I'm involved in a Proof of Existence project and I need to know if it is convenient to create a private blockchain or we can use Bitcoin or Ethereum.
I would like to know the price of storing a SHA3 hash in Bitcoin and Ethereum blockchains.
In Bitcoin, you can store 80 bytes in one transaction with OP_RETURN. SHA-3(256) has a size of 32 bytes and can be stored in one transaction. The fee for the OP_RETURN transaction is 0.0001 BTC. By today's price, it's $0.177 per tx. You can optimize it by using aggregations services, such as Open Timestamps
In Ethereum, there are at two options to store data in the blockchain.
To store one byte in contract's state you need 0.000035 ETH or $0.003115 if 1 ETH worth $89. To store 32 bytes you need $0.09968
to store 32 bytes in a log you need only 8*32=256 gas
Disadvantage of logs is that you can not access logs' data from contracts. Quote from Technical Introduction to Events and Logs in Ethereum
Logs were designed to be a form of storage that costs significantly less gas than contract storage. Logs basically cost 8 gas per byte, whereas contract storage costs 20,000 gas per 32 bytes. Although logs offer gargantuan gas savings, logs are not accessible from any contracts.