# PII Redaction POC

A proof-of-concept .NET solution that redacts personally identifiable information (PII) from user prompts **before** sending them to a large language model (LLM). The design demonstrates enterprise-grade separation of concerns using SOLID principles, dependency injection, and the `Microsoft.Extensions.AI` abstractions.

## Purpose

Financial and customer-service prompts often contain regulated data (names, government IDs, account numbers). This POC shows how to:

1. Accept a console prompt
2. Detect PII using **Regex**, **ONNX NER**, and **domain rules**
3. Replace values with stable placeholders
4. Send only the **sanitized** prompt to an LLM (mocked for now)

## Architecture

For solution design, data-flow diagrams, trust boundaries, and project responsibilities, see **[docs/architecture.md](docs/architecture.md)**.

## Why Three Detection Strategies?

| Strategy | Used For | Rationale |
|----------|----------|-----------|
| **Regex** | Email, phone, PAN, Aadhaar, credit card | Deterministic, format-bound identifiers with stable rules that are easy to audit and test |
| **ONNX NER** | Person names | Contextual entities without rigid formats; names vary widely in surface form |
| **Domain rules** | Loan number, customer ID, account number | Business-specific identifiers defined by internal systems, not inferable from generic models alone |

## Why the LLM Receives Only Sanitized Text

The placeholder map (`<PERSON_1>` → original value) is kept **in-process** for audit or downstream de-tokenization. Only the sanitized prompt crosses the LLM boundary. This reduces data-exposure risk and supports compliance requirements for regulated workloads.

## Project Structure

```
src/
├── PiiRedaction.ConsoleApp/     # Presentation: input/output, DI bootstrap
├── PiiRedaction.Core/           # Business logic: detection, redaction, models
└── PiiRedaction.Infrastructure/ # Technical adapters: ONNX Runtime, mock LLM
models/                          # Optional ONNX model files (gitignored)
```

| Project | Responsibility |
|---------|----------------|
| `PiiRedaction.ConsoleApp` | Read prompt, call sanitizer, display results, call LLM service |
| `PiiRedaction.Core` | PII detection abstractions, redaction, sanitization orchestration |
| `PiiRedaction.Infrastructure` | ONNX model runner, `IChatClient` mock implementation |

## Prerequisites

- [.NET SDK](https://dotnet.microsoft.com/download) 10.x (or compatible SDK for `net10.0`)
- **ONNX NER model** for person-name detection (see [ONNX Model Setup](#onnx-model-setup))
- Python 3.10+ (only for the model download script)

> **Note:** This environment targets `net10.0` because .NET 10 SDK is installed. The architecture is identical to the planned .NET 9 layout; change `TargetFramework` in `.csproj` files if you use .NET 9 SDK.

## Build and Run

From the repository root:

```bash
dotnet restore
dotnet build
dotnet run --project src/PiiRedaction.ConsoleApp
```

By default the console app runs **11 curated sample prompts** covering NER/person names, regex identifiers, domain IDs, combined scenarios, and a clean no-PII ticket.

List available samples:

```bash
dotnet run --project src/PiiRedaction.ConsoleApp -- --list
```

Run a single sample by index or name:

```bash
dotnet run --project src/PiiRedaction.ConsoleApp -- --sample 2
dotnet run --project src/PiiRedaction.ConsoleApp -- --name MrTitlePerson
```

Interactive mode (enter your own prompt):

```bash
dotnet run --project src/PiiRedaction.ConsoleApp -- --interactive
```

### Console sample catalog

Samples are defined in [`SamplePromptCatalog.cs`](src/PiiRedaction.ConsoleApp/Samples/SamplePromptCatalog.cs).

| # | Name | Category | NER / Person example |
|---|------|----------|----------------------|
| 0 | FullFinancialWithCustomer | NER + Regex + Domain | `Customer Ravi Kumar` + email, phone, loan, PAN |
| 1 | CustomerNameOnly | NER | `Customer Anita Sharma` |
| 2 | MrTitlePerson | NER | `Mr. John Smith` |
| 3 | MrsTitlePerson | NER | `Mrs. Lakshmi Reddy` |
| 4 | DrTitlePerson | NER | `Dr. Jane Doe` |
| 5 | TwoCustomersInOnePrompt | NER | `Customer Ravi Kumar` and `Customer Priya Nair` |
| 6 | PersonWithDomainIds | NER + Domain | `Customer Meera Iyer` + CID / ACC |
| 7 | PersonWithEmailNoPhone | NER + Regex | `Customer Arjun Mehta` + email |
| 8 | AllRegexTypes | Regex | email, phone, PAN, Aadhaar, card |
| 9 | AllDomainIds | Domain | LN, CID, ACC |
| 10 | NoPiiCleanTicket | Negative | no redaction |

Person names are detected via **ONNX NER** using `dslim/bert-base-NER` (or a compatible token-classification export). A real model is **required** for person-name detection; there is no regex or heuristic fallback.

## NuGet Packages

```bash
# Infrastructure
dotnet add src/PiiRedaction.Infrastructure package Microsoft.ML.OnnxRuntime
dotnet add src/PiiRedaction.Infrastructure package Microsoft.ML.Tokenizers
dotnet add src/PiiRedaction.Infrastructure package Microsoft.Extensions.AI.Abstractions
dotnet add src/PiiRedaction.Infrastructure package Microsoft.Extensions.AI
dotnet add src/PiiRedaction.Infrastructure package Microsoft.Extensions.Logging.Abstractions
dotnet add src/PiiRedaction.Infrastructure package Microsoft.Extensions.Options

# Core
dotnet add src/PiiRedaction.Core package Microsoft.Extensions.Options

# ConsoleApp
dotnet add src/PiiRedaction.ConsoleApp package Microsoft.Extensions.Hosting
dotnet add src/PiiRedaction.ConsoleApp package Microsoft.Extensions.DependencyInjection
dotnet add src/PiiRedaction.ConsoleApp package Microsoft.Extensions.Configuration.Json
dotnet add src/PiiRedaction.ConsoleApp package Microsoft.Extensions.Configuration.EnvironmentVariables
```

## Configuration

[`appsettings.json`](src/PiiRedaction.ConsoleApp/appsettings.json):

```json
{
  "PiiRedaction": {
    "OnnxModelPath": "models/ner-model.onnx"
  }
}
```

| Setting | Description |
|---------|-------------|
| `OnnxModelPath` | Path to ONNX NER model (relative to working directory or discovered by walking up from the current directory) |

## ONNX Model Setup

Person-name detection requires a token-classification ONNX model and companion tokenizer files in the `models/` directory:

| File | Purpose |
|------|---------|
| `models/ner-model.onnx` | Exported NER model |
| `models/vocab.txt` | BERT WordPiece vocabulary |
| `models/ner-labels.txt` | One BIO label per line (`O`, `B-PER`, `I-PER`, etc.) |

### Download script

From the repository root:

```powershell
.\scripts\download-ner-model.ps1
```

Or with Python directly:

```bash
python scripts/download-ner-model.py
```

The script exports [`dslim/bert-base-NER`](https://huggingface.co/dslim/bert-base-NER) via Hugging Face Optimum when Python is available. Otherwise it downloads the pre-exported ONNX assets from Hugging Face directly.

### Inference pipeline

`OnnxNerModelRunner` performs the full pipeline:

- BERT WordPiece tokenization (`Microsoft.ML.Tokenizers`)
- ONNX Runtime inference (`input_ids`, `attention_mask`, optional `token_type_ids`)
- BIO label decoding (`B-PER` / `I-PER` → `PiiEntityType.Person`)
- Character-span alignment back to the source text

When the model or tokenizer files are missing, person detection returns no results.

## Swapping Mock LLM for Azure OpenAI

The application depends on `ILlmPromptService` (Core) and `IChatClient` (Microsoft.Extensions.AI). To use Azure OpenAI later, replace the mock registration in [`ServiceCollectionExtensions.cs`](src/PiiRedaction.ConsoleApp/DependencyInjection/ServiceCollectionExtensions.cs):

```csharp
// Remove:
// services.AddSingleton<IChatClient, MockChatClient>();

// Add (example — package and API may vary by provider SDK version):
// services.AddAzureOpenAIChatClient(
//     new Uri(configuration["AzureOpenAI:Endpoint"]!),
//     configuration["AzureOpenAI:ApiKey"]!,
//     configuration["AzureOpenAI:DeploymentName"]!);

services.AddSingleton<ILlmPromptService, MockLlmPromptService>(); // unchanged
```

`MockLlmPromptService` already uses `IChatClient`, so it works with any registered chat client implementation.

## Sample Execution Output

```
=== PII Redaction POC ===

Original Prompt:
Customer Ravi Kumar with email ravi.kumar@gmail.com and phone 9876543210 has LoanNumber LN-456789 and PAN ABCDE1234F. Please summarize this customer issue.

Detected PII:
  [PERSON       ] Ravi Kumar             (Ner)
  [EMAIL        ] ravi.kumar@gmail.com   (Regex)
  [PHONE        ] 9876543210             (Regex)
  [LOAN_NUMBER  ] LN-456789              (Domain)
  [PAN          ] ABCDE1234F             (Regex)

Sanitized Prompt:
Customer <PERSON_1> with email <EMAIL_1> and phone <PHONE_1> has LoanNumber <LOAN_NUMBER_1> and PAN <PAN_1>. Please summarize this customer issue.

Internal Placeholder Map (not sent to LLM):
  <EMAIL_1>        -> ravi.kumar@gmail.com
  <LOAN_NUMBER_1>  -> LN-456789
  <PAN_1>          -> ABCDE1234F
  <PERSON_1>       -> Ravi Kumar
  <PHONE_1>        -> 9876543210

Mock LLM Response:
[Mock LLM Response] Received sanitized prompt (146 chars). No original PII was transmitted.
```

## SOLID Principles Applied

| Principle | Application |
|-----------|-------------|
| **Single Responsibility** | Each detector, redactor, and runner has one job; `Program.cs` only orchestrates |
| **Open/Closed** | Add new `IPiiDetector` implementations without changing merge logic |
| **Liskov Substitution** | All detectors are interchangeable via `IPiiDetector` |
| **Interface Segregation** | Separate interfaces for detection, redaction, sanitization, and LLM |
| **Dependency Inversion** | Core defines abstractions; Infrastructure implements them |

## Testing

The solution includes an **NUnit** test suite across two projects:

| Project | Focus |
|---------|-------|
| `tests/PiiRedaction.Core.Tests` | Detectors, redactor, sanitizer, golden pipeline scenarios (fake NER), real-model integration tests |
| `tests/PiiRedaction.Infrastructure.Tests` | Mock LLM, ONNX runner unit tests, real-model NER runner tests |

### Run tests

```bash
dotnet test
dotnet test --filter "FullyQualifiedName~GoldenPromptTests"
dotnet test --filter "Category=RealModel"
dotnet test --logger "console;verbosity=detailed"
```

Fast CI runs without the ONNX model: fake-based tests always execute; tests marked **`Category=RealModel`** are skipped when `models/ner-model.onnx` is absent. Download the model first:

```powershell
.\scripts\download-ner-model.ps1
```

### Test architecture

- **`PromptScenarioCatalog`** — five focused end-to-end scenarios (canonical demo, multi-regex, duplicate people, overlap stress, no-PII negative)
- **`ProductionPipelineFactory`** — builds the same Domain → Regex → OnnxNer composite stack as production DI; `CreateWithRealModel(runner)` wires a real `OnnxNerModelRunner`
- **`FakeOnnxNerModelRunner`** — unit-test double for NER; golden tests inject person spans per scenario
- **`GoldenPromptTests`** — end-to-end sanitization proof across the catalog (fake NER)
- **`RealNerModelFixture`** — shared fixture that loads `models/ner-model.onnx` once per class; skips when model missing
- **`RealNerModelRunnerTests`** — direct ONNX inference with span accuracy checks
- **`RealNerPipelineTests`** — full pipeline with real NER (canonical, multi-person, clean-ticket negative)
- **`OnnxNerModelRunnerTests`** — unit tests for missing/invalid model paths (no download required)
- **`CompositePiiDetectorTests`** — overlap merge and source-priority rules
- **`LlmBoundaryTests`** — verifies raw PII never appears in outbound LLM messages

Assertions use **FluentAssertions** for readable failures on long prompt strings.

## Future Enhancements

- ASP.NET Core API host with request/response middleware
- Persistent audit log of redaction events (without storing raw PII)
- Secure vault for reversible tokenization
- Real Azure OpenAI / OpenAI provider registration