Verifiable alternative to screenshots for web browsers

Question

Screenshots are generally held to be dubious evidence when presented in court because they are susceptible to alteration, especially when it comes to pages rendered in web browsers. You can use the browser's dev tools to alter the content, you can modify the resulting image with MS Paint, you can even be victim to an elaborate MITM attack because you trusted a bad CA.

There are myriad reasons to want non-repudiation and integrity verification for web content. Some people turn to sources like the Wayback Machine, but they have sparse snapshots, and they are unable to record dynamic, per-user content.

Are there tools available to verifiably claim "This is the webpage that was served to me"? Is such a tool even possible on a web client?

Such a tool would need to verify

1. The page's contents
2. Who served the page in question.
3. The page's contents were not modified in transit (eliminate MITM)
4. The tool itself was not tampered with

Requirement 4 alone makes me think that client-side verification tools are impossible, making necessary an external service like a proxy.

Do such services exist?

Answer 1

Yes, this type of product exists. Both Serge's and Steffan's answers and approaches are correct and their concerns are valid, but there is another way to accomplish "proof" (if you drop the requirement to prove no MITM - that can't happen client-side).

There are systems that can be installed at the kernel level of the OS that take screenshots of every page displayed on a browser and the HTTP request string used. These are signed and stored in a central server. I have seen these in corporate settings for end-user monitoring in certain high-risk contexts.

Since this system is operated as a "normal operation" of the client, these screenshots can be used as supporting evidence for a claim made by the end user (tho perhaps not for claims made by the entity running the evidence collection software, but that's a legal matter).

Is it fool-proof? No. But it provides valid, supporting evidence that can be used to force another party to prove that the evidence is invalid.

So, like Steffan says, it requires a third party, but that third party can be close. It's just that the evidence collection needs to be in the hands of someone else.

Answer 2

Such tools would not be able to run on a client since a common client (PC, phone) is not sufficiently trusted for this - the (potentially cheating) user has too much control of the device.

It might be possible for a commonly accepted trusted third party (i.e. a notary) to run a service where users submit a URL and get back some signed container (like a PDF) which contains the image, the URL, a timestamp etc.

The technology for this exists, but I'm not aware of such a service - maybe because it might not be a profitable business model. The approach also has limitations since it can only prove things the service actually has access to, i.e. mostly public content not specific to any user or to any location of access. It can also not prove that the content was actually delivered to the specific user, only that the content was served to the service.

Answer 3

There is a question of trust here. As soon as you are the only provider for a document, you cannot prove that it is authentic because you could as well have forged it. So it can only be a hint that things could have gone that way. The only communication equipment I can remember with a kind of legal value was the good old Telex, yes the one at 50 bauds and ITT2 alphabet with 5 bits characters.

The main difference with Internet of even facs, is that the teleprinters were connected and identified and provided an technical answerback:

A major advantage of telex is that the receipt of the message by the recipient could be confirmed with a high degree of certainty by the "answerback". At the beginning of the message, the sender would transmit a WRU (Who aRe yoU) code, and the recipient machine would automatically initiate a response which was usually encoded in a rotating drum with pegs, much like a music box. The position of the pegs sent an unambiguous identifying code to the sender, so the sender could verify connection to the correct recipient. The WRU code would also be sent at the end of the message, so a correct response would confirm that the connection had remained unbroken during the message transmission. This gave telex a major advantage over group 2 fax, which had no inherent error-checking capability.

So presenting a continuous paper band with an included message and its answerback was a serious evidence that a message was sent to an identified recipient or received from an identified emitter: the operator could confirm that they did not notice any deconnection at that time, and sender of recievers of previous and following message could confirm that they indeed sent or received those messages at those times.

Nowadays only a digitally signed message can ensure non-repudiation, and they are not commonly used...