In addition to signing a message from an address known to be controlled by him, he could also sign a message from
this PGP key, which is the same one he was talking about here:
What's interesting is he never actually signed a message with this key, as far as we know. But signing the same message from both bitcoin address and PGP key would certainly be a good start for proving that he was back.
The problem with that PGP key is that while you can see in the wayback machine the first snapshot of bitcoin.org archived with date 31 Jan 2009 (
https://web.archive.org/web/20090131115053/http://bitcoin.org/ ) , if you click the PGP key link at the bottom of the page it will transfer you to a page that has it archived with date 28 Feb 2011 and not with the same date as the original webpage (
https://web.archive.org/web/20110228054007/http://www.bitcoin.org/Satoshi_Nakamoto.asc ) .
So , to my understanding , that PGP key means nothing as someone with access to bitcoin.org could have change the original and placed there another one .