  Annotating blockchains with auxiliary data is useful for many applications.
For example, e-crime investigations of illegal Tor hidden services, such as
Silk Road, often involve linking Bitcoin addresses, from which money is sent or
received, to user accounts and related online activities. We present BlockTag,
an open-source tagging system for blockchains that facilitates such tasks. We
describe BlockTag's design and present three analyses that illustrate its
capabilities in the context of privacy research and law enforcement.
