How to decrypt password in C#

See more:

C#

ASP.NET

CSS

HTML

Hi,
I am using this code to decrypt my password

public class NetFourMembershipProvider : SqlMembershipProvider
{

public string GetClearTextPassword(string encryptedPwd)
{

    try
    {
        byte[] encodedPassword = Convert.FromBase64String(encryptedPwd);
        byte[] bytes = this.DecryptPassword(encodedPassword);
        if (bytes == null)
        {
            return null;
        }

        return Encoding.Unicode.GetString(bytes, 0x10, bytes.Length - 0x10);
    }
    catch (Exception)
    {

        throw;
    }

   }

}

This is my app.config file.

<configuration>
<system.web>
<machineKey validationKey="XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"        decryptionKey="XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX" decryption="3DES"   validation="SHA1" />

<membership defaultProvider="AspNetSqlMembershipProvider" userIsOnlineTimeWindow="15">
  <providers>
    <clear />
    <add name="AspNetSqlMembershipProvider"
    type="System.Web.Security.SqlMembershipProvider"
    connectionStringName="SiteSqlServer" enablePasswordRetrieval="true"
    enablePasswordReset="true" requiresQuestionAndAnswer="false"
    minRequiredPasswordLength="7" minRequiredNonalphanumericCharacters="0"
    requiresUniqueEmail="false"
    passwordFormat="Encrypted"
    applicationName="DotNetNuke"
    description="Stores and retrieves ......." />
  </providers>
</membership>
<compilation debug="true" targetFramework="4.0" />
</system.web>

At this line

byte[] bytes = this.DecryptPassword(encodedPassword);

I am getting the below error message.

"You must specify a non-autogenerated machine key to store passwords in the encrypted format. Either specify a different passwordFormat, or change the machineKey configuration to use a non-autogenerated decryption key."
How can I convert passwords.

Please help me.

Note:- The validationkey is 40 characters and decryptionKey is a 48 charectors long data.

Thanks in Advance