  We show that superselection rules do not enhance the information-theoretic
security of quantum cryptographic protocols. Our analysis employs two quite
different methods. The first method uses the concept of a reference system --
in a world subject to a superselection rule, unrestricted operations can be
simulated by parties who share access to a reference system with suitable
properties. By this method, we prove that if an n-party protocol is secure in a
world subject to a superselection rule, then the security is maintained even if
the superselection rule is relaxed. However, the proof applies only to a
limited class of superselection rules, those in which the superselection
sectors are labeled by unitary irreducible representations of a compact
symmetry group. The second method uses the concept of the format of a message
sent between parties -- by verifying the format, the recipient of a message can
check whether the message could have been sent by a party who performed
charge-conserving operations. By this method, we prove that protocols subject
to general superselection rules (including those pertaining to nonabelian
anyons in two dimensions) are no more secure than protocols in the unrestricted
world. However, the proof applies only to two-party protocols. Our results show
in particular that, if no assumptions are made about the computational power of
the cheater, then secure quantum bit commitment and strong quantum coin
flipping with arbitrarily small bias are impossible in a world subject to
superselection rules.
