mat2/doc/threat_model.md

Threat Model

The Metadata Anonymisation Toolkit 2 adversary has a number of goals, capabilities, and counter-attack types that can be used to guide us towards a set of requirements for the mat2.

This is an overhaul of MAT's (the first iteration of the software) one.

Warnings

Mat only removes standard metadata from your files, it does not:

• anonymise their content (the substance and the form)
• handle watermarking
• handle steganography nor homoglyphs
• handle stylometry
• handle any non-standard metadata field/system
• handle file-system related metadata

If you really want to be anonymous format that does not contain any metadata, or better : use plain-text ASCII without trailing spaces.

And as usual, think twice before clicking.

Adversary

• Goals:

  • Identifying the source of the document, since a document always has one. Who/where/when/how was a picture taken, where was the document leaked from and by whom, ...

  • Identify the author; in some cases documents may be anonymously authored or created. In these cases, identifying the author is the goal.

  • Identify the equipment/software used. If the attacker fails to directly identify the author and/or source, his next goal is to determine the source of the equipment used to produce, copy, and transmit the document. This can include the model of camera used to take a photo or a film, which software was used to produce an office document, …

• Adversary Capabilities - Positioning

  • The adversary created the document specifically for this user. This is the strongest position for the adversary to have. In this case, the adversary is capable of inserting arbitrary, custom watermarks specifically for tracking the user. In general, mat2 cannot defend against this adversary, but we list it for completeness' sake.

  • The adversary created the document for a group of users. In this case, the adversary knows that they attempted to limit distribution to a specific group of users. They may or may not have watermarked the document for these users, but they certainly know the format used.

    • The adversary did not create the document, the weakest position for the adversary to have. The file format is (most of the time) standard, nothing custom is added: mat2 must be able to remove all metadata from the file.

Requirements

• Processing

  • mat2 should avoid interactions with information. Its goal is to remove metadata, and the user is solely responsible for the information of the file.

  • mat2 must warn when encountering an unknown format. For example, in a zipfile, if mat2 encounters an unknown format, it should warn the user, and ask if the file should be added to the anonymised archive that is produced.

  • mat2 must not add metadata, since its purpose is to anonymise files: every added items of metadata decreases anonymity.

  • mat2 should handle unknown/hidden metadata fields, like proprietary extensions of open formats.

    • mat2 must not fail silently. Upon failure, mat2 must not modify the file in any way.

    • mat2 might leak the fact that mat2 was used on the file, since it might be uncommon for some file formats to come without any kind of metadata, an adversary might suspect that the user used mat2 on certain files.