Does_a_second_matter

Cobb came across a safe, containing the flag for this challenge. On the safe it was written "Your old friend Satoshi". As usual Cobb took Somnacin to initiate the dream, and met Satoshi to get the flag. Satoshi told that he had locked the flag in that digital safe and the key to open it has some relation with his block number 691604, as the key is same as that of hash of the block, if it was mined 5 minutes before, with same nonce. Well it's Inception and Cobb has decided to find it in his dream, if incase he needs some more hints from Satoshi. You are Cob, can you get the flag before the dream finishes?

https://crypto-inst-2.vulnfreak.org/