This is a great question, I'm not 100% sure, but my guess is that it's a proof of age that the PGP key is newer than 2013-10-19
. My reasoning is as follows:
What makes blockchain mining computationally hard is fiddling with the nonce until the hash of that block has a specific number of leading zeros (16 hex zeros, or 64 binary zeros in this case). Once you find such a nonce for that block of transactions, you can publish the block and claim the bitcoin mining reward.
That hash is 64 hex digits = 32 bytes, or 256 bits. Currently there are 410086 block in the blockchain. That means the chances of guessing a random 192 bit hex value (256 - 64 bits of leading zeros) and having it match the hash of an existing block is about 1 / 10^52. So the fact that he embedded in his key the hash of a block from 2013-10-19
means that this block had already been published by the time he published his key; but there's nothing to stop him from having inserted that retro-actively.
In summary: this proves that the key was published sometime between 2013-10-19 and now.
I also notice that the key was first self-signed on 2012-04-25
, but the blockhash wasn't added until 2013-10-19
and then the same blockhash has been carried forward ever since. I'm a little stumped to as to what that's proving. Why would you want to prove that you key is newer than a certain time?
If you want a definitive answer, then email him! I'm sure a bitcoin / PGP nerd would be happy to explain!