[KimDaBa] new feature: IPTC keywords

[Eivind]

On Sunday 23 October 2005 22:02, Marco Molteni wrote:

> My idea was: how to go back to a unique property of film as opposed
> to digital: the fact that if you don't trust a print you can analyze
> the negative to see if there are modifications (putting aside the fact
> that a negative too can be modified).
>
> I though that the only way was for the camera to store a list of serial
> numbers and associated message digest (say MD5 or SHA or whatever).
>
> But then I hit a block because the question is: how do you trust the
> list that comes out of the camera? Did Canon solved this?

Not really. The Canon signed "Digital negatives" essentially work like 
this:

* Each camera comes with an embedded secret cryptographic key

* The corresponding public key is also delivered, aswell as stored by 
Canon, linked to the serial-number of the camera.

* When you tak a photo in raw-mode (asfar as I know only raw images can be 
signed, makes some sense since jpegs are lossy anyway) a secure hash of 
the image-data is computed, and digitally signed with the secret key, the 
resulting signature is stored in the raw-file.

Now, this works fairly well, assuming you can be certain that the 
cryptographic implementation is sane and -- crucially -- that the secret 
key really is known only to the innards of this particular camera.

But can you really ?

A user that -somehow- managed to extract the key from the camera would be 
able to photoshop "evidence" at will. Canon doesn't talk about this 
possibility -- for obvious reasons.

Canon themselves are also in a position to trivially compile a list 
somewhere containing all the secret keys embedded in cameras. It would be 
monumentally stupid of them to do so, but they are, as far as I can see 
(and I spent some time reading a fair portion of the available docs) not 
even anyway explicitly stating that they don't know the secret keys. (even 
if they *did* state it, how do you know it's true ?)

You also have to trust Canons (closed source, not publically reviewed, 
without any certification) cryptographic implementation, aswell as various 
companies delivering parts for Canon.

In short:

You can be fairly sure it'll stop your little sister from modifying one of 
your photos, without you noticiing it. I would *not* feel all that sure 
that Canon are unable to fake "my" photos, nor that the NSA can't produce 
a photoshopped "evidence"-photo that still checks out as legitimate.


	Eivind Kjørstad