Tracing bitcoins has long been easy in theory: The blockchain’s public record allows anyone to follow the trail of coins from one address to another as they’re spent or stolen, though not always to identify who controls those address. But that tracing becomes far dicier when Bitcoin users put their coins through a “mix” or “laundry” service—sometimes in the form of an unregulated exchange—that jumbles up many people’s coins at a single address, and then returns them to confuse anyone trying to trace their path. In other cases, users bundle together their transactions through a process called Coinjoin that gives each spender and recipient deniability about where their money came from or ended up.For companies like Chainanalyis, Coinfirm, and Ciphertrace that offer to trace stolen or “tainted” coins—and who generally don’t make their methodology public— that leaves limited options. They can either treat any coin that comes out of a mix that includes tainted coins as fully “dirty,” or more reasonably, average out the dirt among all the resulting coins; put one stolen coin into a mix address with nine legit ones, and they’re all 10 percent tainted. Some academics have called this the “haircut” method.But Anderson argues that haircut tracing quickly leads to enormous parts of the blockchain being a little bit tainted, with no clear answers about how to treat an infinitesimally unclean coin. Often the fraction can be so small it has to be rounded up, leading to artificial increases in the total “taint” recorded.But when Anderson mentioned this problem in January to David Fox, a professor of law at Edinburgh Law School, Fox pointed out that British law already provides a solution: An 1816 precedent known as Clayton’s Case, which dealt with who should be paid back from the remaining funds of a bankrupted financial firm. The answer, according to the presiding judge, was that whoever put their money in first should take it out first. The resulting first-in-first-out—or FIFO—rule became the standard way under British law to identify whose money is whose in mixed-up assets, whether to resolve debts or reclaim stolen property.