tutorials: add audit trail tutorial (04)

tutorials/04-audit-trail.md

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+# Tutorial 04: Building a Verifiable Audit Trail
+
+OpenGradient's core guarantee is that every LLM inference runs inside a
+Trusted Execution Environment (TEE) and is settled on-chain. This means
+you can prove — cryptographically — what prompt was sent, which model
+processed it, and what it returned.
+
+This tutorial shows how to turn that guarantee into a practical audit
+trail: a tamper-proof log you can store, inspect, and verify at any time.
+
+## Prerequisites
+
+```bash
+pip install opengradient
+export OG_PRIVATE_KEY="0x..."
+```
+
+Get free OPG test tokens at https://faucet.opengradient.ai
+
+## What makes an inference verifiable?
+
+Every response from `llm.chat()` includes:
+
+| Field | What it is |
+|---|---|
+| `transaction_hash` | On-chain record of the inference payment |
+| `tee_signature` | RSA-PSS signature from the TEE enclave |
+| `tee_timestamp` | ISO timestamp from inside the enclave |
+| `tee_id` | On-chain registry ID of the enclave |
+| `tee_payment_address` | Payment address of the TEE node |
+
+Verify any transaction at https://explorer.opengradient.ai
+
+## Step 1: Choose INDIVIDUAL_FULL settlement
+
+For audit trail use cases, use `INDIVIDUAL_FULL` — it records the
+complete input and output on-chain, not just a hash:
+
+```python
+import opengradient as og
+
+result = await llm.chat(
+    model=og.TEE_LLM.GEMINI_2_5_FLASH,
+    messages=[{"role": "user", "content": "Analyze this contract..."}],
+    x402_settlement_mode=og.x402SettlementMode.INDIVIDUAL_FULL,
+)
+
+print(result.transaction_hash)  # verify on explorer
+print(result.tee_signature)     # cryptographic proof
+print(result.tee_id)            # which enclave served this
+```
+
+## Step 2: Build an audit entry
+
+Capture everything needed to prove the inference happened:
+
+```python
+import hashlib
+from dataclasses import dataclass
+from datetime import datetime, timezone
+from typing import Optional
+
+
+@dataclass
+class AuditEntry:
+    turn: int
+    timestamp_utc: str
+    model: str
+    prompt: str
+    response: str
+    transaction_hash: str
+    tee_id: Optional[str]
+    tee_signature: Optional[str]
+    tee_timestamp: Optional[str]
+    content_hash: str
+
+
+def compute_content_hash(prompt: str, response: str) -> str:
+    raw = (prompt + "||" + response).encode("utf-8")
+    return hashlib.sha256(raw).hexdigest()
+```
+
+The `content_hash` is a SHA-256 digest of prompt and response together.
+If anyone modifies the log after the fact, the hash will no longer match.
+
+## Step 3: Persist and verify
+
+Save to JSON and verify integrity at any time:
+
+```python
+import json
+from dataclasses import asdict
+
+
+def save_audit_log(entries, path="audit_log.json"):
+    with open(path, "w") as f:
+        json.dump([asdict(e) for e in entries], f, indent=2)
+
+
+def verify_audit_log(path="audit_log.json"):
+    with open(path) as f:
+        entries = json.load(f)
+    all_ok = True
+    for e in entries:
+        expected = compute_content_hash(e["prompt"], e["response"])
+        ok = expected == e["content_hash"]
+        print("Turn " + str(e["turn"]) + ": " + ("OK" if ok else "TAMPERED"))
+        if not ok:
+            all_ok = False
+    return all_ok
+```
+
+## Step 4: Verify on-chain
+
+Every `transaction_hash` can be looked up on the block explorer:
+
+```
+https://explorer.opengradient.ai/tx/<transaction_hash>
+```
+
+This gives you the full on-chain record: model, timestamp, and
+cryptographic proof that the TEE enclave processed your request.
+
+## Settlement mode comparison
+
+| Mode | On-chain data | Best for |
+|---|---|---|
+| `PRIVATE` | Payment only | Maximum privacy |
+| `BATCH_HASHED` | Merkle hash batch | High volume, cost efficiency |
+| `INDIVIDUAL_FULL` | Full input + output | Compliance, audit trails |
+
+## Full example
+
+See `examples/llm_audit_trail.py` for a complete runnable script
+that combines all the steps above.
+
+## Summary
+
+- Use `INDIVIDUAL_FULL` settlement for maximum on-chain transparency
+- Store `transaction_hash` and `tee_signature` for every inference
+- Add a `content_hash` to detect local log tampering
+- Verify transactions at https://explorer.opengradient.ai