Re:It can all go to zero
by monort on Tuesday January 16, 2018 @04:08PM
(#16162612)
This is not possible because public keys you want to crack are in hashed form RIPE-160(SHA256(public_key)). Grover's search algorithm can help with it, but speedup is much less, than with Shor's algorithm and won't be of much help with sha256.
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You are correct that the blockchain has hashed the public key and so isn't directly attackable, except that the public key must be revealed at some point. Therefore, it is still possible to recover the private key (and if you can do this faster than the transaction can enter the blockchain, you can potentially steal the bitcoin).
However, if you wanted to break the hashed private key, you don't need to solve 256 bits. Grover's algorithm is a search, thus difficulty scales with the final hash key length, not the intermediate hashes. Still, it's only a quadratic speed-up and 160 bits is still pretty tough. But that's fine, as you can still intercept the public keys before they're hashed on the blockchain.