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🎯 What is QWED A2A?

QWED A2A is a verification interceptor for Google's Agent-to-Agent (A2A) protocol. It sits between autonomous agents and verifies every inter-agent payload before it reaches the recipient.

This is NOT a domain guard. Unlike QWED Finance (banking math) or QWED Legal (citations), A2A is infrastructure-level — it intercepts ALL agent communication and routes to the right verification engine.

Without A2A	With A2A
Agent sends wrong total → Propagates downstream	Blocked — math hallucination detected
Agent sends os.system() code → Executes on receiver	Blocked — dangerous pattern detected
Agent makes contradictory claims → Accepted silently	Blocked — logical contradiction caught
Rogue agent floods messages → No limit — DoS possible	Rate-limited — token bucket enforced
No audit trail → Nothing to prove	JWT attestation — cryptographic proof of every decision

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⚡ Quick Start

Prerequisites — Signing Key

QWED A2A requires a persistent ECDSA P-256 signing key for audit continuity (ephemeral per-process keys broke cross-restart verification). Set the key via the QWED_A2A_SIGNING_KEY_PEM environment variable:

# Generate a new P-256 private key (PKCS#8 format):
openssl ecparam -name prime256v1 -genkey -noout | openssl pkcs8 -topk8 -nocrypt > qwed-a2a-key.pem

# Set the environment variable:
export QWED_A2A_SIGNING_KEY_PEM=$(cat qwed-a2a-key.pem)

Installation

# From source (recommended for now)
git clone https://github.com/QWED-AI/qwed-a2a.git
cd qwed-a2a
pip install -e ".[dev]"

Your First Verification

import asyncio
from qwed_a2a.interceptor import A2AVerificationInterceptor
from qwed_a2a.protocol.schema import AgentMessage, PayloadType

interceptor = A2AVerificationInterceptor()

# Trust the communicating agents (Zero-Trust default)
interceptor.trust.trust_agent("procurement-agent")
interceptor.trust.trust_agent("treasury-agent")

message = AgentMessage(
    sender_agent_id="procurement-agent",
    receiver_agent_id="treasury-agent",
    payload_type=PayloadType.FINANCIAL_TRANSACTION,
    payload={
        "data": {
            "claimed_total": 150.00,
            "line_items": [
                {"description": "Widget A", "amount": 50.00, "quantity": 2},
                {"description": "Widget B", "amount": 25.00, "quantity": 2},
            ]
        }
    }
)

async def main():
    verdict = await interceptor.intercept(message, trace_id="demo_001")
    print(f"Status:  {verdict.status.value}")    # forwarded ✅
    print(f"Engine:  {verdict.engine_used}")      # finance_guard
    print(f"JWT:     {verdict.attestation_jwt[:50]}...")

asyncio.run(main())

Output:

Status:  forwarded ✅
Engine:  finance_guard
JWT:     eyJhbGciOiJFUzI1NiIsInR5cCI6InF3ZWQtYTJhLWF0dGVz...

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🔬 How It Works

Every inter-agent message flows through five deterministic stages:

┌──────────────────────────────────────────────────────────────────┐
│                    QWED A2A INTERCEPTOR                          │
│                                                                  │
│  Agent A ──▶ [Schema] ──▶ [Trust] ──▶ [Engine] ──▶ [JWT] ──▶ ? │
│               Validate     Boundary    Verify      Sign          │
│                                                                  │
│                                         │                        │
│                              ┌──────────┴──────────┐            │
│                              ▼                      ▼            │
│                        ✅ FORWARDED             ❌ BLOCKED       │
│                        + attestation            + reason         │
│                        → Agent B                → Agent A        │
└──────────────────────────────────────────────────────────────────┘

Stage	Component	What It Does
0. Schema (endpoint layer)	Pydantic AgentMessage	Validates sender/receiver IDs, payload type, timestamp — runs at FastAPI before intercept()
1. Trust	TrustBoundary	Blocklists, allowlists, pair blocks, token-bucket rate limiting
2. Bypass	Trusted agents check	Skips verification for agents in config.trusted_agents
3. Engine	_route_to_engine()	Routes to finance_guard, logic_guard, code_guard, or passthrough
4. Verdict + JWT	A2ACryptoService	Builds verdict and signs with ES256 JWT (payload hash, trace ID)

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🛡️ Verification Engines

🧮 Finance Guard — Decimal Arithmetic

Recomputes totals from line items using decimal.Decimal. Catches math hallucinations.

# Agent claims total is \$999.99, but line items add up to \$150.00
# → ❌ BLOCKED: "Mathematical hallucination detected: claimed_total=999.99, computed_total=150.00"

Precision: ROUND_HALF_UP quantized to 0.01. Floating-point arithmetic is never used.

⚖️ Logic Guard — Contradiction Detection

Detects logical contradictions using set-based analysis. If a claim is both asserted and negated, the message is blocked.

# Agent says "budget_approved" AND "budget_approved is false"
# → ❌ BLOCKED: "Logical contradiction detected: claims both asserted and negated: ['budget_approved']"

Determinism: Contradictions are sorted() before output — stable across all environments.

🔒 Code Guard — Regex Security Scan

Scans code payloads for dangerous patterns using case-insensitive compiled regex:

Pattern	Catches
eval	eval(, EVAL (, dynamic evaluation
exec	exec(, code execution
subprocess	subprocess.run, import subprocess, from subprocess import
os.system	os.system(, shell command execution
os.popen	Process spawning via popen
__import__	Dynamic imports
compile	Code compilation
importlib	Runtime module loading

Hardened: Subprocess regex catches import statements and aliases, not just subprocess.call().

📦 Passthrough

Messages with payload_type of GENERAL or DATA_QUERY are forwarded without verification.

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🔐 Zero-Trust Boundary

The TrustBoundary enforces deny-all by default. No communication is allowed unless explicitly permitted.

from qwed_a2a.security.trust_boundary import TrustBoundary

# Deny all by default
boundary = TrustBoundary()  # default_allow=False

# Explicitly trust specific agents
boundary.trust_agent("procurement-agent")
boundary.trust_agent("treasury-agent")

# Block a rogue agent globally
boundary.block_agent("rogue-agent-007")

# Block a specific pair
boundary.block_pair("agent-A", "agent-B")

Evaluation Order

Step	Check	On Failure
1	Sender on global blocklist?	BLOCKED
2	Receiver on global blocklist?	BLOCKED
3	Pair explicitly blocked?	BLOCKED
4	(Strict mode) Neither in allowlist?	BLOCKED
5	Token bucket has tokens?	RATE LIMITED
✅	All passed	ALLOWED

Token-Bucket Rate Limiting

• Algorithm: Token bucket (not fixed-window) — smooth, fair enforcement
• Eviction: Cold pairs (no requests for 5 min) are automatically evicted to prevent memory exhaustion
• Map spray prevention: Rate-limit state is only allocated after allowlist checks pass

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🔏 Crypto Attestations

Every verdict includes a signed ES256 JWT attestation:

{
  "iss": "did:qwed:a2a:local",
  "sub": "sha256:e3b0c44298fc1c...",
  "iat": 1711411200,
  "exp": 1711497600,
  "jti": "a2a_trace_001",
  "qwed_a2a": {
    "version": "1.0",
    "verdict": "forwarded",
    "engine": "finance_guard",
    "sender": "procurement-agent",
    "receiver": "treasury-agent"
  }
}

Feature	Details
Algorithm	ECDSA P-256 (ES256)
Tamper detection	sub claim contains SHA-256 hash of original payload
Identity	DID-based issuer (did:qwed:a2a:local)
Expiry	24 hours default
Key persistence	Ephemeral keys replaced by QWED_A2A_SIGNING_KEY_PEM env var for audit continuity

Fail-Closed Attestations

cryptography and PyJWT are required dependencies. If they are unavailable, the interceptor raises at startup instead of returning unsigned verdicts. Signing failures also fail closed so attestation_jwt=None is never emitted as a normal verdict.

The signing key is also fail-closed: if QWED_A2A_SIGNING_KEY_PEM is not set, the first call to sign an attestation raises RuntimeError rather than silently falling back to an ephemeral key.

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🚀 FastAPI Gateway

QWED A2A includes a ready-to-use HTTP gateway. Because the interceptor uses a zero-trust default posture, you must whitelist agents via the QWED_A2A_TRUSTED_AGENTS environment variable.

from fastapi import FastAPI
from qwed_a2a.protocol.endpoints import router, wellknown_router

app = FastAPI(title="QWED A2A Gateway")
app.include_router(router)
app.include_router(wellknown_router)

# QWED_A2A_TRUSTED_AGENTS="agent-A,agent-B" uvicorn main:app --host 0.0.0.0 --port 8000

Endpoints

Endpoint	Method	Description
/a2a/intercept	POST	Verify agent message — returns verdict + attestation
/a2a/health	GET	Service health check
/a2a/metrics	GET	Intercept metrics (forwarded, blocked, errors)
/.well-known/jwks.json	GET	Public JWK set for JWT verification

cURL Test

curl -X POST http://localhost:8000/a2a/intercept \
  -H "Content-Type: application/json" \
  -d '{
    "sender_agent_id": "agent-A",
    "receiver_agent_id": "agent-B",
    "payload_type": "general",
    "payload": {"message": "Hello!"}
  }'

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🏗️ Architecture

qwed-a2a/
├── src/qwed_a2a/
│   ├── interceptor.py          # Core verification pipeline
│   ├── protocol/
│   │   ├── schema.py           # Pydantic models (AgentMessage, Verdict, Config)
│   │   └── endpoints.py        # FastAPI router with /a2a/intercept
│   ├── security/
│   │   ├── trust_boundary.py   # Zero-trust enforcement + token-bucket rate limiter
│   │   └── crypto.py           # ES256 JWT attestations (A2ACryptoService)
│   └── utils/
│       └── telemetry.py        # Sentry integration + structured logging
├── tests/
│   ├── test_interceptor.py     # Financial, logic, code, general test suites
│   └── conftest.py             # Shared fixtures
├── pyproject.toml
└── LICENSE                     # Apache 2.0

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⚙️ Configuration

Field	Type	Default	Description
enable_financial_verification	bool	True	Route financial payloads to math verification
enable_logic_verification	bool	True	Route logic assertions to contradiction checks
enable_code_verification	bool	True	Route code payloads to regex security scanning
block_on_error	bool	True	Block on internal engine errors (set False for shadow mode)
max_payload_size_bytes	int	1,048,576	Maximum payload size (1 MB)
trusted_agents	List[str]	None	Agent IDs that bypass verification

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🧪 Testing

# Run all tests
pytest tests/ -v

# Run specific test suite
pytest tests/test_interceptor.py::TestInterceptorFinancial -v
pytest tests/test_interceptor.py::TestInterceptorCode -v
pytest tests/test_interceptor.py::TestInterceptorLogic -v

All tests use deterministic trace_id injection — no randomness, fully reproducible.

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📋 Dependencies

Package	Version	Purpose
pydantic	≥2.5.0	Schema validation (AgentMessage, Config)
fastapi	≥0.104.0	HTTP gateway for inter-agent routing
cryptography	≥41.0.0	ECDSA P-256 key generation for attestations
PyJWT	≥2.8.0	JWT signing and verification
sentry-sdk	≥2.13.0	Error tracking and telemetry

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🌐 Part of the QWED Ecosystem

QWED A2A is the infrastructure-level verification gateway in the QWED ecosystem:

Package	What It Does	Link
qwed	Core verification engines (Math, Logic, SQL, Code)	GitHub
qwed-a2a	This repo — Agent-to-Agent verification interceptor	—
qwed-finance	Banking, loans, NPV, ISO 20022 verification	GitHub
qwed-legal	Contracts, deadlines, citations, jurisdiction	GitHub
qwed-tax	Tax compliance & withholding verification	GitHub
qwed-infra	IaC verification (Terraform, IAM, Cost)	GitHub
qwed-mcp	Claude Desktop MCP integration	GitHub

📖 Full Documentation · 🎓 Free Course

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📄 License

Apache 2.0 — See LICENSE for details.

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