dotnet-cryptography

Modern .NET cryptography covering hashing (SHA-256/384/512), symmetric encryption (AES-GCM), asymmetric cryptography (RSA, ECDSA), key derivation (PBKDF2, Argon2), and post-quantum algorithms (ML-KEM, ML-DSA, SLH-DSA) for .NET 10+. Includes TFM-aware guidance: what's available on net10.0 vs fallback strategies for net8.0/net9.0.

Out of scope: Secrets management and configuration binding -- see [skill:dotnet-secrets-management]. OWASP vulnerability categories and deprecated security patterns -- see [skill:dotnet-security-owasp]. Authentication/authorization implementation (JWT, OAuth, Identity) -- see [skill:dotnet-api-security] and [skill:dotnet-blazor-auth]. Cloud-specific key management (Azure Key Vault, AWS KMS) -- cloud epics. TLS/HTTPS configuration -- covered by ASP.NET Core middleware.

Cross-references: [skill:dotnet-security-owasp] for OWASP A02 (Cryptographic Failures) and deprecated pattern warnings, [skill:dotnet-secrets-management] for storing keys and secrets securely.

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Scope Boundary

In scope: Algorithm selection, correct usage of System.Security.Cryptography APIs, key derivation, hashing for integrity, symmetric/asymmetric encryption, post-quantum cryptography, and deprecated algorithm warnings.

Not in scope: Key storage (use secrets management), TLS termination (infrastructure), authentication protocols (see [skill:dotnet-api-security]), cloud HSM/KMS services (cloud epics).

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Prerequisites

• .NET 8.0+ (LTS baseline for classical algorithms)
• .NET 10.0+ for post-quantum algorithms (ML-KEM, ML-DSA, SLH-DSA)
• Platform support for PQC: Windows 11 (November 2025+) or OpenSSL 3.5+ on Linux/macOS

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Hashing (SHA-2 Family)

Use SHA-256/384/512 for integrity verification, checksums, and content-addressable storage. Never use hashing alone for passwords (see Key Derivation below).

using System.Security.Cryptography;

// Hash a byte array
byte[] data = "Hello, world"u8.ToArray();
byte[] hash = SHA256.HashData(data);

// Hash a stream (efficient for large files)
await using var stream = File.OpenRead("largefile.bin");
byte[] fileHash = await SHA256.HashDataAsync(stream);

// Compare hashes securely (constant-time comparison prevents timing attacks)
bool isEqual = CryptographicOperations.FixedTimeEquals(hash1, hash2);

// HMAC for authenticated hashing (message authentication codes)
byte[] key = RandomNumberGenerator.GetBytes(32); // 256-bit key
byte[] mac = HMACSHA256.HashData(key, data);

// Verify HMAC
byte[] computedMac = HMACSHA256.HashData(key, receivedData);
if (!CryptographicOperations.FixedTimeEquals(mac, computedMac))
{
    throw new CryptographicException("Message authentication failed");
}

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Symmetric Encryption (AES-GCM)

AES-GCM is the recommended symmetric encryption for .NET. It provides both confidentiality and authenticity (authenticated encryption with associated data -- AEAD).

using System.Security.Cryptography;

public static class AesGcmEncryptor
{
    private const int NonceSize = 12; // 96-bit nonce (required by GCM)
    private const int TagSize = 16;   // 128-bit authentication tag

    public static byte[] Encrypt(byte[] plaintext, byte[] key)
    {
        var nonce = RandomNumberGenerator.GetBytes(NonceSize);
        var ciphertext = new byte[plaintext.Length];
        var tag = new byte[TagSize];

        using var aes = new AesGcm(key, TagSize);
        aes.Encrypt(nonce, plaintext, ciphertext, tag);

        // Prepend nonce + append tag for transport
        var result = new byte[NonceSize + ciphertext.Length + TagSize];
        nonce.CopyTo(result, 0);
        ciphertext.CopyTo(result, NonceSize);
        tag.CopyTo(result, NonceSize + ciphertext.Length);
        return result;
    }

    public static byte[] Decrypt(byte[] encryptedData, byte[] key)
    {
        var nonce = encryptedData.AsSpan(0, NonceSize);
        var ciphertext = encryptedData.AsSpan(NonceSize, encryptedData.Length - NonceSize - TagSize);
        var tag = encryptedData.AsSpan(encryptedData.Length - TagSize);
        var plaintext = new byte[ciphertext.Length];

        using var aes = new AesGcm(key, TagSize);
        aes.Decrypt(nonce, ciphertext, tag, plaintext);
        return plaintext;
    }
}

// ASP.NET Core Data Protection API -- preferred for web application scenarios
// Handles key management, rotation, and storage automatically
using Microsoft.AspNetCore.DataProtection;

public sealed class TokenProtector(IDataProtectionProvider provider)
{
    private readonly IDataProtector _protector =
        provider.CreateProtector("Tokens.V1");

    public string Protect(string plaintext) => _protector.Protect(plaintext);
    public string Unprotect(string ciphertext) => _protector.Unprotect(ciphertext);
}

// Registration:
builder.Services.AddDataProtection()
    .SetApplicationName("MyApp")
    .PersistKeysToFileSystem(new DirectoryInfo("/keys"));

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Asymmetric Cryptography (RSA, ECDSA)

RSA

Use RSA for encryption of small payloads (key wrapping) and digital signatures. Minimum 2048-bit keys; prefer 4096-bit for new systems.

using System.Security.Cryptography;

// Generate an RSA key pair
using var rsa = RSA.Create(4096);

// Sign data
byte[] signature = rsa.SignData(data, HashAlgorithmName.SHA256, RSASignaturePadding.Pss);

// Verify signature (with public key)
byte[] publicKeyBytes = rsa.ExportRSAPublicKey();
using var rsaPublic = RSA.Create();
rsaPublic.ImportRSAPublicKey(publicKeyBytes, out _);
bool valid = rsaPublic.VerifyData(data, signature, HashAlgorithmName.SHA256, RSASignaturePadding.Pss);

// Encrypt with OAEP padding (never use PKCS#1 v1.5 for new code)
byte[] encrypted = rsaPublic.Encrypt(smallPayload, RSAEncryptionPadding.OaepSHA256);
byte[] decrypted = rsa.Decrypt(encrypted, RSAEncryptionPadding.OaepSHA256);

ECDSA

Prefer ECDSA over RSA for digital signatures in new projects -- smaller keys with equivalent security.

using System.Security.Cryptography;

// Generate ECDSA key (P-256 = NIST curve, widely supported)
using var ecdsa = ECDsa.Create(ECCurve.NamedCurves.nistP256);

// Sign data
byte[] signature = ecdsa.SignData(data, HashAlgorithmName.SHA256);

// Export public key for verification
byte[] publicKey = ecdsa.ExportSubjectPublicKeyInfo();

// Import and verify
using var ecdsaPublic = ECDsa.Create();
ecdsaPublic.ImportSubjectPublicKeyInfo(publicKey, out _);
bool valid = ecdsaPublic.VerifyData(data, signature, HashAlgorithmName.SHA256);

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Key Derivation (Password Hashing)

PBKDF2 (Built-in)

PBKDF2 is built into .NET and acceptable for password hashing. Use at least 600,000 iterations with SHA-256 (OWASP recommendation).

using System.Buffers.Binary;
using System.Security.Cryptography;

public static class PasswordHasher
{
    private const int SaltSize = 16;       // 128-bit salt
    private const int HashSize = 32;       // 256-bit derived key
    private const int Iterations = 600_000; // OWASP 2023 recommendation for SHA-256
    private const int PayloadSize = 4 + SaltSize + HashSize; // iteration count + salt + hash

    public static string HashPassword(string password)
    {
        byte[] salt = RandomNumberGenerator.GetBytes(SaltSize);
        byte[] hash = Rfc2898DeriveBytes.Pbkdf2(
            password,
            salt,
            Iterations,
            HashAlgorithmName.SHA256,
            HashSize);

        // Store iteration count (fixed little-endian), salt, and hash together
        byte[] result = new byte[PayloadSize];
        BinaryPrimitives.WriteInt32LittleEndian(result, Iterations);
        salt.CopyTo(result.AsSpan(4));
        hash.CopyTo(result.AsSpan(4 + SaltSize));
        return Convert.ToBase64String(result);
    }

    public static bool VerifyPassword(string password, string stored)
    {
        // Defensive parsing: reject malformed input without exceptions
        Span<byte> decoded = stackalloc byte[PayloadSize];
        if (!Convert.TryFromBase64String(stored, decoded, out int bytesWritten)
            || bytesWritten != PayloadSize)
        {
            return false;
        }

        int iterations = BinaryPrimitives.ReadInt32LittleEndian(decoded);
        if (iterations <= 0)
            return false;

        var salt = decoded.Slice(4, SaltSize);
        var expectedHash = decoded.Slice(4 + SaltSize, HashSize);

        byte[] actualHash = Rfc2898DeriveBytes.Pbkdf2(
            password,
            salt,
            iterations,
            HashAlgorithmName.SHA256,
            HashSize);

        return CryptographicOperations.FixedTimeEquals(expectedHash, actualHash);
    }
}

Argon2 (via NuGet)

Argon2id is the recommended algorithm for password hashing when a NuGet dependency is acceptable. It is memory-hard, resisting GPU/ASIC attacks better than PBKDF2.

// Requires: <PackageReference Include="Konscious.Security.Cryptography.Argon2" Version="1.*" />
using Konscious.Security.Cryptography;

public static byte[] HashWithArgon2(string password, byte[] salt)
{
    using var argon2 = new Argon2id(Encoding.UTF8.GetBytes(password))
    {
        Salt = salt,
        DegreeOfParallelism = 4,  // threads
        MemorySize = 65536,       // 64 MB
        Iterations = 3
    };
    return argon2.GetBytes(32); // 256-bit hash
}

Prefer ASP.NET Core Identity's PasswordHasher<T> for web applications -- it handles PBKDF2 with correct parameters and format versioning automatically. Use custom hashing only for non-Identity scenarios.

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Post-Quantum Cryptography (.NET 10+)

.NET 10 introduces post-quantum cryptography (PQC) through the System.Security.Cryptography namespace. These algorithms resist attacks from both classical and quantum computers.

Platform Requirements

PQC APIs require OS-level support:

• Windows: Windows 11 (November 2025 update) or Windows Server 2025 with PQC updates
• Linux/macOS: OpenSSL 3.5 or newer

Always check IsSupported before using PQC types. On unsupported platforms, fall back to classical algorithms.

ML-KEM (FIPS 203) -- Key Encapsulation

ML-KEM replaces classical key exchange (ECDH) for establishing shared secrets. It is the most mature .NET 10 PQC API (not marked [Experimental] at class level).

#if NET10_0_OR_GREATER
using System.Security.Cryptography;

if (!MLKem.IsSupported)
{
    Console.WriteLine("ML-KEM not available on this platform");
    return;
}

// Generate a key pair
using MLKem privateKey = MLKem.GenerateKey(MLKemAlgorithm.MLKem768);

// Export public encapsulation key (share with peer)
byte[] publicKeyBytes = privateKey.ExportEncapsulationKey();

// Peer: import public key and encapsulate a shared secret
using MLKem publicKey = MLKem.ImportEncapsulationKey(
    MLKemAlgorithm.MLKem768, publicKeyBytes);
publicKey.Encapsulate(out byte[] ciphertext, out byte[] sharedSecret1);

// Original holder: decapsulate to recover the same shared secret
byte[] sharedSecret2 = privateKey.Decapsulate(ciphertext);

// Both parties now have the same shared secret for symmetric encryption
bool match = sharedSecret1.AsSpan().SequenceEqual(sharedSecret2);
#endif

Parameter sets:

Parameter Set	Security Level	Encapsulation Key	Ciphertext
MLKemAlgorithm.MLKem512	NIST Level 1 (128-bit)	800 bytes	768 bytes
MLKemAlgorithm.MLKem768	NIST Level 3 (192-bit)	1,184 bytes	1,088 bytes
MLKemAlgorithm.MLKem1024	NIST Level 5 (256-bit)	1,568 bytes	1,568 bytes

Prefer MLKem768 for general use (balances security and performance).

ML-DSA (FIPS 204) -- Digital Signatures

ML-DSA replaces RSA/ECDSA for quantum-resistant digital signatures.

#if NET10_0_OR_GREATER
using System.Security.Cryptography;

if (!MLDsa.IsSupported)
{
    Console.WriteLine("ML-DSA not available on this platform");
    return;
}

// Generate signing key
using MLDsa key = MLDsa.GenerateKey(MLDsaAlgorithm.MLDsa65);

// Sign data
byte[] data = "Document to sign"u8.ToArray();
byte[] signature = new byte[key.Algorithm.SignatureSizeInBytes];
key.SignData(data, signature);

// Export public key for verification
byte[] publicKeyBytes = key.ExportMLDsaPublicKey();

// Verify with public key
using MLDsa publicKey = MLDsa.ImportMLDsaPublicKey(
    MLDsaAlgorithm.MLDsa65, publicKeyBytes);
bool valid = publicKey.VerifyData(data, signature);
#endif

Parameter sets:

Parameter Set	Security Level	Public Key	Signature
MLDsaAlgorithm.MLDsa44	NIST Level 2	1,312 bytes	2,420 bytes
MLDsaAlgorithm.MLDsa65	NIST Level 3	1,952 bytes	3,309 bytes
MLDsaAlgorithm.MLDsa87	NIST Level 5	2,592 bytes	4,627 bytes

SLH-DSA (FIPS 205) -- Hash-Based Signatures

SLH-DSA (Stateless Hash-Based Digital Signature Algorithm) provides extremely conservative long-term signatures. Use when mathematical structure of lattice-based schemes (ML-DSA) is a concern. The entire SlhDsa class is [Experimental] (SYSLIB5006) -- Windows has not yet added native support.

#if NET10_0_OR_GREATER
using System.Security.Cryptography;

// SlhDsa is [Experimental] -- suppress SYSLIB5006 only when intentional
#pragma warning disable SYSLIB5006
if (SlhDsa.IsSupported)
{
    using SlhDsa key = SlhDsa.GenerateKey(SlhDsaAlgorithm.SlhDsaSha2_128s);
    byte[] data = "Long-term document"u8.ToArray();
    byte[] signature = new byte[key.Algorithm.SignatureSizeInBytes];
    key.SignData(data, signature);
    bool valid = key.VerifyData(data, signature);
}
#pragma warning restore SYSLIB5006
#endif

Fallback Strategy for net8.0/net9.0

Post-quantum algorithms are only available in .NET 10+. For applications targeting earlier TFMs:

1. Use classical algorithms now: ECDSA (P-256/P-384) for signatures, ECDH + AES-GCM for key exchange/encryption. These remain secure against classical attacks.
2. Prepare for migration: Isolate cryptographic operations behind interfaces so algorithm swaps require minimal code changes.
3. Multi-target when ready: Use #if NET10_0_OR_GREATER conditionals or separate assemblies per TFM to add PQC support alongside classical fallbacks.
4. Harvest-now-decrypt-later: For data that must remain confidential for 10+ years, consider migrating to .NET 10 sooner to protect against future quantum decryption of captured ciphertext.

Interoperability Caveats

• Key and signature sizes: PQC keys and signatures are significantly larger than classical equivalents (e.g., ML-DSA-65 signature is 3,309 bytes vs ECDSA P-256 at 64 bytes). This affects storage, bandwidth, and protocol message sizes.
• No cross-platform PQC yet: PQC APIs depend on OS crypto libraries. An app compiled for net10.0 will fail at runtime on older OS versions. Always gate behind IsSupported.
• PKCS#8/X.509 formats are experimental: Import/export of PQC keys in standard certificate formats is [Experimental] pending IETF RFC finalization. Do not persist PQC keys in PKCS#8 format in production yet.
• Composite/hybrid signatures: CompositeMLDsa (hybrid ML-DSA + classical) is fully [Experimental] with no native OS support. Use it only for prototyping.
• TLS integration: ML-DSA and SLH-DSA certificates work in TLS 1.3+ via SslStream, but only when the OS crypto library supports PQC in TLS. Verify with your deployment target.
• Performance: ML-KEM and ML-DSA are fast. SLH-DSA is significantly slower for signing (seconds, not milliseconds) -- use it only when hash-based security guarantees are required.

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Deprecated Cryptographic APIs

The following cryptographic algorithms are broken or obsolete. Do not use them in new code.

Algorithm	Replacement	Reason
MD5	SHA-256+	Collision attacks since 2004; trivially broken
SHA-1	SHA-256+	Collision attacks demonstrated (SHAttered, 2017)
DES	AES-GCM	56-bit key; brute-forceable in hours
3DES (TripleDES)	AES-GCM	Deprecated by NIST (2023); Sweet32 attack
RC2	AES-GCM	Weak key schedule; effective key length < advertised
RSA PKCS#1 v1.5 encryption	RSA-OAEP	Bleichenbacher padding oracle attacks

For the full list of deprecated security patterns beyond cryptography (CAS, APTCA, .NET Remoting, DCOM, BinaryFormatter), see [skill:dotnet-security-owasp] which is the canonical owner of deprecated security pattern warnings.

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Agent Gotchas

1. Never reuse a nonce with AES-GCM -- reusing a nonce with the same key breaks both confidentiality and authenticity. Always generate a fresh random nonce per encryption operation.
2. Never use ECB mode -- ECB encrypts identical plaintext blocks to identical ciphertext blocks, leaking patterns. .NET's Aes.Create() defaults to CBC, but prefer AES-GCM for authenticated encryption.
3. Never compare hashes with == -- use CryptographicOperations.FixedTimeEquals to prevent timing side-channel attacks.
4. Never use MD5 or SHA-1 for security purposes -- they are broken. SHA-1 is acceptable only for non-security checksums (e.g., git object hashes) where collision resistance is not a security requirement.
5. Never hardcode encryption keys -- use [skill:dotnet-secrets-management] for key storage. Generate keys with RandomNumberGenerator.GetBytes.
6. Minimum RSA key size is 2048 bits -- NIST deprecated 1024-bit RSA keys. Use 4096 for new systems.
7. PBKDF2 iteration count must be high -- OWASP recommends 600,000 iterations with SHA-256 (as of 2023). Lower counts are brute-forceable.
8. PQC IsSupported checks are mandatory -- calling PQC APIs on unsupported platforms throws PlatformNotSupportedException. Always check before use.
9. Do not suppress SYSLIB5006 globally -- suppress the experimental diagnostic only at the specific call site where you intentionally use experimental PQC APIs.

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References

• ASP.NET Core Security
• Security in .NET
• Secure Coding Guidelines for .NET
• Cryptography Model in .NET
• Post-Quantum Cryptography in .NET
• ASP.NET Core Data Protection
• OWASP Password Storage Cheat Sheet
• NIST FIPS 203 (ML-KEM)
• NIST FIPS 204 (ML-DSA)
• NIST FIPS 205 (SLH-DSA)