I am thinking if secure encryption like one time pad (OTP) is achievable using bitcoin blockchain? The reason is bitcoin blockchain size is over 70 GB, which could potentially encrypt/decrypt 70 GB worth of data. All that is needed by the sender and receiver is blockchain and an application that encrypts and decrypts the data on the sender and receiver end respectively. Please share your thoughts if it is a bad or wild idea and why? How about using other public blockchains like Ethereum or private ones like Eris or Hyperledger. What would make it work?
The requirement for a one time pad to be secure is that it consists of uniformly random data, only known to the participants that should be able to encrypt/decrypt.
The blockchain is not random (it's created by miners and users creating transactions), certainly not uniformly random (blocks and transactions have a very strict syntactic structure), and worst of all it is designed to be known to the entire world (or in the case of private blockchains, to all those who should be able to audit).
Short answer: bad idea.