Can Blockchain miners see confidential transaction information? Since they have the public key, they decrypt the transaction for the purpose of verification. So if I am mining in Ethereum, I can tweak the node code to log the transaction details on my hard disk. Is Blockchain not suitable where confidentiality is important?
Ethereum transactions are not encrypted, so no decryption is necessary. Logging all transactions to disk is part of being a node (miner or any full node), so no tweaking is necessary. You can see every historic transaction in the clear at sites like Etherscan.
So, as it stands, there is no transaction confidentiality on the public Ethereum blockchain.
This may change in future if/when ZK Snark technology gets implemented. Variants of Ethereum exist (such as Quorum) with mechanisms for private transactions, but these don't transfer over to the public Ethereum blockchain.
Anything that is broadcast onto the network, which includes all transactions, is public to all nodes. The only question is whether that information is in cleartext or not. You haven't defined "confidential transaction information". If you mean that a transaction has taken place at all (which some people might consider to be confidential), then no. If you mean things like the recipient or amount, the introduction of zkSNARKs in Metropolis are the beginning of adding privacy for those. If using a custom token (not a currently ERC-compliant one), then zkSNARKs should enable that kind of privacy to be added to the token.
If you mean some other data you want in your transaction, specific to a contract you are writing, you could employ standard encryption techniques (with enough gas) or even homomorphic encryption if necessary. This can be done at present.
Obfuscation techniques like tumbling may also satisfy your needs.
Just like the others explained on this thread, there's no way to hide transactions on Ethereum using the primitive tools available out there.
However, two years after this question has been posted, the AZTEC protocol has been launched and I published How to Code Your Own Confidential Token on Ethereum - an article that describes how to link a normal ERC20 token to a confidential version.
Disclaimer: I work at AZTEC as a full-time blockchain engineer.