Financial privacy is an essential criteria for the efficient operation of a free market: if you run a business, you cannot effectively set prices if your suppliers and customers can see all your transactions against your will. You cannot compete effectively if your competition is tracking your sales. Individually your informational leverage is lost in your private dealings if you don't have privacy over your accounts: if you pay your landlord in Bitcoin without enough privacy in place, your landlord will see when you've received a pay raise and can hit you up for more rent. Financial privacy is essential for personal safety: if thieves can see your spending, income, and holdings, they can use that information to target and exploit you. Without privacy malicious parties have more ability to steal your identity, snatch your large purchases off your doorstep, or impersonate businesses you transact with towards you... they can tell exactly how much to try to scam you for. Financial privacy is essential for human dignity: no one wants the snotty barista at the coffee shop or their nosy neighbors commenting on their income or spending habits. No one wants their baby-crazy in-laws asking why they're buying contraception (or sex toys). Your employer has no business knowing what church you donate to. Only in a perfectly enlightened discrimination free world where no one has undue authority over anyone else could we retain our dignity and make our lawful transactions freely without self-censorship if we don't have privacy.

Total anonymity, in the sense that when you spend money there is no trace of where it came from or where it's going, is theoretically possible by using the cryptographic technique of zero-knowledge proofs.

In order for Bitcoin's idea to work, you must have coins that can only be spent by the owner of a given private key. This means that whatever you send to must be tied, in some way, to a public key. Using arbitrary pseudonyms (e.g. user names) would mean that you'd have to then somehow link the pseudonym to a public key in order to enable public/private key crypto. This would remove the ability to securely create addresses/pseudonyms offline (e.g. before someone could send money to the user name "tdumidu", you'd have to announce in the blockchain that "tdumidu" is owned by public key "a1c...", and include a fee so others have a reason to announce it), reduce anonymity (by encouraging you to reuse pseudonyms), and needlessly bloat the size of the blockchain. It would also create a false sense of security that you're sending to who you think you are (if I take the name "Linus Torvalds" before he does, then it's mine and people might send money thinking they're paying the creator of Linux, not me).

As an additional firewall, a new key pair should be used for each transaction to keep them from being linked to a common owner. Some linking is still unavoidable with multi-input transactions, which necessarily reveal that their inputs were owned by the same owner. The risk is that if the owner of a key is revealed, linking could reveal other transactions that belonged to the same owner.

This actually works in practice, too... A nice whitehat hacker on IRC was playing around with brainwallet cracking and hit a phrase with ~250 BTC in it. We were able to identify the owner from just the address alone, because they'd been paid by a Bitcoin service that reused addresses and he was able to talk them into giving up the users contact information. He actually got the user on the phone, they were shocked and confused— but grateful to not be out their coin. A happy ending there. (This isn't the only example of it, by far ... but its one of the more fun ones).

If the adversary sees a transaction or block coming out of your node which did not previously enter, then it can know with near-certainty that the transaction was made by you or the block was mined by you. As internet connections are involved, the adversary will be able to link the IP address with the discovered bitcoin information.

Financial privacy is an essential element to fungibility in Bitcoin: if you can meaningfully distinguish one coin from another, then their fungibility is weak. If our fungibility is too weak in practice, then we cannot be decentralized: if someone important announces a list of stolen coins they won't accept coins derived from, you must carefully check coins you accept against that list and return the ones that fail. Everyone gets stuck checking blacklists issued by various authorities because in that world we'd all not like to get stuck with bad coins. This adds friction and transactional costs and makes Bitcoin less valuable as a money.

You need fungibility for bitcoin to function. If you receive coins and can’t spend them, then you start to doubt whether you can spend them. If there are doubts about coins you receive, then people are going to go to taint services and check whether "are these coins blessed" and then people are going to refuse to trade. What this does is it transitions bitcoin from a decentralized permissionless system into a centralized permissioned system where you have an "IOU" from the blacklist providers.

They were a bitcoin casino. Online gambling is not allowed in the US. Any customers of Coinbase that deposited straight to Bustabit would have their accounts shutdown because Coinbase was monitoring for this. Bustabit did a few things. They did something called change avoidance where you go through– and you see if you can construct a transaction that has no change output. This saves miner fees and also hinders analysis. Also, they imported their heavily-used reused deposit addresses into joinmarket. At this point, coinbase.com customers never got banned. It seems Coinbase’s surveillance service was unable to do the analysis after this, so it is possible to break these algorithms.

Thankfully there are some options out there to purchase Bitcoin via no KYC sources. These are all P2P (peer to peer) exchanges where you are trading directly with another individual and not a centralised third party. Unfortunately some sell other coins as well as bitcoin so we urge you to take care.