Scenario 11.4: Key Backup

This scenario describes the secure backup of cryptographic keys. A well-designed backup concept is essential for business continuity and disaster recovery.

Backup Strategies:

• Encrypted Backup - AES-256 or PQ-hybrid
• Key Splitting - Shamir's Secret Sharing
• Geographic Distribution - Multiple locations
• Offline Backup - Air-gapped systems
• HSM Backup - Hardware-based

using WvdS.Security.Cryptography.X509Certificates.Extensions.PQ;
using System.Security.Cryptography;
 
public class KeyBackupService
{
    public void CreateEncryptedBackup(
        AsymmetricAlgorithm privateKey,
        string backupPath,
        string backupPassword,
        BackupOptions options = null)
    {
        options ??= BackupOptions.Default;
        using var ctx = PqCryptoContext.Initialize();
 
        // 1. Export private key as PKCS#8
        byte[] pkcs8 = privateKey switch
        {
            RSA rsa => rsa.ExportPkcs8PrivateKey(),
            ECDsa ecdsa => ecdsa.ExportPkcs8PrivateKey(),
            _ => ctx.ExportPrivateKey(privateKey)
        };
 
        // 2. Derive Key Encryption Key (KEK)
        var salt = RandomNumberGenerator.GetBytes(32);
        var kek = ctx.DeriveKey(
            backupPassword,
            salt,
            outputLength: 32,
            algorithm: KeyDerivationAlgorithm.Argon2id,
            iterations: 4,
            memoryKiB: 65536,
            parallelism: 4
        );
 
        // 3. Encrypt with AES-256-GCM
        var nonce = RandomNumberGenerator.GetBytes(12);
        var ciphertext = new byte[pkcs8.Length];
        var tag = new byte[16];
 
        using var aes = new OpenSslAesGcm(kek);
        aes.Encrypt(nonce, pkcs8, ciphertext, tag);
 
        // 4. Create backup file
        var backup = new KeyBackup
        {
            Version = 1,
            Algorithm = "ML-DSA-65",
            CreatedAt = DateTime.UtcNow,
            KdfAlgorithm = "Argon2id",
            KdfSalt = Convert.ToBase64String(salt),
            KdfIterations = 4,
            KdfMemoryKiB = 65536,
            EncryptionAlgorithm = "AES-256-GCM",
            Nonce = Convert.ToBase64String(nonce),
            Tag = Convert.ToBase64String(tag),
            EncryptedKey = Convert.ToBase64String(ciphertext),
            Checksum = ComputeChecksum(pkcs8)
        };
 
        // 5. Save as JSON
        var json = JsonSerializer.Serialize(backup, new JsonSerializerOptions
        {
            WriteIndented = true
        });
        File.WriteAllText(backupPath, json);
 
        // 6. Clear memory
        CryptographicOperations.ZeroMemory(pkcs8);
        CryptographicOperations.ZeroMemory(kek);
 
        Console.WriteLine($"Backup created: {backupPath}");
        Console.WriteLine($"  Checksum: {backup.Checksum}");
    }
 
    public AsymmetricAlgorithm RestoreFromBackup(
        string backupPath,
        string backupPassword)
    {
        using var ctx = PqCryptoContext.Initialize();
 
        // 1. Load backup
        var json = File.ReadAllText(backupPath);
        var backup = JsonSerializer.Deserialize<KeyBackup>(json);
 
        // 2. Derive KEK (same parameters as backup)
        var salt = Convert.FromBase64String(backup.KdfSalt);
        var kek = ctx.DeriveKey(
            backupPassword,
            salt,
            outputLength: 32,
            algorithm: KeyDerivationAlgorithm.Argon2id,
            iterations: backup.KdfIterations,
            memoryKiB: backup.KdfMemoryKiB,
            parallelism: 4
        );
 
        // 3. Decrypt
        var nonce = Convert.FromBase64String(backup.Nonce);
        var tag = Convert.FromBase64String(backup.Tag);
        var ciphertext = Convert.FromBase64String(backup.EncryptedKey);
        var pkcs8 = new byte[ciphertext.Length];
 
        using var aes = new OpenSslAesGcm(kek);
        aes.Decrypt(nonce, ciphertext, tag, pkcs8);
 
        // 4. Verify checksum
        var checksum = ComputeChecksum(pkcs8);
        if (checksum != backup.Checksum)
        {
            throw new CryptographicException("Backup checksum invalid");
        }
 
        // 5. Import key
        var key = ctx.ImportPrivateKey(pkcs8, backup.Algorithm);
 
        CryptographicOperations.ZeroMemory(pkcs8);
        CryptographicOperations.ZeroMemory(kek);
 
        Console.WriteLine($"Backup restored: {backup.Algorithm}");
        return key;
    }
 
    private string ComputeChecksum(byte[] data)
    {
        var hash = SHA256.HashData(data);
        return Convert.ToHexString(hash).Substring(0, 16);
    }
}

using SecretSharingDotNet.Cryptography;
 
public class ShamirKeyBackup
{
    public ShamirShares SplitKey(
        byte[] privateKey,
        int totalShares,
        int threshold)
    {
        // Shamir's Secret Sharing: n-of-m scheme
        // e.g., 3-of-5: At least 3 of 5 shares needed
 
        var gf = new ExtendedEuclideanAlgorithm<BigInteger>();
        var shamir = new ShamirsSecretSharing<BigInteger>(gf);
 
        // Key as BigInteger
        var secret = new BigInteger(privateKey, isUnsigned: true);
 
        // Split into shares
        var shares = shamir.MakeShares(
            (uint)threshold,
            (uint)totalShares,
            secret
        );
 
        var result = new ShamirShares
        {
            Threshold = threshold,
            TotalShares = totalShares,
            Shares = shares.Select((s, i) => new Share
            {
                Index = i + 1,
                Value = Convert.ToBase64String(s.Y.ToByteArray())
            }).ToList()
        };
 
        Console.WriteLine($"Key split into {totalShares} shares (Threshold: {threshold})");
        return result;
    }
 
    public byte[] RecombineKey(ShamirShares shares, List<int> shareIndices)
    {
        if (shareIndices.Count < shares.Threshold)
        {
            throw new ArgumentException(
                $"At least {shares.Threshold} shares needed, but only {shareIndices.Count} provided"
            );
        }
 
        var gf = new ExtendedEuclideanAlgorithm<BigInteger>();
        var shamir = new ShamirsSecretSharing<BigInteger>(gf);
 
        // Collect shares
        var selectedShares = shareIndices
            .Select(i => shares.Shares.First(s => s.Index == i))
            .Select(s => new FinitePoint<BigInteger>(
                s.Index,
                new BigInteger(Convert.FromBase64String(s.Value), isUnsigned: true)
            ))
            .ToArray();
 
        // Reconstruct secret
        var secret = shamir.Reconstruction(selectedShares);
 
        return secret.ToByteArray();
    }
 
    public void DistributeShares(ShamirShares shares, string[] recipients)
    {
        // Distribute shares to different persons/locations
        for (int i = 0; i < shares.Shares.Count && i < recipients.Length; i++)
        {
            var share = shares.Shares[i];
            var recipient = recipients[i];
 
            // In practice: Send encrypted to recipient
            Console.WriteLine($"Share {share.Index} → {recipient}");
 
            // Example: As QR code or USB stick
            var shareFile = $"share-{share.Index}-{recipient.Replace(" ", "_")}.json";
            File.WriteAllText(shareFile, JsonSerializer.Serialize(share));
        }
    }
}

« ← 11.3 Key Rotation | ↑ Key Overview | 11.5 Key Destruction → »

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