gistfile1.cs

static byte[] EncryptAsymmetric(string message, byte[] publicKey, byte[] secretKey, byte[] nonce)
    {
        // First convert the string to a byte array:
        byte[] messageBuffer = UTF8Encoding.UTF8.GetBytes(message);
        // Preallocate array and accomodate space for zero padding.
        var unencryptedData = new byte[CryptoBox.ZeroBytes + messageBuffer.Length];
        Array.Copy(messageBuffer, 0, unencryptedData, CryptoBox.ZeroBytes, messageBuffer.Length);
        // Preallocate output buffer
        var encryptedData = new byte[unencryptedData.Length];
        CryptoBox.Box(encryptedData, 0, unencryptedData, 0, unencryptedData.Length, nonce, publicKey, secretKey);
        return encryptedData;
    }
 
static string DecryptAsymmetric(byte[] encryptedData, byte[] publicKey, byte[] secretKey, byte[] nonce)
    {
        // First preallocate the data array
        var decryptedData = new byte[encryptedData.Length];
        if (CryptoBox.Open(decryptedData, 0, encryptedData, 0, encryptedData.Length, nonce, publicKey, secretKey)
                != 0)
        {
            throw new ArgumentException("Could not decrypt data");
        }
        // Message could be decrypted, convert byte array to string
        int dataLength = (decryptedData.Length - CryptoBox.ZeroBytes);
        char[] buffer = UTF8Encoding.UTF8.GetChars(decryptedData, CryptoBox.ZeroBytes, dataLength);
        return new string(buffer);
    }