The security source in blockchain is the fact multiple entities verify the transactions.
But that does not require for computers to crack the same problem. Reverse verification is a computationally trivial for these proof of work blockchains. Any computer can do it nearly instantly.
ex: ( SHA512 ):
Text: "This is a bunch of stuff."
Number (Hash): 8ffa5045150bbdc16be0c530ab3665e718973b2b501100b61c0a5c6bdc4e0fc498da034813f97c812303deec3cf73bfb9edf02c789c479b3eb13ac7bb7fe3022
These block chains are basically just given the number, and forced to figure out the text. This is done by simply guessing random strings of text and seeing if it generates the correct number.
But if you have the text, it is trivial to generate the number to verify the correct text was found.
The computer that guesses the right text gets the privilege of making the next block in the chain. It usually takes multiple blocks in the chain to finalize a transaction. Since a different computer guesses the right text (or answer) every time, security and safety is provided by having unrelated* computers create the block chain and audit each other.
*when people lose interest in mining a block chain currency ( ex: bitcoin gold ), 51% attacks occur, where one organization/entity is able to make >51% of the guesses, allowing it to both make and audit malicious blocks as valid, and steal from the block chain.