A Confidence-Based Classification System for Digital Credentials Time-Decay Mechanics, Cryptographic Heartbeats, Zero-Knowledge Risk Proofs, and Privacy-Preserving Trust Assessment
Sirraya Labs • Open Specification • April 2026 Version 1.0.0 — Editor's Draft
The Trust Decay Framework (TDF) defines a standardized approach for assessing and expressing the current trustworthiness of a digital credential over time. Trust is not static—it naturally diminishes without ongoing evidence of security and identity consistency. This specification provides a mechanism for credentials to carry a confidence level that decays according to predictable rules and can be refreshed through cryptographic proofs, heartbeat mechanisms, and risk assessment.
Built on established standards including 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, zero-knowledge proofs, and risk signals.
About This Document
This is an open specification developed by Sirraya Labs. It is intended for implementation experimentation and community feedback. All content is licensed under the MIT License.
In digital identity systems, trust in a credential must not be treated as permanent. A credential issued today may demonstrate diminished trustworthiness tomorrow if the holder’s device changes, if suspicious activity is detected, or simply because time has elapsed without re-verification.
The Trust Decay Framework addresses this challenge by introducing:
Confidence Levels — A four-tier trust hierarchy (Levels 1-4)
Time-Based Decay — Trust naturally diminishes according to defined half-life parameters
Refresh Mechanisms — Heartbeats and cryptographic proofs restore or maintain trust
Privacy by Default — No global identifiers; only pairwise relationships are established
Decentralized Trust — No central authority is required for verification
Cryptographic Agility — Support for both classical and post-quantum algorithms
Deterministic Calculation — Trust levels are computed algorithmically, not subjectively assigned
Open Standard — Freely implementable without licensing restrictions
Example: Financial Services Application
A banking institution requires Level 3 (High Trust) for wire transfers. A user holds a Level 4 (Maximum Trust) credential active for 45 days. The system calculates the current trust level, determines it has decayed to Level 3, and grants access accordingly.
Confidence Level
A four-tier classification (Level 1 through Level 4) indicating the current trustworthiness of a credential. Level 4 represents the highest degree of trust.
Trust Half-Life
The period after which confidence decreases by 50% in the absence of refreshment activity.
Heartbeat
A cryptographic proof demonstrating that the credential holder retains control, serving to slow or reverse trust decay.
Risk Signal
An observable event that may indicate compromised security, triggering immediate trust reduction according to severity classification.
Zero-Knowledge Proof (ZKP)
A cryptographic method enabling proof of a statement’s validity without revealing the underlying data. Applied to verify risk status while preserving information confidentiality.
Pairwise Identifier
A unique identifier derived for each relationship context, preventing correlation across different services.
The framework defines four confidence levels. Each level possesses distinct characteristics, proof requirements, and decay properties.
Level
Name
Half-Life
Typical Use Cases
Required Proofs
Level 1
Basic Trust
7 days
Anonymous access, low-risk services
None
Level 2
Standard Trust
90 days
Standard enterprise, retail banking
No recent high-risk activity (24h)
Level 3
High Trust
90 days
High-value banking, corporate access
No high-risk + device consistency (7d)
Level 4
Maximum Trust
30 days
Government services, critical systems
No high-risk + device consistency (30d) + stable location (72h)
Rationale for Variable Half-Lives
Higher trust levels mandate more frequent refreshment because they authorize access to increasingly sensitive resources. Level 4 decays within 30 days, whereas Level 2 may persist for 90 days without renewal.
Demotion: When trust decays or risk signals are detected, the confidence level decreases. A Level 4 credential transitions to Level 3 after sufficient decay has accumulated.
Promotion: When valid heartbeats are provided, trust may increase. A Level 2 credential accompanied by a valid biometric heartbeat may be promoted to Level 3.
Promotion is bounded by the credential’s original issuance level. A Level 2 credential cannot attain Level 4 through heartbeat mechanisms alone.
Implementations SHOULD support hybrid cryptographic modes combining traditional and quantum-resistant algorithms. Cryptographic profiles affect trust calculations as follows:
Quantum-Resistant — No additional decay multiplier applied
Quantum-Vulnerable — 5× decay multiplier enforced (trust decays five times faster)
Deprecated/Revoked — Immediate demotion to Level 1
Implementers SHOULD employ pairwise identifiers unique to each relationship rather than global identifiers. This prevents correlation across different services.
The same holder interacting with distinct services presents different identifiers, rendering cross-service correlation infeasible without access to the root key.
Each proof includes a nullifier to prevent replay attacks while preserving unlinkability. Nullifiers are derived from the holder’s root key and a context value, ensuring they cannot be correlated across different contexts.
