~~NOTOC~~
====== Scenario 11.5: Key Destruction ======

<WRAP round info>
**Category:** [[.:start|Key Management]] \\
**Complexity:** ⭐⭐⭐⭐ (High) \\
**Prerequisites:** Key no longer needed, backups verified \\
**Estimated Time:** 20-30 Minutes
</WRAP>

----

===== Description =====

This scenario describes the **secure and irreversible destruction of cryptographic keys**. Proper key destruction is the final step in the key lifecycle and critical for security.

**Destruction Reasons:**
  * **End of Life** - Key lifecycle ended
  * **Compromise** - Key possibly known
  * **Compliance** - Retention periods expired
  * **Decommissioning** - System being retired
  * **Algorithm Migration** - Old keys after PQ transition

<WRAP round important>
**WARNING:** Key destruction is irreversible! Ensure:
  * All encrypted data is decrypted or no longer needed
  * All signatures are validated or no longer relevant
  * Backups have also been destroyed
</WRAP>

----

===== Workflow =====

<mermaid>
flowchart TD
    INIT[Initiate destruction] --> CHECK{Checks}
    CHECK --> DEP[Check dependencies]
    DEP --> BAK[Locate backups]
    BAK --> APPROVE{Approval?}
    APPROVE -->|No| ABORT[Abort]
    APPROVE -->|Yes| DESTROY[Overwrite key]
    DESTROY --> BACKUP_DEL[Destroy backups]
    BACKUP_DEL --> HSM_DEL[Delete HSM keys]
    HSM_DEL --> LOG[Audit log]
    LOG --> CERT[Certificate]

    style DESTROY fill:#ffebee
    style BACKUP_DEL fill:#ffebee
    style HSM_DEL fill:#ffebee
</mermaid>

----

===== Code Example: Secure Memory Deletion =====

<code csharp>
using System.Security.Cryptography;
using System.Runtime.InteropServices;

public class SecureKeyDestruction
{
    public void DestroyKeyInMemory(byte[] keyMaterial)
    {
        if (keyMaterial == null || keyMaterial.Length == 0)
            return;

        // 1. Overwrite with random data
        RandomNumberGenerator.Fill(keyMaterial);

        // 2. Overwrite with zeros
        Array.Clear(keyMaterial, 0, keyMaterial.Length);

        // 3. Overwrite again with random data
        RandomNumberGenerator.Fill(keyMaterial);

        // 4. Final overwrite with zeros
        CryptographicOperations.ZeroMemory(keyMaterial);

        Console.WriteLine($"Memory cleared: {keyMaterial.Length} bytes");
    }

    public void DestroyKeyOnDisk(string keyFilePath)
    {
        if (!File.Exists(keyFilePath))
        {
            Console.WriteLine($"File not found: {keyFilePath}");
            return;
        }

        var fileInfo = new FileInfo(keyFilePath);
        var fileLength = fileInfo.Length;

        // Multiple overwrites (DoD 5220.22-M Standard)
        using (var fs = new FileStream(keyFilePath, FileMode.Open, FileAccess.Write))
        {
            var buffer = new byte[4096];

            // Pass 1: Overwrite with 0x00
            OverwriteFile(fs, fileLength, 0x00);

            // Pass 2: Overwrite with 0xFF
            OverwriteFile(fs, fileLength, 0xFF);

            // Pass 3: Overwrite with random data
            OverwriteFileRandom(fs, fileLength);

            // Pass 4: Again with 0x00
            OverwriteFile(fs, fileLength, 0x00);

            fs.Flush();
        }

        // Rename file (obscure metadata)
        var randomName = Path.Combine(
            Path.GetDirectoryName(keyFilePath),
            Guid.NewGuid().ToString()
        );
        File.Move(keyFilePath, randomName);

        // Delete file
        File.Delete(randomName);

        Console.WriteLine($"Key file securely deleted: {keyFilePath}");
    }

    private void OverwriteFile(FileStream fs, long length, byte value)
    {
        fs.Seek(0, SeekOrigin.Begin);
        var buffer = new byte[4096];
        Array.Fill(buffer, value);

        long remaining = length;
        while (remaining > 0)
        {
            int toWrite = (int)Math.Min(remaining, buffer.Length);
            fs.Write(buffer, 0, toWrite);
            remaining -= toWrite;
        }
    }

    private void OverwriteFileRandom(FileStream fs, long length)
    {
        fs.Seek(0, SeekOrigin.Begin);
        var buffer = new byte[4096];

        long remaining = length;
        while (remaining > 0)
        {
            RandomNumberGenerator.Fill(buffer);
            int toWrite = (int)Math.Min(remaining, buffer.Length);
            fs.Write(buffer, 0, toWrite);
            remaining -= toWrite;
        }
    }
}
</code>

----

===== Code Example: Complete Destruction with Protocol =====

<code csharp>
public class KeyDestructionService
{
    private readonly ILogger _logger;
    private readonly IAuditLog _auditLog;

    public async Task<DestructionCertificate> DestroyKeyAsync(
        string keyId,
        string reason,
        string authorizedBy,
        DestructionOptions options = null)
    {
        options ??= DestructionOptions.Default;

        var certificate = new DestructionCertificate
        {
            KeyId = keyId,
            DestructionId = Guid.NewGuid().ToString(),
            InitiatedAt = DateTime.UtcNow,
            Reason = reason,
            AuthorizedBy = authorizedBy
        };

        try
        {
            // 1. Check dependencies
            await ValidateDestructionAsync(keyId, certificate);

            // 2. Destroy active key
            await DestroyPrimaryKeyAsync(keyId, certificate);

            // 3. Destroy all backups
            if (options.DestroyBackups)
            {
                await DestroyBackupsAsync(keyId, certificate);
            }

            // 4. Destroy HSM keys
            if (options.DestroyHsmKeys)
            {
                await DestroyHsmKeyAsync(keyId, certificate);
            }

            // 5. Create audit log
            certificate.CompletedAt = DateTime.UtcNow;
            certificate.Status = DestructionStatus.Completed;

            await _auditLog.LogAsync(new AuditEntry
            {
                Action = "KEY_DESTRUCTION",
                KeyId = keyId,
                Details = certificate,
                Timestamp = DateTime.UtcNow
            });

            _logger.LogInformation(
                "Key {KeyId} successfully destroyed. Certificate: {DestructionId}",
                keyId, certificate.DestructionId);

            return certificate;
        }
        catch (Exception ex)
        {
            certificate.Status = DestructionStatus.Failed;
            certificate.Error = ex.Message;

            _logger.LogError(ex, "Key destruction failed: {KeyId}", keyId);
            throw;
        }
    }

    private async Task ValidateDestructionAsync(string keyId, DestructionCertificate cert)
    {
        // Check if active certificates still exist
        var activeCerts = await _certStore.FindByKeyIdAsync(keyId);
        if (activeCerts.Any(c => c.NotAfter > DateTime.UtcNow))
        {
            throw new InvalidOperationException(
                $"Key {keyId} is still used by {activeCerts.Count} active certificates");
        }

        cert.ValidationsPassed.Add("No active certificates");

        // Check if retention period expired
        var metadata = await _keyStore.GetMetadataAsync(keyId);
        if (metadata.RetentionUntil > DateTime.UtcNow)
        {
            throw new InvalidOperationException(
                $"Retention period runs until {metadata.RetentionUntil:d}");
        }

        cert.ValidationsPassed.Add("Retention period expired");
    }
}

public class DestructionCertificate
{
    public string DestructionId { get; set; }
    public string KeyId { get; set; }
    public string KeyFingerprint { get; set; }
    public DateTime InitiatedAt { get; set; }
    public DateTime? CompletedAt { get; set; }
    public string Reason { get; set; }
    public string AuthorizedBy { get; set; }
    public DestructionStatus Status { get; set; }
    public List<string> ValidationsPassed { get; set; } = new();
    public bool PrimaryKeyDestroyed { get; set; }
    public int BackupsDestroyed { get; set; }
    public bool HsmKeyDestroyed { get; set; }
    public List<string> DestroyedLocations { get; set; } = new();
    public string Error { get; set; }
}

public enum DestructionStatus
{
    InProgress,
    Completed,
    Failed
}
</code>

----

===== Destruction Standards =====

^  Standard  ^  Overwrites  ^  Pattern  ^  Application  ^
| **DoD 5220.22-M** | 3 | 0x00, 0xFF, Random | US Military |
| **NIST SP 800-88** | 1-3 | Depends on media type | US Government |
| **BSI-TL 03423** | 3 | Random, 0x00, 0xFF | German Authorities |
| **Gutmann** | 35 | Various patterns | Paranoid |

<WRAP round tip>
For SSDs and flash storage: Secure deletion requires special firmware commands (TRIM, Secure Erase). Software overwriting is not reliable!
</WRAP>

----

===== Industry-Specific Destruction Requirements =====

^  Industry  ^  Requirement  ^  Witnesses  ^  Documentation  ^
| **Financial Sector** | PCI-DSS 3.6.6 | 2 persons | Destruction protocol |
| **Healthcare** | HIPAA | 1 witness | Audit log |
| **Government** | VS-NfD | Security Officer | Destruction certificate |
| **KRITIS** | BSI | 2 persons | Certificate + Protocol |

----

===== Related Scenarios =====

^  Relationship  ^  Scenario  ^  Description  ^
| **Prerequisite** | [[.:backup|11.4 Key Backup]] | Identify all backups |
| **Prerequisite** | [[.:rotation|11.3 Key Rotation]] | New keys before destruction |
| **Related** | [[en:int:pqcrypt:szenarien:widerruf:zertifikat_widerrufen|6.4 Revoke Certificate]] | Revoke before destruction |
| **Related** | [[en:int:pqcrypt:szenarien:verwaltung:archivierung|4.3 Archival]] | Alternative: Archive instead of destroy |

----

<< [[.:backup|← 11.4 Key Backup]] | [[.:start|↑ Key Overview]] | [[en:int:pqcrypt:szenarien:start|→ All Scenarios]] >>

{{tag>scenario key destruction security compliance audit}}

----
//Wolfgang van der Stille @ EMSR DATA d.o.o. - Post-Quantum Cryptography Professional//