Save user credentials using isolated storage and ProtectedData

Question

I want to save user details in a secure way so my intention is to take a class that contains the credentials, serialize it, encrypt it using protectedData and then save this new encrypted data in isolated storage. I have the following save method

    public bool SaveCredentials(ILoginCredentials credentials)
    {
        try
        {
            //CredentialStorage implements ILoginCredentials 
            CredentialStorage storage = new CredentialStorage(credentials);
            byte[] lastEncryptedData = ToByteArray(storage);
            lastEncryptedData = ProtectedData.Protect(lastEncryptedData, AditionalEntropy, DataProtectionScope.CurrentUser);

            IsolatedStorageFile isoStore = IsolatedStorageFile.GetStore(IsolatedStorageScope.User | IsolatedStorageScope.Assembly, null, null);
            IsolatedStorageFileStream isoStream = new IsolatedStorageFileStream("ExternalSSOProvider", FileMode.Create,
                                                                                 FileAccess.Write, isoStore);
            isoStream.Write(lastEncryptedData, 0, lastEncryptedData.Length);
            isoStream.Close();
            return true;
        }
        catch (Exception)
        {
            return false;
        }
    }

    private static byte[] ToByteArray(object source)
    {
        var formatter = new BinaryFormatter();
        using (var stream = new MemoryStream())
        {
            formatter.Serialize(stream, source);
            return stream.ToArray();
        }
    }

This bit of code seems to work no problem

Then I have the code that restores to object

    private CredentialStorage GetCredentials()
    {
        try
        {
            IsolatedStorageFile isoStore = IsolatedStorageFile.GetStore(IsolatedStorageScope.User | IsolatedStorageScope.Assembly, null, null);
            if (isoStore.FileExists("ExternalSSOProvider"))
            {
                using (IsolatedStorageFileStream stream = new IsolatedStorageFileStream("ExternalSSOProvider", FileMode.Open, isoStore))
                {
                    using (StreamReader reader = new StreamReader(stream))
                    {
                       using(MemoryStream ms = new MemoryStream())
                       {
                           reader.BaseStream.CopyTo(ms);
                           byte[] protectedMemory = ms.ToArray();
                           ms.Close();
                           ProtectedData.Unprotect(protectedMemory, AditionalEntropy, DataProtectionScope.CurrentUser);
                           return ToCredentials(protectedMemory);
                       }
                    }
                }
            }
        }
        catch (Exception)
        {
            return null;
        }
        return null;
    }

    private static CredentialStorage ToCredentials(byte[] source)
    {
        var formatter = new BinaryFormatter();
        using (var stream = new MemoryStream(source))
        {
            var x = formatter.Deserialize(stream); //My exception occurs here
            return x as CredentialStorage;
        }
    }

When I attempt to deserialize the object in the ToCredentials method I get the following error

Binary stream 'n' does not contain a valid BinaryHeader. Possible causes are invalid stream or object version change between serialization and deserialization.

Any help would be gratefully appreciated!

FYI this is the ILoginCredentials interface

 public interface ILoginCredentials
 {
     string Username { get; }
     string Password { get; }
 }

Answer 1

Ok I found the problem. In the GetCredentials method I had the line

    ProtectedData.Unprotect(protectedMemory, AditionalEntropy, DataProtectionScope.CurrentUser); 

I have changed this to

    protectedMemory = ProtectedData.Unprotect(protectedMemory, AditionalEntropy, DataProtectionScope.CurrentUser); 

Becuase I was never updating the protectedMemory variable with the return value I was attempting to deserialize from the still encrypted data