  Security trade-offs have been established for one-way bit commitment in
quant-ph/0106019. We study this trade-off in two superselection settings. We
show that for an `abelian' superselection rule (exemplified by particle
conservation) the standard trade-off between sealing and binding properties
still holds. For the non-abelian case (exemplified by angular momentum
conservation) the security trade-off can be more subtle, which we illustrate by
showing that if the bit-commitment is forced to be ancilla-free an
asymptotically secure quantum bit commitment is possible.
