  Spacetime-constrained oblivious transfer (SCOT) extends the fundamental
primitive of oblivious transfer to Minkowski space. SCOT and location oblivious
data transfer (LODT) are the only known cryptographic tasks with classical
inputs and outputs for which unconditional security needs both quantum theory
and relativity. We give an unconditionally secure SCOT protocol that,
contrasting previous SCOT and LODT protocols, is practical to implement with
current technology, where distant agents need only communicate classical
information, while quantum communication occurs at a single location. We also
show that our SCOT protocol can be used to implement unconditionally secure
quantum relativistic bit commitment.
