Once an address has been doxxed, you've got a lead on any address used in a common transaction. It doesn't conclusively unmask these other addresses, so whether

Once an address has been doxxed, you've got a lead on any address used in a common transaction. It doesn't conclusively unmask these other addresses, so whether this counts as “really that difficult” depends on your investigative powers and your definition of “difficult”. Because of KYC[1] and AML[2], an address belonging to an exchange wallet should probably be considered doxxed. An authority can say “Hey Coinbase, I see you sent coin from address A on this date. Show me what account that came from.” CoinJoin[3] was intended to solve this problem, but apparently doesn't[6]. I was going to say that a decentralized cross-chain trading scheme[4][5] to launder through an anonymous currency such as Monero or ZCash might provide anonymity, but it looks like there's enough information to correlate the transactions across chains. (Even if there weren't, cross-chain trading might practically have one of the same issues as CoinJoin: that the transaction volume is just too low to provide anonymity.) There's other cryptocurrencies designed to provide anonymity[8]. The Lighting Network[10] might add this to Bitcoin[11]; I don't know if that's still one of their goals. EDIT: I somehow missed EthanHeilman's comment, above, when I wrote this. He says pretty much everything I said about why it's hard to keep Bitcoin activity private, more clearly. I included more links, though :-) [1] "Know Your Customer", https://en.wikipedia.org/wiki/Know_your_customer https://en.wikipedia.org/wiki/Know_your_customer [2] "Anti Money-Laundering", https://en.wikipedia.org/wiki/Money_laundering#Anti-money-laundering_measures_by_region https://en.wikipedia.org/wiki/Money_laundering#Anti-money-la... [3] "CoinJoin", https://en.wikipedia.org/wiki/CoinJoin https://en.wikipedia.org/wiki/CoinJoin [4] "Atomic cross-chain trading", https://en.bitcoin.it/wiki/Atomic_cross-chain_trading https://en.bitcoin.it/wiki/Atomic_cross-chain_trading [5] "First Ever Cross Chain Atomic Swap Between Bitcoin and Litecoin a Success", http://bitcoinist.com/first-ever-cross-chain-atomic-swap-between-bitcoin-and-litecoin-has-now-taken-place/ http://bitcoinist.com/first-ever-cross-chain-atomic-swap-bet... [6] S. Goldfeder et al, "When the cookie meets the blockchain: Privacy risks of web payments via cryptocurrencies" (2017) https://arxiv.org/pdf/1708.04748.pdf https://arxiv.org/pdf/1708.04748.pdf. [7] "Bitcoin Transactions Aren’t as Anonymous as Everyone Hoped ", Technology Review Aug, 2013, https://www.technologyreview.com/s/608716/bitcoin-transactions-arent-as-anonymous-as-everyone-hoped/ https://www.technologyreview.com/s/608716/bitcoin-transactio... summarizes [6]. [8] "Keybase chooses Zcash" https://keybase.io/blog/keybase-and-zcash https://keybase.io/blog/keybase-and-zcash has a fun example of address linkage. [9] Sudhir Khatwani, "9 Anonymous Cryptocurrencies You Should Know About", 2/02/18. https://coinsutra.com/anonymous-cryptocurrencies/ https://coinsutra.com/anonymous-cryptocurrencies/ [10] Lightning Network, https://lightning.network https://lightning.network. [11] Aaron van Wirdum, "How the Lightning Network Layers Privacy on Top of Bitcoin", Bitcoin Magazine, Dec 19 2016. https://bitcoinmagazine.com/articles/how-the-lightning-network-layers-privacy-on-top-of-bitcoin-1482183775/ https://bitcoinmagazine.com/articles/how-the-lightning-netwo...