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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Trust Decay Framework (TDF) — Open Specification</title>
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</head>
<body>
<div class="container">
<!-- Header -->
<div class="spec-header">
<h1>Trust Decay Framework (TDF)</h1>
<div class="subtitle">A Confidence-Based Classification System for Digital Credentials</div>
<div>
<span class="version">Version 1.0.0</span>
<span class="status">Editor's Draft</span>
</div>
<div style="margin-top: 1rem; font-size: 0.875rem;">
<strong>Sirraya Labs</strong> • Open Specification • April 2026
</div>
</div>
<!-- Navigation -->
<div class="spec-nav">
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#core-concepts">Core Concepts</a></li>
<li><a href="#trust-levels">Trust Levels</a></li>
<li><a href="#credential-format">Credential Format</a></li>
<li><a href="#decay-calculation">Decay Calculation</a></li>
<li><a href="#proofs">Proofs & Heartbeats</a></li>
<li><a href="#risk-signals">Risk Signals</a></li>
<li><a href="#cryptography">Cryptography</a></li>
<li><a href="#privacy">Privacy</a></li>
<li><a href="#implementation">Implementation</a></li>
</ul>
</div>
<!-- Abstract -->
<section id="abstract">
<h2>Abstract</h2>
<p>
The <strong>Trust Decay Framework (TDF)</strong> 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.
</p>
<p>
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,
zero-knowledge proofs, and risk signals.
</p>
<div class="note">
<strong>📘 About This Document</strong><br>
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.
</div>
</section>
<!-- Introduction -->
<section id="introduction">
<h2>1. Introduction</h2>
<p>
In digital identity systems, trust in a credential should not be permanent. A credential
issued today may be less trustworthy tomorrow if the holder's device changes, if
suspicious activity is detected, or simply because time has passed without re-verification.
</p>
<p>
The Trust Decay Framework addresses this by introducing:
</p>
<ul>
<li><strong>Confidence Levels</strong> — A four-tier trust hierarchy (Levels 1-4)</li>
<li><strong>Time-Based Decay</strong> — Trust naturally diminishes over time</li>
<li><strong>Refresh Mechanisms</strong> — Heartbeats and proofs can restore trust</li>
<li><strong>Risk Signals</strong> — Anomalies can reduce trust immediately</li>
<li><strong>Privacy Preservation</strong> — Using pairwise identifiers and zero-knowledge proofs</li>
</ul>
<h3>1.1 Design Principles</h3>
<ul>
<li><strong>Privacy by Default</strong> — No global identifiers; pairwise relationships only</li>
<li><strong>Decentralized Trust</strong> — No central authority required for verification</li>
<li><strong>Cryptographic Agility</strong> — Support for both classical and post-quantum algorithms</li>
<li><strong>Deterministic Calculation</strong> — Trust levels are computed, not subjective</li>
<li><strong>Open Standard</strong> — Freely implementable without licensing restrictions</li>
</ul>
<div class="example">
<strong>💡 Example: Banking Application</strong><br>
A bank requires Level 3 (High Trust) for wire transfers. The user holds a Level 4
(Maximum Trust) credential that has been active for 45 days. The system calculates
the current trust level, finds it at Level 3, and grants access.
</div>
</section>
<!-- Core Concepts -->
<section id="core-concepts">
<h2>2. Core Concepts</h2>
<h3>2.1 Terminology</h3>
<dl style="margin-left: 1rem;">
<dt><strong>Confidence Level</strong></dt>
<dd>A four-tier classification (Level 1 to Level 4) indicating the current
trustworthiness of a credential, with Level 4 being the highest.</dd>
<dt><strong>Trust Half-Life</strong></dt>
<dd>The period after which confidence decreases by 50% without refreshment.</dd>
<dt><strong>Heartbeat</strong></dt>
<dd>A cryptographic proof that the credential holder remains in control, used to
slow or reverse trust decay.</dd>
<dt><strong>Risk Signal</strong></dt>
<dd>An observable event that may indicate compromised security, triggering
immediate trust reduction.</dd>
<dt><strong>Zero-Knowledge Proof (ZKP)</strong></dt>
<dd>A cryptographic method that proves a statement is true without revealing the
underlying data. Used to verify risk status without exposing sensitive information.</dd>
<dt><strong>Pairwise Identifier</strong></dt>
<dd>A unique identifier derived for each relationship, preventing correlation
across different services.</dd>
</dl>
</section>
<!-- Trust Levels -->
<section id="trust-levels">
<h2>3. Trust Levels</h2>
<p>
The framework defines four confidence levels. Each level has distinct
characteristics, proof requirements, and decay properties.
</p>
<table>
<thead>
<tr>
<th>Level</th>
<th>Name</th>
<th>Half-Life</th>
<th>Typical Use Cases</th>
<th>Required Proofs</th>
</tr>
</thead>
<tbody>
<tr>
<td><span class="trust-badge trust-1">Level 1</span></td>
<td>Basic Trust</td>
<td>7 days</td>
<td>Anonymous access, low-risk services</td>
<td>None</td>
</tr>
<tr>
<td><span class="trust-badge trust-2">Level 2</span></td>
<td>Standard Trust</td>
<td>90 days</td>
<td>Standard enterprise, retail banking</td>
<td>No recent high-risk activity (24h)</td>
</tr>
<tr>
<td><span class="trust-badge trust-3">Level 3</span></td>
<td>High Trust</td>
<td>90 days</td>
<td>High-value banking, corporate access</td>
<td>No high-risk + device consistency (7d)</td>
</tr>
<tr>
<td><span class="trust-badge trust-4">Level 4</span></td>
<td>Maximum Trust</td>
<td>30 days</td>
<td>Government services, critical systems</td>
<td>No high-risk + device consistency (30d) + stable location (72h)</td>
</tr>
</tbody>
</table>
<div class="note">
<strong>Why Different Half-Lives?</strong> Higher trust levels require more frequent
refreshment because they grant access to more sensitive resources. Level 4 decays
in 30 days, while Level 2 can last 90 days without renewal.
</div>
<h3>3.1 Trust Operations</h3>
<p><strong>Demotion:</strong> When trust decays or risks are detected, the level
decreases. A Level 4 credential becomes Level 3 after sufficient decay.</p>
<p><strong>Promotion:</strong> When valid heartbeats are provided, trust may increase.
A Level 2 credential with a valid biometric heartbeat may be promoted to Level 3.</p>
<div class="note">
Promotion is limited to the credential's original issuance level—a Level 2 credential
cannot become Level 4 through heartbeats alone.
</div>
</section>
<!-- Credential Format -->
<section id="credential-format">
<h2>4. Credential Format</h2>
<p>
A Trust Decay credential extends the standard Verifiable Credential format with confidence
level information.
</p>
<pre>{
"@context": [
"https://www.w3.org/ns/credentials/v2",
"https://spec.sirraya.com/tdf/v1"
],
"id": "urn:uuid:3c4c7e5d-8b2a-4f1e-9c6d-7a3b8e2f5c1d",
"type": ["VerifiableCredential", "TrustDecayCredential"],
"issuer": "did:example:issuer123",
"issuanceDate": "2026-04-01T10:00:00Z",
"expirationDate": "2027-04-01T10:00:00Z",
"credentialSubject": {
"id": "did:example:holder456",
"confidenceLevel": "Level4",
"assessmentPolicy": "https://spec.sirraya.com/tdf/policies/v1",
"validFrom": "2026-04-01T10:00:00Z",
"validUntil": "2027-04-01T10:00:00Z"
},
"proof": {
"type": "Ed25519Signature2020",
"created": "2026-04-01T10:00:00Z",
"verificationMethod": "did:example:issuer123#key-1",
"proofPurpose": "assertionMethod",
"proofValue": "z5hNqKjVkG8P3qR2sT1uVwX9yZ4aB6cD7eF8gH9iJ0kL1"
}
}</pre>
<h3>4.1 Credential Fields</h3>
<ul>
<li><code>confidenceLevel</code> — The initial trust level (Level1, Level2, Level3, Level4)</li>
<li><code>assessmentPolicy</code> — URL pointing to the policy used to assess trust</li>
<li><code>validFrom</code> — Timestamp from which trust decay is calculated</li>
<li><code>validUntil</code> — Absolute expiration after which credential cannot be used</li>
</ul>
</section>
<!-- Decay Calculation -->
<section id="decay-calculation">
<h2>5. Trust Decay Calculation</h2>
<p>
The current trust level is calculated using a deterministic formula that considers
age, cryptographic strength, and available proofs.
</p>
<h3>5.1 Decay Formula</h3>
<pre>DecayFactor = 2 ^ (-(EffectiveAge × CryptoMultiplier) / HalfLife)</pre>
<p>Where:</p>
<ul>
<li><strong>EffectiveAge</strong> = Actual age minus time offset from valid heartbeats</li>
<li><strong>CryptoMultiplier</strong> = 1 for quantum-resistant algorithms, 5 for quantum-vulnerable</li>
<li><strong>HalfLife</strong> = Based on initial confidence level (30, 90, or 180 days)</li>
</ul>
<p>The resulting trust level is:</p>
<ul>
<li>DecayFactor > 0.75 → No demotion</li>
<li>DecayFactor > 0.5 → Demote one level</li>
<li>DecayFactor > 0.25 → Demote two levels</li>
<li>DecayFactor ≤ 0.25 → Demote three levels (minimum trust)</li>
</ul>
<div class="example">
<strong>📐 Decay Example</strong><br>
A Level 4 credential (half-life 30 days) issued 45 days ago with no heartbeats:<br><br>
EffectiveAge = 45 days<br>
HalfLife = 30 days<br>
DecayExponent = -(45 × 1) / 30 = -1.5<br>
DecayFactor = 2 ^ (-1.5) = 0.3535<br><br>
0.3535 > 0.25 → Demote two levels<br>
Level 4 → Level 2
</div>
</section>
<!-- Proofs and Heartbeats -->
<section id="proofs">
<h2>6. Proofs and Heartbeats</h2>
<p>
Credential holders can provide cryptographic proofs to slow decay or restore trust.
</p>
<h3>6.1 Proof Types</h3>
<table>
<thead>
<tr>
<th>Proof Type</th>
<th>What It Proves</th>
<th>Effect on Trust</th>
<th>Validity Period</th>
</tr>
</thead>
<tbody>
<tr>
<td><strong>No High-Risk Activity</strong></td>
<td>No security alerts in the last 24 hours</td>
<td>Required for Level 2+</td>
<td>24 hours</td>
</tr>
<tr>
<td><strong>Device Consistency (7d)</strong></td>
<td>Same device used for 7 days</td>
<td>Enables Level 3</td>
<td>7 days</td>
</tr>
<tr>
<td><strong>Device Consistency (30d)</strong></td>
<td>Same device used for 30 days</td>
<td>Enables Level 4</td>
<td>30 days</td>
</tr>
<tr>
<td><strong>Stable Location (72h)</strong></td>
<td>Geographic location stable for 72 hours</td>
<td>Enables Level 4</td>
<td>72 hours</td>
</tr>
</tbody>
</table>
<h3>6.2 Heartbeat Types</h3>
<ul>
<li><strong>Biometric Heartbeat</strong> — Zero-knowledge proof confirming recent biometric verification. Resets trust decay (max 3 resets).</li>
<li><strong>Proof of Possession</strong> — Demonstrates continued control of private key. Can promote trust one level.</li>
<li><strong>Liveness Check</strong> — Confirms physical presence. Restores trust to initial level.</li>
</ul>
<pre>{
"method": "ZkBiometric",
"issuedAt": "2026-04-01T12:00:00Z",
"validUntil": "2026-04-08T12:00:00Z",
"walletSignature": "0x7a3b8e2f5c1d9e4f...",
"nonce": "0x8b2a4f1e9c6d...",
"zkCommitment": "0x3c4c7e5d8b2a..."
}</pre>
</section>
<!-- Risk Signals -->
<section id="risk-signals">
<h2>7. Risk Signals</h2>
<p>
Risk signals indicate potential security concerns and can trigger immediate trust reduction.
</p>
<h3>7.1 Signal Types</h3>
<table>
<thead>
<tr>
<th>Signal Type</th>
<th>Description</th>
<th>Default Action</th>
</tr>
</thead>
<tbody>
<tr>
<td><strong>Device Change</strong></td>
<td>Authentication from a new device</td>
<td>Demote to Level 2</td>
</tr>
<tr>
<td><strong>Geographic Anomaly</strong></td>
<td>Location inconsistent with normal behavior</td>
<td>Temporary demotion (2 hours)</td>
</tr>
<tr>
<td><strong>Behavioral Anomaly</strong></td>
<td>Unusual usage patterns detected</td>
<td>Require heartbeat (15 min)</td>
</tr>
<tr>
<td><strong>Concurrent Session</strong></td>
<td>Multiple simultaneous logins</td>
<td>Depends on severity</td>
</tr>
<tr>
<td><strong>Velocity Anomaly</strong></td>
<td>Unusually rapid actions</td>
<td>Depends on severity</td>
</tr>
</tbody>
</table>
<h3>7.2 Severity Levels</h3>
<ul>
<li><strong>Low</strong> — Minor anomaly, requires additional verification</li>
<li><strong>Medium</strong> — Notable deviation, temporary demotion</li>
<li><strong>High</strong> — Significant risk, immediate demotion</li>
<li><strong>Critical</strong> — Severe threat, demote to Level 1 immediately</li>
</ul>
</section>
<!-- Cryptography -->
<section id="cryptography">
<h2>8. Cryptographic Requirements</h2>
<h3>8.1 Supported Algorithms</h3>
<table>
<thead>
<tr>
<th>Algorithm</th>
<th>Status</th>
<th>Quantum Resistance</th>
<th>Requirement</th>
</tr>
</thead>
<tbody>
<tr>
<td>Ed25519</td>
<td>Approved</td>
<td>No</td>
<td>REQUIRED</td>
</tr>
<tr>
<td>ML-DSA-44</td>
<td>Quantum Resistant</td>
<td>Yes</td>
<td>RECOMMENDED</td>
</tr>
<tr>
<td>ML-DSA-65</td>
<td>Quantum Resistant</td>
<td>Yes</td>
<td>OPTIONAL</td>
</tr>
<tr>
<td>Hybrid Ed25519-ML-DSA</td>
<td>Hybrid</td>
<td>Yes</td>
<td>OPTIONAL</td>
</tr>
</tbody>
</table>
<h3>8.2 Quantum Considerations</h3>
<p>
Implementations SHOULD support hybrid cryptographic modes combining traditional and
quantum-resistant algorithms. Cryptographic profiles affect trust calculations:
</p>
<ul>
<li><strong>Quantum-Resistant</strong> — No additional decay multiplier</li>
<li><strong>Quantum-Vulnerable</strong> — 5× decay multiplier (trust decays 5× faster)</li>
<li><strong>Deprecated/Revoked</strong> — Immediate demotion to Level 1</li>
</ul>
</section>
<!-- Privacy -->
<section id="privacy">
<h2>9. Privacy Considerations</h2>
<p>
Privacy is a foundational concern in trust assessment. This framework incorporates
privacy-preserving mechanisms throughout.
</p>
<h3>9.1 Pairwise Identifiers</h3>
<p>
Rather than using a global identifier, implementers SHOULD use pairwise identifiers
that are unique per relationship. This prevents correlation across different services.
</p>
<pre>PairwiseID = HMAC(RootKey, RelationshipContext || ServiceIdentifier)</pre>
<p>The same holder interacting with different services uses different identifiers, making it impossible to link those interactions without the root key.</p>
<h3>9.2 Zero-Knowledge Proofs</h3>
<p>
Risk proofs SHOULD be implemented as zero-knowledge proofs:
</p>
<ul>
<li><strong>NoHighRisk24h</strong> — Proves no high-risk alerts exist without revealing what alerts</li>
<li><strong>DeviceConsistency</strong> — Proves device stability without revealing device fingerprint</li>
<li><strong>GeoStable</strong> — Proves location stability without revealing actual location</li>
</ul>
<h3>9.3 Nullifiers</h3>
<p>
Each proof includes a nullifier that prevents replay attacks while maintaining
unlinkability. Nullifiers are derived from the holder's root key and a context
value, ensuring they cannot be correlated across different contexts.
</p>
</section>
<!-- Implementation -->
<section id="implementation">
<h2>10. Implementation Guidelines</h2>
<h3>10.1 State Management</h3>
<p>Implementations SHOULD maintain credential state including:</p>
<ul>
<li>The credential itself and its current trust level</li>
<li>All active ZK risk proofs with their validity periods</li>
<li>Heartbeat proofs and reset counts</li>
<li>Active risk indicators and temporary demotions</li>
<li>Timestamps of last evaluation</li>
</ul>
<h3>10.2 Caching Strategy</h3>
<ul>
<li><strong>L1 (Memory)</strong> — Frequently accessed credentials, < 1ms</li>
<li><strong>L2 (Local)</strong> — Recently accessed credentials, < 10ms</li>
<li><strong>L3 (Distributed)</strong> — Network cache, < 100ms</li>
</ul>
<h3>10.3 Policy Evaluation</h3>
<pre>{
"serviceId": "HighValueBanking",
"requiredLevel": "Level3",
"requiredProofs": ["NoHighRisk24h", "DeviceConsistency30d"],
"maxCredentialAge": 604800,
"acceptHeartbeats": true,
"requireQuantumReady": false
}</pre>
<h3>10.4 Decision Types</h3>
<ul>
<li><strong>Granted</strong> — All requirements satisfied. Access permitted.</li>
<li><strong>Reauthentication Required</strong> — Credentials expired or missing proofs.</li>
<li><strong>Denied</strong> — Access refused with fallback action.</li>
</ul>
</section>
<!-- Security -->
<section id="security">
<h2>11. Security Considerations</h2>
<h3>11.1 Attack Mitigations</h3>
<table>
<thead>
<tr>
<th>Attack Type</th>
<th>Mitigation</th>
</tr>
</thead>
<tbody>
<tr>
<td>Replay Attacks</td>
<td>Nonces in heartbeats, timestamps, nullifiers in ZK proofs</td>
</tr>
<tr>
<td>Key Compromise</td>
<td>Key rotation protocol with grace periods and revocation</td>
</tr>
<tr>
<td>Sybil Attacks</td>
<td>Proof-of-work for node identifiers, reputation systems</td>
</tr>
<tr>
<td>Man-in-the-Middle</td>
<td>End-to-end encryption with authenticated key exchange</td>
</tr>
</tbody>
</table>
<h3>11.2 Key Rotation</h3>
<ol>
<li><strong>Pre-rotation</strong> — Generate new keys, create updated DID Document</li>
<li><strong>Rotation</strong> — Sign rotation proof, accept both old and new keys</li>
<li><strong>Post-rotation</strong> — Remove old key after grace period (7 days)</li>
</ol>
</section>
<!-- References -->
<section id="references">
<h2>12. References</h2>
<h3>12.1 Normative References</h3>
<ul>
<li><a href="https://www.w3.org/TR/did-core/">W3C Decentralized Identifiers (DIDs) v1.0</a></li>
<li><a href="https://www.w3.org/TR/vc-data-model-2.0/">W3C Verifiable Credentials Data Model v2.0</a></li>
<li><a href="https://www.rfc-editor.org/rfc/rfc8032">RFC 8032: Edwards-Curve Digital Signature Algorithm (EdDSA)</a></li>
</ul>
<h3>12.2 Informative References</h3>
<ul>
<li><a href="https://github.com/sirrayalabs/trust-decay-framework">Sirraya Labs — Trust Decay Framework Repository</a></li>
<li><a href="https://www.w3.org/TR/vc-zkp/">W3C Verifiable Credentials with Zero-Knowledge Proofs</a></li>
</ul>
</section>
<!-- Footer -->
<div class="footer">
<p>Trust Decay Framework (TDF) — Version 1.0.0</p>
<p>Developed by Sirraya Labs • Open Source • MIT License</p>
<p><a href="https://github.com/sirrayalabs/trust-decay-framework" style="color: #3b82f6;">GitHub Repository</a> |
<a href="https://spec.sirraya.com/tdf" style="color: #3b82f6;">Specification Home</a></p>
</div>
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