FieldEncryptionDecryption.cs

using System;
using System.ComponentModel;
using System.IO;
using System.Security.Cryptography;

namespace CryptoShredding.Serialization.JsonConverters;

public class FieldEncryptionDecryption
{
    private const string EncryptionPrefix = "crypto.";
        
    public object GetEncryptedOrDefault(object value, ICryptoTransform encryptor)
    {
        if (encryptor is null) 
            throw new ArgumentNullException(nameof(encryptor));
        
        var isEncryptionNeeded = value != null;
        
        if (!isEncryptionNeeded) return default;
        
        using var memoryStream = new MemoryStream();
        using var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);
        using var writer = new StreamWriter(cryptoStream);
        var valueAsText = value.ToString();
        writer.Write(valueAsText);
        writer.Flush();
        cryptoStream.FlushFinalBlock();

        var encryptedData = memoryStream.ToArray();
        var encryptedText = Convert.ToBase64String(encryptedData);
        var result = $"{EncryptionPrefix}{encryptedText}";
        
        return result;

    }

    public object GetDecryptedOrDefault(object value, ICryptoTransform decryptor, Type destinationType)
    {
        if (value is null)
            throw new ArgumentNullException(nameof(value));
        
        var isText = value is string;
        if (!isText) return value;
        
        var valueAsText = (string)value;
        var isEncrypted = valueAsText.StartsWith(EncryptionPrefix);

        if (!isEncrypted)
        {
            var valueParsed = Parse(destinationType, valueAsText);
            return valueParsed;
        }

        var isDecryptorAvailable = decryptor != null;
        if (!isDecryptorAvailable)
        {
            var maskedValue = GetMaskedValue(destinationType);
            return maskedValue;
        }

        var startIndex = EncryptionPrefix.Length;
        var valueWithoutPrefix = valueAsText.Substring(startIndex);
        var encryptedValue = Convert.FromBase64String(valueWithoutPrefix);
        
        using var memoryStream = new MemoryStream(encryptedValue);
        using var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read);
        using var reader = new StreamReader(cryptoStream);
        
        var decryptedText = reader.ReadToEnd();
        var result = Parse(destinationType, decryptedText);
        return result;
    }

    private object Parse(Type outputType, string valueAsString)
    {
        var converter = TypeDescriptor.GetConverter(outputType);
        var result = converter.ConvertFromString(valueAsString);
        return result;
    }
        
    private object GetMaskedValue(Type destinationType)
    {
        if (destinationType == typeof(string))
        {
            const string templateText = "***";
            return templateText;
        }
        var defaultValue = Activator.CreateInstance(destinationType);
        return defaultValue;
    }
}