When I was doing security research I determined that a necessary component was a "Device for Human Signatures". This could make it pretty easy to build one. From http://grampsgrumps.blogspot.com.au/2013/11/the-key2key-project.html:
Device for Human Signatures
We want to make it possible for real world legal entities to interact via the network. What is needed is a way to link people to the network in a way that makes legal sense. The proposed solution will work for an individual representing themself, or for an employee with some delegated ability to act for the employer. We don't consider the possibility of combining these in a single physical device.
The solution is a Device for Human Signature, DHS. The DHS requirements mean that it must be a separate device, not part of a more complex device. The proposed device has the following characteristics:
It has biometric authentication which is unchangeably linked to the owner.
It has a private key that is generated when first activated. Only the public key ever leaves the device.
It has a black and white screen and a mechanism for scrolling the image left-right and up-down.
It has a way that the owner can agree to sign what is displayed on the screen. This is such that it can't be done accidentally, nor can it be done without simultaneous biometric authentication.
There is another mechanism to clear the current image without signing it.
The device is connected to the world by wireless mechanisms and/or cable. If a cable is plugged in then it only uses that, which is desirable for signing things that have privacy restrictions. Either way it displays any offered image and, if signed, it sends the signature back on the reverse route.
The user signs the extended black and white image. She is not able to sign it till she has used the scroll control to view all of it.
The image will always be created, by a defined and public process, from information in a computer friendly format (such as XML). For example one of the known processes will be "English". The information in computer format, and the well know translation process will be sent with the signature of the text when it is used for internal computer purposes. For legal purposes only the actual visible text applies.
Any computer software can "understand" the signed text by using the conversion process on the computer friendly variant and checking that the resultant image is the one that the user signed. E.g. the user might sign "pay $1000 from my account 061234567 to Example Company (ABN 1234) account 0698765". What they actually sign is an array of black and white dots which has the appearance of this sentence. However the receiving computer (presumably the bank) doesn't have to understand the visual dots because such signed documents always come with an accompanying computer friendly structure which converts to the image in a well defined mechanical way. The signed document comes with an accompanying solution to the problem of determining its meaning.
It is important to sign a picture rather than "text", because it removes questions about how the text was rendered, and as we see it works just as well.
The signing device is only intended to be used for important things, or to create a temporary delegation to some more practical computer system which will sign as needed to act on the network within that delegation.