Gravatar: Hash from image?

Question

Given a pre-saved generated Gravatar image in high resolution, is it possible to get original MD5 hash from it?

Answer 1

The algorithm Gravatar uses to generate images is not publicly available and I doubt they will reveal their secrets here. Even in the unlikely case one can reconstruct it, it could change any second, making your solution temporary, at most.

However, what would you want to do with the hash corresponding to an image? As you cannot get information out of that (by all assumptions of cryptography, although MD5 in particular is not so secure anymore), the only thing I could image doing with it is comparing it to another given hash to see if the underlying email addresses/URLs/whatever are equal. Yet, this can be easily achieved "the other way round": given an email, get the corresponding Gravatar image (by applying MD5 and querying for the image like in this answer) and compare it to your given Gravatar. If they are equal pixel-by-pixel, chances are very high you got the corresponding email, otherwise not.