SENTINEL Phase 1 Defense System

IndabaX Tunisia 2026 AI Agent Security Challenge
Deterministic, Typed, Fail-Closed Security Architecture for Autonomous LLM Agents

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🛡️ Architecture Overview

The Phase 1 defense is a 100% deterministic, zero-ML/zero-LLM, fail-closed security pipeline built strictly around the official SENTINEL v1 defense contract.

flowchart TD
    Req[DefenseRequest from Evaluator] --> API[FastAPI /v1/decision]
    API --> Ctx[Context Builder & Normalizer]
    Ctx --> Prov[Provenance Graph Builder]
    Ctx --> Taint[Pessimistic Taint Propagator]
    Ctx --> Flow[Data-Flow & Canary Analyzer]
    Ctx --> Cap[Capability Checker]
    
    Prov --> Taint
    Taint --> Flow
    Taint --> Policy[Policy Engine]
    Flow --> Policy
    Cap --> Policy
    
    Policy --> DecEngine[Decision Engine: 10-Step Precedence]
    DecEngine --> Resp[DefenseDecision: ALLOW / BLOCK / ESCALATE / REWRITE]
    Resp --> Trace[Structured JSON Trace to stderr]

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Core Security Principles

1. Never Trust Untrusted Sources: Any content originating from UNTRUSTED_INTERNAL, UNTRUSTED_EXTERNAL, or ADVERSARY_CONTROLLED is tracked pessimistically across derivations.
2. Memory Does Not Upgrade Trust: Untrusted content stored in agent memory remains untrusted when subsequently retrieved.
3. No Unconfirmed Consequential Actions: Consequential actions (financial transfers, status closures, config mutations) require recorded human confirmations.
4. Data-Flow Boundary Enforcement: Sensitive information (CONFIDENTIAL, RESTRICTED) is prevented from flowing to external sinks (email_send, webhooks) in plain or encoded formats (Base64, Hex, Spaced, Reversed).
5. Fail-Closed Guarantee: Any unexpected exception or validation issue immediately yields a BLOCK decision with INTERNAL_ERROR.

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📁 Repository Structure

sentinel-defense/
├── api/
│   ├── __init__.py
│   └── defense.py             # Phase1Defense (implements Defense ABC) & FastAPI service
├── context/
│   ├── __init__.py
│   ├── builder.py             # Normalizes DefenseRequest into typed NormalizedContext
│   └── normalizer.py          # String squash, destination extraction, domain matching
├── provenance/
│   ├── __init__.py
│   ├── nodes.py               # ProvenanceNode (source, content, action, sink)
│   ├── edges.py               # ProvenanceEdge (derived_from, used_by, reaches)
│   ├── graph.py               # ProvenanceGraph with BFS ancestor queries
│   └── taint.py               # Pessimistic trust & sensitivity taint propagation
├── dataflow/
│   ├── __init__.py
│   ├── labels.py              # FlowLabel attached to propagating data
│   ├── sinks.py               # SinkCategory classification (external, financial, etc.)
│   ├── transforms.py          # Multi-encoding sensitive canary detection
│   └── flow.py                # DataFlowAnalyzer: sliding-window & canary leak detection
├── policy/
│   ├── __init__.py
│   ├── capabilities.py        # Tool permission & confirmation requirements
│   ├── engine.py              # PolicyResult evaluator against evaluator rules
│   └── rules.yaml             # Generic parameter thresholds
├── decision/
│   ├── __init__.py
│   ├── actions.py             # ReasonCode enum and DefenseDecision constructor
│   └── engine.py              # 10-step deterministic precedence decision engine
├── config/
│   └── defense.yaml           # Sink mappings, confidence scores, and thresholds
├── tests/
│   ├── __init__.py
│   ├── conftest.py            # Test request, action, and provenance builders
│   ├── test_provenance.py     # 9 tests: graph construction, BFS, taint rules
│   ├── test_dataflow.py       # 11 tests: sinks, encodings, sliding-window leaks
│   ├── test_policy.py         # 7 tests: permissions, consequential requirements
│   ├── test_decision.py       # 10 tests: decision precedence & score clamping
│   └── test_api.py            # 7 tests: FastAPI HTTP endpoints & health check
├── pyproject.toml
├── requirements.txt
├── pytest.ini
└── sentinel-submission.yaml   # Official evaluator submission manifest

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⚙️ Decision Precedence Chain

When evaluating a DefenseRequest, the DecisionEngine evaluates security signals in strict order:

Step	Rule	Condition	Decision	Reason Code
1	Invalid Action	TOOL_CALL action missing tool name	BLOCK	INVALID_ACTION
2	Unauthorized Tool	Tool not in allowed_tools	BLOCK	UNAUTHORIZED_TOOL
3	Adversary + Consequential	Action influenced by adversary_controlled source & action is consequential	BLOCK	ADVERSARY_CONTROLLED_SOURCE POLICY_VIOLATION
4	Forbidden Data Flow	Sensitive content detected in payload to external destination	BLOCK	FORBIDDEN_DATA_FLOW
5	Untrusted External Flow	Untrusted content influences a consequential external action	BLOCK	UNTRUSTED_SOURCE FORBIDDEN_DATA_FLOW
6	Adversary Influenced	Any action derived from an adversary-controlled source	BLOCK	ADVERSARY_CONTROLLED_SOURCE
7	Untrusted Memory Write	MEMORY_WRITE derived from untrusted source	BLOCK	UNTRUSTED_SOURCE
8	Confirmation Request	Agent issues REQUEST_CONFIRMATION for consequential action	ALLOW	CONFIRMATION_REQUESTED
9	Missing Confirmation	Consequential tool without confirmation: • email_send with email_draft allowed • Other consequential tools	REWRITE ESCALATE	REWRITE_TO_DRAFT MISSING_CONFIRMATION
10	Permitted Action	All security checks pass: • Non-tool RESPOND • Read-only tool • Confirmed consequential • General permitted tool	ALLOW ALLOW ALLOW ALLOW	USER_GOAL_ALIGNED READ_ONLY_ACTION USER_GOAL_ALIGNED USER_GOAL_ALIGNED

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🔍 Data-Flow & Encoding Detection

The data-flow analyzer inspects free-text payloads for confidential data leaks without hardcoding scenario secrets:

• Sliding-Window Text Overlap: Evaluates 32-character sliding window n-grams across normalized strings.
• Multi-Encoding Canary Matching: Automatically detects canary secrets and tags in:
  • Plain: Exact normalized string matching.
  • Spaced: Interleaved whitespace (s e c r e t).
  • Base64: Standard and unpadded Base64 representations.
  • Hex: Hex-encoded byte representations.
  • Reversed: Reversed character sequences.

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🚀 Running and Testing

1. Installation

pip install -r requirements.txt
pip install -e ../Sentinel_Starter_Kit

2. Running Test Suite

$env:PYTHONPATH = "."
python -m pytest tests/ -v

Current test suite: 46 passed (100% pass rate).

3. Starting Defense Service

python -m uvicorn api.defense:app --host 0.0.0.0 --port 8081

4. Running Official Evaluator

sentinel eval run --defense-url http://localhost:8081 scenarios/public/enterprise

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📊 Structured Tracing & Observability

Every decision emits a structured, single-line JSON log to stderr with zero secret exposure:

{
  "request_id": "eval_run_01:1",
  "action_type": "tool_call",
  "tool": "ticket_read",
  "decision": "allow",
  "reason_codes": ["READ_ONLY_ACTION"],
  "risk_score": 0.05,
  "confidence": 0.8,
  "worst_trust_level": "authenticated_user",
  "worst_sensitivity": "public",
  "action_depends_on_untrusted": false,
  "action_depends_on_adversary": false,
  "sink_category": "read_only",
  "has_sensitive_to_external": false,
  "tool_allowed": true,
  "confirmation_required": false,
  "confirmation_present": false,
  "policy_findings": [],
  "latency_ms": 0.22
}

Average decision latency is < 1.0 ms, providing high-throughput inline protection for agent evaluation.