Redflag/docs/4_LOG/November_2025/Security-Documentation/SECURITY.md

RedFlag Security Architecture

This document outlines the security architecture and implementation details for RedFlag's Ed25519-based cryptographic update system.

Overview

RedFlag implements a defense-in-depth security model for agent updates using:

• Ed25519 Digital Signatures for binary authenticity
• Runtime Public Key Distribution via Trust-On-First-Use (TOFU)
• Nonce-based Replay Protection for command freshness (<5min freshness)
• Atomic Update Process with automatic rollback and watchdog

Architecture Overview

graph TB
    A[Server Signs Package] --> B[Ed25519 Signature]
    B --> C[Package Distribution]
    C --> D[Agent Downloads]
    D --> E[Signature Verification]
    E --> F[AES-256-GCM Decryption]
    F --> G[Checksum Validation]
    G --> H[Atomic Installation]
    H --> I[Service Restart]
    I --> J[Update Confirmation]

    subgraph "Security Layers"
        K[Nonce Validation]
        L[Signature Verification]
        M[Encryption]
        N[Checksum Validation]
    end

Threat Model

Protected Against

• Package Tampering: Ed25519 signatures prevent unauthorized modifications
• Replay Attacks: Nonce-based validation ensures command freshness (< 5 minutes)
• Eavesdropping: AES-256-GCM encryption protects transit
• Rollback Protection: Version-based updates prevent downgrade attacks
• Privilege Escalation: Atomic updates with proper file permissions

Assumptions

• Server private key is securely stored and protected
• Agent system has basic file system protections
• Network transport uses HTTPS/TLS
• Initial agent registration is secure

Cryptographic Operations

Key Generation (Server Setup)

# Generate Ed25519 key pair for RedFlag
go run scripts/generate-keypair.go

# Output:
# REDFLAG_SIGNING_PRIVATE_KEY=c038751ba992c9335501a0853b83e93190021075...
# REDFLAG_PUBLIC_KEY=37f6d2a4ffe0f83bcb91d0ee2eb266833f766e8180866d31...

# Add the private key to server environment
# (Public key is distributed to agents automatically via API)

Package Signing Flow

sequenceDiagram
    participant S as Server
    participant PKG as Update Package
    participant A as Agent

    S->>PKG: 1. Generate Package
    S->>PKG: 2. Calculate SHA-256 Checksum
    S->>PKG: 3. Sign with Ed25519 Private Key
    S->>PKG: 4. Add Metadata (version, platform, etc.)
    S->>PKG: 5. Encrypt with AES-256-GCM (optional)
    PKG->>A: 6. Distribute Package

    A->>A: 7. Verify Signature
    A->>A: 8. Validate Nonce (< 5min)
    A->>A: 9. Decrypt Package (if encrypted)
    A->>A: 10. Verify Checksum
    A->>A: 11. Atomic Installation
    A->>S: 12. Update Confirmation

Implementation Details

1. Ed25519 Signature System

Server-side (signing.go)

// SignFile creates Ed25519 signature for update packages
func (s *SigningService) SignFile(filePath string) (*models.AgentUpdatePackage, error) {
    content, err := io.ReadAll(file)
    hash := sha256.Sum256(content)
    signature := ed25519.Sign(s.privateKey, content)

    return &models.AgentUpdatePackage{
        Signature: hex.EncodeToString(signature),
        Checksum:  hex.EncodeToString(hash[:]),
        // ... other metadata
    }, nil
}

// VerifySignature validates package authenticity
func (s *SigningService) VerifySignature(content []byte, signatureHex string) (bool, error) {
    signature, _ := hex.DecodeString(signatureHex)
    return ed25519.Verify(s.publicKey, content, signature), nil
}

Agent-side (subsystem_handlers.go)

// Fetch and cache public key at agent startup
publicKey, err := crypto.FetchAndCacheServerPublicKey(serverURL)
// Cached to /etc/aggregator/server_public_key

// Signature verification during update
signature, _ := hex.DecodeString(params["signature"].(string))
if valid := ed25519.Verify(publicKey, packageContent, signature); !valid {
    return fmt.Errorf("invalid package signature")
}

Public Key Distribution (TOFU Model)

Server provides public key via API

// GET /api/v1/public-key (no authentication required)
{
  "public_key": "37f6d2a4ffe0f83bcb91d0ee2eb266833f766e8180866d31...",
  "fingerprint": "37f6d2a4ffe0f83b",
  "algorithm": "ed25519",
  "key_size": 32
}

Agent fetches and caches at startup

// During agent registration
publicKey, err := crypto.FetchAndCacheServerPublicKey(serverURL)
// Cached to /etc/aggregator/server_public_key for future use

Security Model: Trust-On-First-Use (TOFU)

• Like SSH fingerprints - trust the first connection
• Protected by HTTPS/TLS during initial fetch
• Cached locally for all future verifications
• Optional: Manual fingerprint verification (out-of-band)

2. Nonce-Based Replay Protection

Server-side Nonce Generation

// Generate and sign nonce for update command
func (s *SigningService) SignNonce(nonceUUID uuid.UUID, timestamp time.Time) (string, error) {
    nonceData := fmt.Sprintf("%s:%d", nonceUUID.String(), timestamp.Unix())
    signature := ed25519.Sign(s.privateKey, []byte(nonceData))
    return hex.EncodeToString(signature), nil
}

// Verify nonce freshness and signature
func (s *SigningService) VerifyNonce(nonceUUID uuid.UUID, timestamp time.Time,
    signatureHex string, maxAge time.Duration) (bool, error) {
    if time.Since(timestamp) > maxAge {
        return false, fmt.Errorf("nonce expired")
    }
    // ... signature verification
}

Agent-side Validation

// Extract nonce parameters from command
nonceUUIDStr := params["nonce_uuid"].(string)
nonceTimestampStr := params["nonce_timestamp"].(string)
nonceSignature := params["nonce_signature"].(string)

// TODO: Implement full validation
// - Parse timestamp
// - Verify < 5min freshness
// - Verify Ed25519 signature
// - Prevent replay attacks

3. AES-256-GCM Encryption

Key Derivation

// Derive AES-256 key from nonce
func deriveKeyFromNonce(nonce string) []byte {
    hash := sha256.Sum256([]byte(nonce))
    return hash[:] // 32 bytes for AES-256
}

Decryption Process

// Decrypt update package with AES-256-GCM
func decryptAES256GCM(encryptedData, nonce string) ([]byte, error) {
    key := deriveKeyFromNonce(nonce)
    data, _ := hex.DecodeString(encryptedData)

    block, _ := aes.NewCipher(key)
    gcm, _ := cipher.NewGCM(block)

    // Extract nonce and ciphertext
    nonceSize := gcm.NonceSize()
    nonceBytes, ciphertext := data[:nonceSize], data[nonceSize:]

    // Decrypt and verify
    return gcm.Open(nil, nonceBytes, ciphertext, nil)
}

Update Process Flow

1. Server Startup

1. Load Private Key: From REDFLAG_SIGNING_PRIVATE_KEY environment variable
2. Initialize Signing Service: Ed25519 operations ready
3. Serve Public Key: Available at GET /api/v1/public-key

2. Agent Installation (One-Liner)

curl -sSL https://redflag.example/install.sh | bash

1. Download Agent: Pre-built binary from server
2. Start Agent: Automatic startup
3. Register: Agent ↔ Server authentication
4. Fetch Public Key: From GET /api/v1/public-key
5. Cache Key: Saved to /etc/aggregator/server_public_key

3. Package Preparation (Server)

1. Build: Compile agent binary for target platform
2. Sign: Create Ed25519 signature using server private key
3. Store: Persist package with signature + metadata in database

4. Command Distribution (Server → Agent)

1. Generate Nonce: Create UUID + timestamp for freshness (<5min)
2. Sign Nonce: Ed25519 sign nonce for authenticity
3. Create Command: Bundle update parameters with signed nonce
4. Distribute: Send command to target agents

5. Package Reception (Agent)

1. Validate Nonce: Check timestamp < 5 minutes, verify Ed25519 signature
2. Download: Fetch package from secure URL
3. Verify Signature: Validate Ed25519 signature against cached public key
4. Verify Checksum: SHA-256 integrity check

6. Atomic Installation (Agent)

1. Backup: Copy current binary to .bak
2. Install: Atomically replace with new binary
3. Restart: Restart agent service (systemd/service/Windows service)
4. Watchdog: Poll server every 15s for version confirmation (5min timeout)
5. Confirm or Rollback:
   • ✓ Success → cleanup backup
   • ✗ Timeout/Failure → automatic rollback from backup

Security Best Practices

Server Operations

• ✅ Private key stored in secure environment (hardware security module recommended)
• ✅ Regular key rotation (see TODO in signing.go)
• ✅ Audit logging of all signing operations
• ✅ Network access controls for signing endpoints

Agent Operations

• ✅ Public key fetched via TOFU (Trust-On-First-Use)
• ✅ Nonce validation prevents replay attacks (<5min freshness)
• ✅ Signature verification prevents tampering
• ✅ Watchdog polls server for version confirmation
• ✅ Atomic updates prevent partial installations
• ✅ Automatic rollback on watchdog timeout/failure

Network Security

• ✅ HTTPS/TLS for all communications
• ✅ Package integrity verification
• ✅ Timeout controls for downloads
• ✅ Rate limiting on update endpoints

Key Rotation Strategy

Planned Implementation (TODO)

graph LR
    A[Key v1 Active] --> B[Generate Key v2]
    B --> C[Dual-Key Period]
    C --> D[Sign with v1+v2]
    D --> E[Phase out v1]
    E --> F[Key v2 Active]

Rotation Steps

1. Generate: Create new Ed25519 key pair (v2)
2. Distribute: Add v2 public key to agents
3. Transition: Sign packages with both v1 and v2
4. Verify: Agents accept signatures from either key
5. Phase-out: Gradually retire v1
6. Cleanup: Remove v1 from agent trust store

Migration Considerations

• Backward compatibility during transition
• Graceful period for key rotation (30 days recommended)
• Monitoring for rotation completion
• Emergency rollback procedures

Vulnerability Management

Known Mitigations

• Supply Chain: Ed25519 signatures prevent package tampering
• Replay Attacks: Nonce validation ensures freshness
• Privilege Escalation: Atomic updates with proper permissions
• Information Disclosure: AES-256-GCM encryption for transit

Security Monitoring

• Monitor for failed signature verifications
• Alert on nonce replay attempts
• Track update success/failure rates
• Audit signing service access logs

Incident Response

1. Compromise Detection: Monitor for signature verification failures
2. Key Rotation: Immediate rotation if private key compromised
3. Agent Update: Force security updates to all agents
4. Investigation: Audit logs for unauthorized access

Compliance Considerations

• Cryptography: Uses FIPS-validated algorithms (Ed25519, AES-256-GCM, SHA-256)
• Audit Trail: Complete logging of all signing and update operations
• Access Control: Role-based access to signing infrastructure
• Data Protection: Encryption in transit and at rest

Future Enhancements

Planned Security Features

• Hardware Security Module (HSM) integration for private key protection
• Certificate-based agent authentication
• Mutual TLS for server-agent communication
• Package reputation scoring
• Zero-knowledge proof-based update verification

Performance Optimizations

• Parallel signature verification
• Cached public key validation
• Optimized crypto operations
• Delta update support

Testing and Validation

Security Testing

• Unit Tests: 80% coverage for crypto operations
• Integration Tests: Full update cycle simulation
• Penetration Testing: Regular third-party security assessments
• Fuzz Testing: Cryptographic input validation

Test Scenarios

1. Valid Update: Normal successful update flow
2. Invalid Signature: Tampered package rejection
3. Expired Nonce: Replay attack prevention
4. Corrupted Package: Checksum validation
5. Service Failure: Automatic rollback
6. Network Issues: Timeout and retry handling

References

• Ed25519 Specification
• AES-GCM Specification
• NIST Cryptographic Standards

Reporting Security Issues

Please report security vulnerabilities responsibly:

• Email: security@redflag-project.org
• PGP Key: Available on request
• Response time: Within 48 hours

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Last updated: v0.1.21 Security classification: Internal use