timestamp.verisign.com has been doing something like this for the past decade, though it's primarily used for code signing (aka Authenticode).

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Indeed. You commented on Authenticode for my previous entry on this, which I totally failed to comment on. I read the web page you linked to, and indeed it is similar to what I want. However, it relies on a centralized public key infrastructure (i.e., not PGP). I assume it also requires the user to pay for each timestamp. If I wanted to timestamp all my photos as a routine part of my workflow, I would prefer to not have to go through the hassle of paying every time. But perhaps they have a subscription thing, too.

I'd really like to see this as a free (as in freedom) service, though. Perhaps some day I will have time to implement the spec I've started. --liw