WCF Service Design Pattern for File Storage / Encryption

I'm starting a project that is required to take a document (pdf, jpeg, etc) via a WCf service and store it in with encryption in a location.

My question is one of fundamental class layout / design.

My intention is to have the WCF service function (SaveDocument) create call a worker class that handles the encryption / saving. Both the encryption / saving need to be changeable for future proofing i.e. change encryption method or how / where the data is saved (DB / File). Does my design below make sense:-

class DocumentManager
{
    private IEncryptionService encrypt;
    private IStoreService store;

    private Document doc;

    public DocumentManager(Document _doc)
    {
        encrypt = new XXXEncryptionService();
        store = new FileStoreService();
        doc = _doc;
    }

    public bool Save()
    {
        byte[] data = encrypt.Encrypt(doc.data);
        return store.Save(data);
    }
}

class IEncryptionService
{
    public byte[] Encrypt(byte[] stream);
    public byte[] Decrypt(byte[] stream);
}

class IStoreManager
{
    public bool Save(byte[] stream);       
}

Is there any use in the DocumentManager class being interfaced? I assume it would help for unit testing? Any other recommendations?

Many Thanks