[Ethereum] ny way to hide a transaction

Theoretically you could make an Ethereum mixer but I don't think it would be productive. This stems from 2 major flaws with mixers.

1. Mixers aren't truly anonymous it obfuscates data but with enough computing power it can be decoded.
2. You're spending a lot of Ethereum on transactions when you don't need to. This is true with Bitcoin as well where mixers end up costing 2%-3%.

In my opion the best way to 'clean' ethereum or bitcoin if you want to is to use a site like ShapeShift and change your ethereum into Monero (a crypto specializing in anonymity). Move that Monero to a second wallet and then transfer it back out (possibly through a second website to avoid timing attacks). The costs of these transactions are minimal compared to the costs of mixing.

Eth #1 -> Shapeshift -> Monero #1 -> Monero #2 -> Shapeshift -> Eth #2

The transfer between Monereo #1 and #2 breaks the chain creating anonymity.

To elaborate on the timing attack bit. This would where you are 'cleaning' coin and it becomes obvious because the amount coming in is distinctly similar to an amount leaving. If you see someone bring in 37 ether and 5 minutes later the same amount goes back out to another address you might suspect what happened. To avoid this just stagger payments out over time and split it up between addresses for separate transfers.

Roughly, there are three aspects relative to privacy of transactions.

1. Hiding the sender of the transaction -> Ring Signatures. See Will be a ring signature scheme possible to implement at Ethereum? for a PoC in Ethereum.
2. Hiding the receiver of the transaction -> Stealth Addresses (like used in Monero). See https://monero.stackexchange.com/questions/1500/what-is-a-stealth-address. I am not aware of a PoC in Ethereum.
3. Hiding the content of the transaction -> Zero-Knowledge-Proofs. See What are zkSNARKs? How do they work?, zk-SNARKs vs. Zk-STARKs vs. BulletProofs? (Updated), and Is there a decentralized zk-SNARK (zero-knowledge Succinct Non-Interactive ARgument of Knowledge)?

Now, there is a point where I am not 100% sure. zcash has the concept of shielded addresses which is similar to ring signatures / stealth addresses from monero. From this blog post https://medium.com/coinmonks/monero-vs-zcash-and-the-race-to-anonymity-4322b0a9bd90 it reads like it is possible to implement shielded address with ZK-SNARKs (?):

Zcash enables optional features to enable privacy for transactions including the sender, receiver, and amount. Zcash addresses beginning with a “t” (t-addrs) are considered transparent, and are similar to bitcoin transactions. “Shielded” transactions are used with addresses beginning with a “z” (z-addrs), and these are considered to be fully anonymous. It’s important to note that as of December 2017 only 4% of Zcash transactions were being shielded.

With a z-addr, zk-SNARKs are used to hide the outputs to and from the z-addr. Therefore a transaction from a t-addr to a z-addr would show the ZEC leaving the t-addr but the output would be hidden to the z-addr.

UPDATE: I just stumbled upon this one. It is possible to achieve privacy in all three aspects (sender, receiver, value) with ZK-SNARKS only. See https://medium.com/@atvanguard/zkdai-private-dai-transactions-on-ethereum-using-zk-snarks-9e3ef4676e22 for an example using the ZoKrates library.