Attacking Blochcain/Bitcoin by hash Assuming that SHA-256 is not save anymore. How could the Bitcoin blockchain get attacked? Ok I could get the input value for the hash function but could I now easily add my own blocks to the chain? I'm currently confused why there are theses hashes used...

How long would it take to attack the hash now, with a working SHA-256. Could you provide me a calculation with GFLOPS or something?

best regards
i can not calculate gflops  but...

some easy thoughts:

if sha2 is not safe anymore you can not easily steal bitcoin from an existing address by calculating the private key from any address because to generate the address ripemd160 is also used. and you did not assume that ripemd160 is also not safe anymore. 

if everyone would know sha2 is not safe anymore then bitcoin will be worthless because everybody can mine without doing any work of the proof-of-work-process.

if only the attacker knows how to calculate the input of an sha2 function, an intelligent attacker would not attack the system. he would use the system: 
the easiest way to make money when you can calculate nearly arbitrary sha2-hashs with nearly arbitrary input: mining. you can mine blocks and so you can earn 12.5btc several times every hour (furthermore you earn the transactionfees) without paying many money for electricity because you don't need to "work" in the sense of proof of work anymore. the advantage: you do not destroying the system because no one can really realize that you can "hack" the system and so you could theoretically earn many bitcoins.