using System;

using System.Security.Cryptography;

using NSec.Cryptography;

​

public static class Program

{

    public static void Main() {

    Console.WriteLine("ChaCha20-Poly1305 String encryption with random key full");

    // you need the library https://www.nuget.org/packages/NSec.Cryptography version 20.2.0

    // documentation: https://nsec.rocks/

    // https://github.com/ektrah/nsec

​

    string plaintext = "The quick brown fox jumps over the lazy dog";

    Console.WriteLine("plaintext: " + plaintext);

​

    byte[] encryptionKey = GenerateRandomKey();

    string encryptionKeyBase64 = Base64Encoding(encryptionKey);

    Console.WriteLine("encryptionKey (Base64): " + encryptionKeyBase64);

​

    // encrypt

​

    string ciphertextBase64 = Chacha20Poly1305EncryptToBase64(encryptionKey, plaintext);

    //string ciphertextBase64 = SecretboxEncryptionToBase64(encryptionKeyBase64, plaintext);

    Console.WriteLine("\n* * * encryption * * *");

    Console.WriteLine("all data are in Base64 encoding");

    Console.WriteLine("ciphertext:  " + ciphertextBase64);

    Console.WriteLine("output is (Base64) nonce : (Base64) ciphertext");

​

    // decrypt

    string ciphertextReceivedBase64 = ciphertextBase64;

    string decryptionKeyBase64 = encryptionKeyBase64;

    byte[] decryptionKey = Base64Decoding(decryptionKeyBase64);

    string decryptedtext = Chacha20Poly1305DecryptFromBase64(decryptionKey, ciphertextReceivedBase64);

    Console.WriteLine("\n* * * decryption * * *");

    Console.WriteLine("ciphertext:  " + ciphertextReceivedBase64);

    Console.WriteLine("input is (Base64) nonce : (Base64) ciphertext");

    Console.WriteLine("decrypt.text:" + decryptedtext);

}

​

    static string Chacha20Poly1305EncryptToBase64(byte[] encryptionKey, string plaintext) {

        byte[] nonce = GenerateRandomNonce();

        //byte[] nonce = new byte[12];

        byte[] data = System.Text.Encoding.UTF8.GetBytes(plaintext);

        byte[] aad = new byte[0];

        // Create the primitive

        // select the Ed25519 signature algorithm

        var alg = AeadAlgorithm.ChaCha20Poly1305;

        using var key = Key.Import(alg, encryptionKey, KeyBlobFormat.RawSymmetricKey);

        var ciphertextWithTag = alg.Encrypt(key, new Nonce(nonce, 0), aad, data);

        // split ciphertextWitTag in ciphertext and poly1305Tag

        int ciphertextWithTagLength = ciphertextWithTag.Length;

        byte[] ciphertext = new byte[ciphertextWithTagLength - 16];

        byte[] poly1305Tag = new byte[16];

        Array.Copy(ciphertextWithTag, 0, ciphertext, 0, (ciphertextWithTagLength - 16));

        Array.Copy(ciphertextWithTag, (ciphertextWithTagLength - 16), poly1305Tag, 0, 16);

        return Base64Encoding(nonce) + ":" + Base64Encoding(ciphertext) + ":" + Base64Encoding(poly1305Tag);

    }

​

    static string Chacha20Poly1305DecryptFromBase64(byte[] encryptionKey, string ciphertextReceivedBase64) {

        String[] parts = ciphertextReceivedBase64.Split(':');

        byte[] nonce = Base64Decoding(parts[0]);

        byte[] ciphertext = Base64Decoding(parts[1]);

        byte[] poly1305Tag = Base64Decoding(parts[2]);

        byte[] ciphertextWithTag = new byte[(ciphertext.Length + poly1305Tag.Length)];

        Array.Copy(ciphertext, 0, ciphertextWithTag, 0, ciphertext.Length);

        Array.Copy(poly1305Tag, 0, ciphertextWithTag, ciphertext.Length, poly1305Tag.Length);

        byte[] aad = new byte[0];

        var alg = AeadAlgorithm.ChaCha20Poly1305;

        using var key = Key.Import(alg, encryptionKey, KeyBlobFormat.RawSymmetricKey);

        byte[] decrypted = new byte[ciphertext.Length];

        bool success = alg.Decrypt(key, new Nonce(nonce, 0), aad, ciphertextWithTag, decrypted);

        return System.Text.Encoding.UTF8.GetString(decrypted, 0, decrypted.Length);;

    }

​

    static byte[] GenerateRandomKey() {

        RNGCryptoServiceProvider rngCsp = new RNGCryptoServiceProvider();

        byte[] key = new byte[32];

        rngCsp.GetBytes(key);

        return key;

    }

​

    static byte[] GenerateRandomNonce() {

       RNGCryptoServiceProvider rngCsp = new RNGCryptoServiceProvider();

       byte[] nonce = new byte[12];

       rngCsp.GetBytes(nonce);

       return nonce;

    }

​

    private static string Base64Encoding(byte[] input) {

        return Convert.ToBase64String(input);

    }

​

    private static byte[] Base64Decoding(String input) {

        return Convert.FromBase64String(input);

    }

}

​