In a remarkable exchange that — per Laszlo Hanyecz’s later recall — occurred after Hanyecz shared his GPU miner code with Satoshi, Satoshi reciprocated by sharing his own independently developed GPU mining code. As Hanyecz tells it, this revelation showed that Satoshi had been working on GPU mining before Hanyecz’s public announcement — not to mine coins, but as a contingency plan to defend the network against potential 51% attacks.
As Hanyecz later recounted:
And he actually shared with me his version of it. So even though it wasn’t in Bitcoin, he did have GPU mining code and he said he was just keeping it ready in case he had to defend the network with it.
Satoshi had developed several versions of the GPU mining algorithm but deliberately chose not to include it in the public Bitcoin software. His strategic reasoning was twofold: he wanted to keep GPU mining capability as a defensive weapon against potential attackers, and he did not want to prematurely increase network difficulty, which would have discouraged ordinary CPU miners from participating.
Hanyecz noted that his own GPU code actually performed better than Satoshi’s version, but that Satoshi had intentionally avoided optimizing it:
And I got the feeling, that part of it was that he didn’t want to prematurely optimize it because he didn’t want to run up the difficulty on the network.
The contrast is the point: in public Satoshi was slowing GPU mining down to keep ordinary CPUs in the game, while in private he kept a GPU miner ready — and deliberately un-optimized — as something to deploy only if the network ever needed defending against a 51% attack.
Keeping this capability private depended on the public stance recorded in Satoshi’s pushback on Hanyecz’s May 10 GPU-mining announcement, where — again per Hanyecz’s recall — he asked Hanyecz to slow down so ordinary CPU miners would not be discouraged from participating.