# Encryption Strategy
## AI Tax Agent System

**Document Version:** 1.0  
**Date:** 2024-01-31  
**Owner:** Security Architecture Team  

## 1. Executive Summary

This document defines the comprehensive encryption strategy for the AI Tax Agent System, covering data at rest, in transit, and in use. The strategy implements defense-in-depth with multiple encryption layers and key management best practices.

## 2. Encryption Requirements

### 2.1 Regulatory Requirements
- **GDPR Article 32**: Appropriate technical measures including encryption
- **UK Data Protection Act 2018**: Security of processing requirements
- **HMRC Security Standards**: Government security classifications
- **ISO 27001**: Information security management requirements
- **SOC 2 Type II**: Security and availability controls

### 2.2 Business Requirements
- **Client Data Protection**: Financial and personal information
- **Intellectual Property**: Proprietary algorithms and models
- **Regulatory Compliance**: Audit trail and evidence integrity
- **Business Continuity**: Key recovery and disaster recovery

## 3. Encryption Architecture

### 3.1 Encryption Layers

```mermaid
graph TB
    A[Client Browser] -->|TLS 1.3| B[Traefik Gateway]
    B -->|mTLS| C[Application Services]
    C -->|Application-Level| D[Database Layer]
    D -->|Transparent Data Encryption| E[Storage Layer]
    E -->|Volume Encryption| F[Disk Storage]
    
    G[Key Management] --> H[Vault HSM]
    H --> I[Encryption Keys]
    I --> C
    I --> D
    I --> E
```

### 3.2 Encryption Domains

| Domain | Technology | Key Size | Algorithm | Rotation |
|--------|------------|----------|-----------|----------|
| **Transport** | TLS 1.3 | 256-bit | AES-GCM, ChaCha20-Poly1305 | Annual |
| **Application** | AES-GCM | 256-bit | AES-256-GCM | Quarterly |
| **Database** | TDE | 256-bit | AES-256-CBC | Quarterly |
| **Storage** | LUKS/dm-crypt | 256-bit | AES-256-XTS | Annual |
| **Backup** | GPG | 4096-bit | RSA-4096 + AES-256 | Annual |

## 4. Data Classification and Encryption

### 4.1 Data Classification Matrix

| Classification | Examples | Encryption Level | Key Access |
|----------------|----------|------------------|------------|
| **PUBLIC** | Marketing materials, documentation | TLS only | Public |
| **INTERNAL** | System logs, metrics | TLS + Storage | Service accounts |
| **CONFIDENTIAL** | Client names, addresses | TLS + App + Storage | Authorized users |
| **RESTRICTED** | Financial data, UTR, NI numbers | TLS + App + Field + Storage | Need-to-know |
| **SECRET** | Encryption keys, certificates | HSM + Multiple layers | Key custodians |

### 4.2 Field-Level Encryption

**Sensitive Fields Requiring Field-Level Encryption:**
```python
ENCRYPTED_FIELDS = {
    'taxpayer_profile': ['utr', 'ni_number', 'full_name', 'address'],
    'financial_data': ['account_number', 'sort_code', 'iban', 'amount'],
    'document_content': ['ocr_text', 'extracted_fields'],
    'authentication': ['password_hash', 'api_keys', 'tokens']
}
```

**Implementation Example:**
```python
from cryptography.fernet import Fernet
import vault_client

class FieldEncryption:
    def __init__(self, vault_client):
        self.vault = vault_client
        
    def encrypt_field(self, field_name: str, value: str) -> str:
        """Encrypt sensitive field using Vault transit engine"""
        key_name = f"field-{field_name}"
        response = self.vault.encrypt(
            mount_point='transit',
            name=key_name,
            plaintext=base64.b64encode(value.encode()).decode()
        )
        return response['data']['ciphertext']
    
    def decrypt_field(self, field_name: str, ciphertext: str) -> str:
        """Decrypt sensitive field using Vault transit engine"""
        key_name = f"field-{field_name}"
        response = self.vault.decrypt(
            mount_point='transit',
            name=key_name,
            ciphertext=ciphertext
        )
        return base64.b64decode(response['data']['plaintext']).decode()
```

## 5. Key Management Strategy

### 5.1 Key Hierarchy

```
Root Key (HSM)
├── Master Encryption Key (MEK)
│   ├── Data Encryption Keys (DEK)
│   │   ├── Database DEK
│   │   ├── Application DEK
│   │   └── Storage DEK
│   └── Key Encryption Keys (KEK)
│       ├── Field Encryption KEK
│       ├── Backup KEK
│       └── Archive KEK
└── Signing Keys
    ├── JWT Signing Key
    ├── Document Signing Key
    └── API Signing Key
```

### 5.2 HashiCorp Vault Configuration

**Vault Policies:**
```hcl
# Database encryption policy
path "transit/encrypt/database-*" {
  capabilities = ["create", "update"]
}

path "transit/decrypt/database-*" {
  capabilities = ["create", "update"]
}

# Application encryption policy
path "transit/encrypt/app-*" {
  capabilities = ["create", "update"]
}

path "transit/decrypt/app-*" {
  capabilities = ["create", "update"]
}

# Field encryption policy (restricted)
path "transit/encrypt/field-*" {
  capabilities = ["create", "update"]
  allowed_parameters = {
    "plaintext" = []
  }
  denied_parameters = {
    "batch_input" = []
  }
}
```

**Key Rotation Policy:**
```hcl
# Automatic key rotation
path "transit/keys/database-primary" {
  min_decryption_version = 1
  min_encryption_version = 2
  deletion_allowed = false
  auto_rotate_period = "2160h"  # 90 days
}
```

### 5.3 Hardware Security Module (HSM)

**HSM Configuration:**
- **Type**: AWS CloudHSM / Azure Dedicated HSM
- **FIPS Level**: FIPS 140-2 Level 3
- **High Availability**: Multi-AZ deployment
- **Backup**: Encrypted key backup to secure offline storage

## 6. Transport Layer Security

### 6.1 TLS Configuration

**Traefik TLS Configuration:**
```yaml
tls:
  options:
    default:
      minVersion: "VersionTLS13"
      maxVersion: "VersionTLS13"
      cipherSuites:
        - "TLS_AES_256_GCM_SHA384"
        - "TLS_CHACHA20_POLY1305_SHA256"
        - "TLS_AES_128_GCM_SHA256"
      curvePreferences:
        - "X25519"
        - "secp384r1"
      sniStrict: true
      
  certificates:
    - certFile: /certs/wildcard.crt
      keyFile: /certs/wildcard.key
```

### 6.2 Certificate Management

**Certificate Lifecycle:**
- **Issuance**: Let's Encrypt with DNS challenge
- **Rotation**: Automated 30-day renewal
- **Monitoring**: Certificate expiry alerts
- **Backup**: Encrypted certificate backup

**Internal PKI:**
```bash
# Vault PKI setup
vault secrets enable -path=pki-root pki
vault secrets tune -max-lease-ttl=87600h pki-root

vault write pki-root/root/generate/internal \
    common_name="AI Tax Agent Root CA" \
    ttl=87600h \
    key_bits=4096

vault secrets enable -path=pki-int pki
vault secrets tune -max-lease-ttl=43800h pki-int

vault write pki-int/intermediate/generate/internal \
    common_name="AI Tax Agent Intermediate CA" \
    ttl=43800h \
    key_bits=4096
```

## 7. Database Encryption

### 7.1 PostgreSQL Encryption

**Transparent Data Encryption (TDE):**
```sql
-- Enable pgcrypto extension
CREATE EXTENSION IF NOT EXISTS pgcrypto;

-- Create encrypted table
CREATE TABLE taxpayer_profiles (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    utr_encrypted BYTEA NOT NULL,
    ni_number_encrypted BYTEA NOT NULL,
    name_encrypted BYTEA NOT NULL,
    created_at TIMESTAMP DEFAULT NOW()
);

-- Encryption functions
CREATE OR REPLACE FUNCTION encrypt_pii(data TEXT, key_id TEXT)
RETURNS BYTEA AS $$
BEGIN
    -- Use Vault transit engine for encryption
    RETURN vault_encrypt(data, key_id);
END;
$$ LANGUAGE plpgsql;
```

**Column-Level Encryption:**
```python
class EncryptedTaxpayerProfile(Base):
    __tablename__ = 'taxpayer_profiles'
    
    id = Column(UUID, primary_key=True, default=uuid.uuid4)
    utr_encrypted = Column(LargeBinary, nullable=False)
    ni_number_encrypted = Column(LargeBinary, nullable=False)
    
    @hybrid_property
    def utr(self):
        return vault_client.decrypt('field-utr', self.utr_encrypted)
    
    @utr.setter
    def utr(self, value):
        self.utr_encrypted = vault_client.encrypt('field-utr', value)
```

### 7.2 Neo4j Encryption

**Enterprise Edition Features:**
```cypher
// Enable encryption at rest
CALL dbms.security.setConfigValue('dbms.security.encryption.enabled', 'true');

// Create encrypted property
CREATE CONSTRAINT encrypted_utr IF NOT EXISTS
FOR (tp:TaxpayerProfile)
REQUIRE tp.utr_encrypted IS NOT NULL;

// Encryption UDF
CALL apoc.custom.asFunction(
    'encrypt',
    'RETURN apoc.util.md5([text, $key])',
    'STRING',
    [['text', 'STRING'], ['key', 'STRING']]
);
```

## 8. Application-Level Encryption

### 8.1 Microservice Encryption

**Service-to-Service Communication:**
```python
import httpx
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import rsa, padding

class SecureServiceClient:
    def __init__(self, service_url: str, private_key: rsa.RSAPrivateKey):
        self.service_url = service_url
        self.private_key = private_key
        
    async def make_request(self, endpoint: str, data: dict):
        # Encrypt request payload
        encrypted_data = self.encrypt_payload(data)
        
        # Sign request
        signature = self.sign_request(encrypted_data)
        
        async with httpx.AsyncClient() as client:
            response = await client.post(
                f"{self.service_url}/{endpoint}",
                json={"data": encrypted_data, "signature": signature},
                headers={"Content-Type": "application/json"}
            )
            
        # Decrypt response
        return self.decrypt_response(response.json())
```

### 8.2 Document Encryption

**Document Storage Encryption:**
```python
class DocumentEncryption:
    def __init__(self, vault_client):
        self.vault = vault_client
        
    def encrypt_document(self, document_content: bytes, doc_id: str) -> dict:
        """Encrypt document with unique DEK"""
        # Generate document-specific DEK
        dek = self.vault.generate_data_key('document-master-key')
        
        # Encrypt document with DEK
        cipher = Fernet(dek['plaintext_key'])
        encrypted_content = cipher.encrypt(document_content)
        
        # Store encrypted DEK
        encrypted_dek = dek['ciphertext_key']
        
        return {
            'encrypted_content': encrypted_content,
            'encrypted_dek': encrypted_dek,
            'key_version': dek['key_version']
        }
```

## 9. Backup and Archive Encryption

### 9.1 Backup Encryption Strategy

**Multi-Layer Backup Encryption:**
```bash
#!/bin/bash
# Backup encryption script

# 1. Database dump with encryption
pg_dump tax_system | gpg --cipher-algo AES256 --compress-algo 2 \
    --symmetric --output backup_$(date +%Y%m%d).sql.gpg

# 2. Neo4j backup with encryption
neo4j-admin backup --backup-dir=/backups/neo4j \
    --name=graph_$(date +%Y%m%d) --encrypt

# 3. Document backup with encryption
tar -czf - /data/documents | gpg --cipher-algo AES256 \
    --symmetric --output documents_$(date +%Y%m%d).tar.gz.gpg

# 4. Upload to encrypted cloud storage
aws s3 cp backup_$(date +%Y%m%d).sql.gpg \
    s3://tax-agent-backups/ --sse aws:kms --sse-kms-key-id alias/backup-key
```

### 9.2 Archive Encryption

**Long-Term Archive Strategy:**
- **Encryption**: AES-256 with 10-year key retention
- **Integrity**: SHA-256 checksums with digital signatures
- **Storage**: Geographically distributed encrypted storage
- **Access**: Multi-person authorization for archive access

## 10. Key Rotation and Recovery

### 10.1 Automated Key Rotation

**Rotation Schedule:**
```python
ROTATION_SCHEDULE = {
    'transport_keys': timedelta(days=365),      # Annual
    'application_keys': timedelta(days=90),     # Quarterly  
    'database_keys': timedelta(days=90),        # Quarterly
    'field_encryption_keys': timedelta(days=30), # Monthly
    'signing_keys': timedelta(days=180),        # Bi-annual
}

class KeyRotationManager:
    def __init__(self, vault_client):
        self.vault = vault_client
        
    async def rotate_keys(self):
        """Automated key rotation process"""
        for key_type, rotation_period in ROTATION_SCHEDULE.items():
            keys = await self.get_keys_due_for_rotation(key_type, rotation_period)
            
            for key in keys:
                await self.rotate_key(key)
                await self.update_applications(key)
                await self.verify_rotation(key)
```

### 10.2 Key Recovery Procedures

**Emergency Key Recovery:**
1. **Multi-Person Authorization**: Require 3 of 5 key custodians
2. **Secure Communication**: Use encrypted channels for coordination
3. **Audit Trail**: Log all recovery activities
4. **Verification**: Verify key integrity before use
5. **Re-encryption**: Re-encrypt data with new keys if compromise suspected

## 11. Monitoring and Compliance

### 11.1 Encryption Monitoring

**Key Metrics:**
- Key rotation compliance rate
- Encryption coverage percentage
- Failed encryption/decryption attempts
- Key access patterns and anomalies
- Certificate expiry warnings

**Alerting Rules:**
```yaml
groups:
  - name: encryption_alerts
    rules:
      - alert: KeyRotationOverdue
        expr: vault_key_age_days > 90
        for: 1h
        labels:
          severity: warning
        annotations:
          summary: "Encryption key rotation overdue"
          
      - alert: EncryptionFailure
        expr: rate(encryption_errors_total[5m]) > 0.1
        for: 2m
        labels:
          severity: critical
        annotations:
          summary: "High encryption failure rate detected"
```

### 11.2 Compliance Reporting

**Quarterly Encryption Report:**
- Encryption coverage by data classification
- Key rotation compliance status
- Security incidents related to encryption
- Vulnerability assessment results
- Compliance gap analysis

## 12. Incident Response

### 12.1 Key Compromise Response

**Response Procedures:**
1. **Immediate**: Revoke compromised keys
2. **Assessment**: Determine scope of compromise
3. **Containment**: Isolate affected systems
4. **Recovery**: Generate new keys and re-encrypt data
5. **Lessons Learned**: Update procedures and controls

### 12.2 Encryption Failure Response

**Failure Scenarios:**
- HSM hardware failure
- Key corruption or loss
- Encryption service outage
- Certificate expiry

**Recovery Procedures:**
- Activate backup HSM
- Restore keys from secure backup
- Implement manual encryption processes
- Emergency certificate issuance

---

**Document Classification**: CONFIDENTIAL  
**Next Review Date**: 2024-07-31  
**Approval**: Security Architecture Team