Login system for anonymous crime reporting service

I work in the digital team for a UK-based crime prevention charity. The charity has an online form where people can give information about crime anonymously, and they can chose when completing the form to create an anonymous login so that they can add details at a later point/answer other questions that we might have for them. I am helping a digital agency to redesign a) the form and b) the login system.

My primary interest at the moment is the login side of this project. Does anyone here have experience of creating a similar login system, or advice on improving UX for such a login system? High security systems can have UX as a secondary consideration but I think it's worth attempting to simplify the system we are current expecting users to go through.

Here is some useful background info -

Key requirements of the system:

• Has to be secure, but anonymous (we don't capture email addresses, names, phone numbers or anything else that could identify the person giving information)

Current login system:

• We ask for:

  1. Pass phrase (user created - at least six characters long and contain at least 1 numeric character, 1 capital letter and 1 special character (eg. &@%*?!))
  2. The user to remember an image they selected from a choice of 8 images
  3. The user to remember a colour they chose from a predefined list of 8 colours
  4. (optional) A unique code generated when the user sent the initial form

Thanks!