We consider the problem of hiding sender and receiver of classical and
quantum bits (qubits), even if all physical transmissions can be monitored. We
present a quantum protocol for sending and receiving classical bits
anonymously, which is completely traceless: it successfully prevents later
reconstruction of the sender. We show that this is not possible classically. It
appears that entangled quantum states are uniquely suited for traceless
anonymous transmissions. We then extend this protocol to send and receive
qubits anonymously. In the process we introduce a new primitive called
anonymous entanglement, which may be useful in other contexts as well.Comment: 18 pages, LaTeX. Substantially updated version. To appear at
ASIACRYPT '0