TPTB_need_war, you cannot prove nor disprove that the Sartre text Craig Wright supposedly hashed is a collision for SHA256. The hash that he published is the exact hash that is signed by the signature that spent the Block 9 coinbase. Because calculating that hash is trivial and the signature is already public, it is reasonable and safe to assume that Craig Wright simply took that hash and claimed that it was the hash of the sartre text.

You also pointed out that he supposedly has access to a supercomputer. Even with access to a supercomputer, he would not be able to find a collision as other researchers have already tried. Simply having a lot of computing power does not mean that he can find a collision.

Alternatively, Craig could have found a vulnerability in sha256, in which case a lot more things than just Bitcoin is screwed. If Craig did not responsibly disclose such a vulnerability and instead exploited it, this would be incredibly sketchy and dishonest behavior.

The theory that the sha256 double hash is weaker than sha256 is false. It has been proven that performing multiple iterations of a hash is more secure than just one iteration. Specifically, many websites will store users passwords in the form of a multiple iteration hash. This is significantly more secure than a single iteration hash. The resulting hash of a multihash function (including multiple iterations) has the same collision resistance as the collision resistance of the weakest hash. This means that sha256d has the same collision resistance as sha256. What multiple hashes protect against is a preimage attack.

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It isn't really, it started as a complaint against the removal of his thread and then he promptly continued the thread here.