Encrypting and decrypting data using NHibernate

Question

I'm writing a publicly accessible web application which will contain personal user data, such as names and birth dates, and I'm required to encrypt this data in a form that will be difficult for a human who might access the raw data to decrypt. I'm using Fluent NHibernate, mySQL, and C# 3.5.

1. What method should I use for industry standard encryption and decryption of user information? The method of encryption should not be database dependent.

2. How do I tell nHibernate to do transparent encryption/decryption on certain mapped classes with a simple property, like StorageType = StorageType.Encrypted. I don't mind if the resulting database table has just one or two columns, or one for each encrypted field. From what I've found, I should create my own data type from IUserDataType and encrypt the data in the constructor. Is this correct?

Answer 1

In true Blue Peter fashion, here's one I created earlier to do just this. It relies on a provider pattern to get the encryption algorithm but you could replace this with whatever you want.

This exposes a string property in your domain object, but persists it as a binary (array of bytes) representing the encrypted form. In my provider pattern code, Encrypt takes a string and returns a byte array, and Decrypt does the opposite.

[Serializable]
public class EncryptedStringType : PrimitiveType
{
    public EncryptedStringType() : this(new BinarySqlType()) {}

    public EncryptedStringType(SqlType sqlType) : base(sqlType) {}

    public override string Name
    {
        get { return "String"; }
    }

    public override Type ReturnedClass
    {
        get { return typeof (string); }
    }

    public override Type PrimitiveClass
    {
        get { return typeof (string); }
    }

    public override object DefaultValue
    {
        get { return null; }
    }

    public override void Set(IDbCommand cmd, object value, int index)
    {
        if (cmd == null) throw new ArgumentNullException("cmd");
        if (value == null)
        {
            ((IDataParameter)cmd.Parameters[index]).Value = null;
        }
        else
        {
            ((IDataParameter)cmd.Parameters[index]).Value = EncryptionManager.Provider.Encrypt((string)value);
        }
    }

    public override object Get(IDataReader rs, int index)
    {
        if (rs == null) throw new ArgumentNullException("rs");
        var encrypted = rs[index] as byte[];
        if (encrypted == null) return null;
        return EncryptionManager.Provider.Decrypt(encrypted);
    }

    public override object Get(IDataReader rs, string name)
    {
        return Get(rs, rs.GetOrdinal(name));
    }

    public override object FromStringValue(string xml)
    {
        if (xml == null)
        {
            return null;
        }

        if (xml.Length % 2 != 0)
        {
            throw new ArgumentException(
                "The string is not a valid xml representation of a binary content.",
                "xml");
        }

        var bytes = new byte[xml.Length / 2];
        for (int i = 0; i < bytes.Length; i++)
        {
            string hexStr = xml.Substring(i * 2, (i + 1) * 2);
            bytes[i] = (byte)(byte.MinValue
                              + byte.Parse(hexStr, NumberStyles.HexNumber, CultureInfo.InvariantCulture));
        }

        return EncryptionManager.Provider.Decrypt(bytes);
    }

    public override string ObjectToSQLString(object value, Dialect dialect)
    {
        var bytes = value as byte[];
        if (bytes == null)
        {
            return "NULL";
        }
        var builder = new StringBuilder();
        for (int i = 0; i < bytes.Length; i++)
        {
            string hexStr = (bytes[i] - byte.MinValue).ToString("x", CultureInfo.InvariantCulture);
            if (hexStr.Length == 1)
            {
                builder.Append('0');
            }
            builder.Append(hexStr);
        }
        return builder.ToString();
    }
}

Answer 2

I would create an EncryptionService that encrypts strings using whatever Key you'd like. Then I would make 2 properties in your entity. One that NHibernate interacts with (Encrypted values) and another that you (or other developers) interact with that will automatically encrypt the values.

See: http://kockerbeck.blogspot.com/2009/08/fluent-nhibernate-encrypting-values.html

A sample EncryptionService, User entity and UserMap are below.

public class User
{
   private readonly EncryptionService _encryptionService =
                   new EncryptionService();
   public virtual int Id { get; set; }
   public virtual DateTime? DateOfBirth
   {
     get
     {
       return _encryptionService.DecryptObject<DateTime?>(DateOfBirthEncrypted);
     }
     set
     {
       DateOfBirthEncrypted= _encryptionService.EncryptString(value.Value
                                   .ToString("yyyy-MM-dd HH:mm:ss"));
     }
   }
   [Obsolete("Use the 'DateOfBirth' property -- this property is only to be used by NHibernate")]
   public virtual string DateOfBirthEncrypted { get; set; }
}


public sealed class UserMap : ClassMap<User>
{
  public UserMap()
  {
    WithTable("dbo.[User]");
    Id(x => x.Id, "[ID]");
    Map(x => x.DateOfBirthEncrypted, "DOB");
  }
}

And the EncryptionService:

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

namespace Services
{
    public class EncryptionService : IEncryptionService 
    {
        /// <summary>
        /// Decrypts a string 
        /// </summary>
        /// <param name="encryptedString"></param>
        /// <returns></returns>
        public String DecryptString(string encryptedString)
        {
            if (String.IsNullOrEmpty(encryptedString)) return String.Empty;

            try
            {
                using (TripleDESCryptoServiceProvider cypher = new TripleDESCryptoServiceProvider())
                {
                    PasswordDeriveBytes pdb = new PasswordDeriveBytes("ENTERAKEYHERE", new byte[0]);
                    cypher.Key = pdb.GetBytes(16);
                    cypher.IV = pdb.GetBytes(8);

                    using (MemoryStream ms = new MemoryStream())
                    {
                        using (CryptoStream cs = new CryptoStream(ms, cypher.CreateDecryptor(), CryptoStreamMode.Write))
                        {
                            byte[] data = Convert.FromBase64String(encryptedString);
                            cs.Write(data, 0, data.Length);
                            cs.Close();

                            return Encoding.Unicode.GetString(ms.ToArray());
                        }
                    }
                }
            }
            catch
            {
                return String.Empty;
            }
        }

        /// <summary>
        /// Encrypts a string
        /// </summary>
        /// <param name="decryptedString"
        /// <returns></returns>
        public String EncryptString(string decryptedString)
        {
            if (String.IsNullOrEmpty(decryptedString)) return String.Empty;

            using (TripleDESCryptoServiceProvider cypher = new TripleDESCryptoServiceProvider())
            {
                PasswordDeriveBytes pdb = new PasswordDeriveBytes("ENTERAKEYHERE", new byte[0]);

                cypher.Key = pdb.GetBytes(16);
                cypher.IV = pdb.GetBytes(8);

                using (MemoryStream ms = new MemoryStream())
                {
                    using (CryptoStream cs = new CryptoStream(ms, cypher.CreateEncryptor(), CryptoStreamMode.Write))
                    {
                        byte[] data = Encoding.Unicode.GetBytes(decryptedString);

                        cs.Write(data, 0, data.Length);
                        cs.Close();

                        return Convert.ToBase64String(ms.ToArray());
                    }
                }
            }
        }

        /// <summary>
        /// Decrypts a given value as type of T, if unsuccessful the defaultValue is used
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="value"></param>
        /// <param name="defaultValue"></param>
        /// <returns></returns>
        public T DecryptObject<T>(object value, T defaultValue)
        {
            if (value == null) return defaultValue;

            try
            {
            Type conversionType = typeof(T);

            // Some trickery for Nullable Types
            if (conversionType.IsGenericType && conversionType.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
            {
                conversionType = new NullableConverter(conversionType).UnderlyingType;
            }

            return (T)Convert.ChangeType(DecryptString(Convert.ToString(value)), conversionType);
            }
            catch 
            {
                // Do nothing
            }

            return defaultValue;
        }
    }
}