[betterunix]

"Satoshi didn't try to create his own hashing algorithm or anything like that."

No, he tried to create his own digital cash system, and digital cash is a cryptography problem that has been extensively studied by cryptographers (and had been studied for decades prior to Bitcoin). Bitcoin is also a system that involves multiparty computation, and secure multiparty computation has also been studied extensively by cryptographers, also going back decades. It is a mistake to think that the only relevant cryptography in Bitcoin are digital signatures and hash functions.

This is really the crux of the issue here. Bitcoin is not a hash function. It is not a digital signature system. The security of hash functions and digital signatures is not in question here; Bitcoin could be vulnerable to attack even if it is built using secure hash functions and secure signature systems. The point of having a security definition is to be clear about these things. We need to be clear about what the meaning of "security" is in the case of Bitcoin if we want to make any statements about whether or not Bitcoin actually achieves that security goal. It is not hard to see that the definition of security for a hash function or a digital signature system is not what we want for Bitcoin; what is not so clear is what we actually do want.