W3C VC Data Model 2.0 | Verifiable Credential structure and proofs |
| RFC 8032 (Ed25519) | Digital signatures |
This specification and related materials are licensed under the MIT License — freely implementable without restrictions.
- W3C DID Working Group for Decentralized Identifiers
- W3C VC Working Group for Verifiable Credentials
- UDNA Community Group for decentralized addressing standards
- Sirraya Labs for open-source commitment
Trust Decay Framework — Making digital trust dynamic, privacy-preserving, and quantum-ready.
Version 1.0.0 • Editor's Draft • April 2026