There have been some efforts out there to provide completely anonymous bitcoin transactions, namely ZeroCoin and bitcoin mixers like Silk Road used. After these

There have been some efforts out there to provide completely anonymous bitcoin transactions, namely ZeroCoin and bitcoin mixers like Silk Road used. After these services are used, t becomes impossible to track the coins origins. http://isi.jhu.edu/~mgreen/ZerocoinOakland.pdf http://isi.jhu.edu/~mgreen/ZerocoinOakland.pdf