Transmission opportunities in delay- and disruption-tolerant networks (DTNs) may be scarce and short-lived. In consequence, the fragmentation of larger messages at intermediate nodes is an important requirement to efficiently utilize any available connectivity. At the same time, bandwidth must be protected against any unauthorized transmission attempt, which implies that source authentication mechanisms are needed. However, naive solutions for supporting both message fragmentation and authentication are inefficient in terms of bandwidth or computational requirements. The problem has been clearly identified in the literature and various solutions have been suggested, but a systematic treatment of the problem has not been carried out so far. In this work, we approach the problem of authenticated fragmentation by rephrasing it as a multicast authentication problem. We identify a number of computationally efficient multicast authentication protocols that are suitable for DTN scenarios and highlight known computational or bandwidth optimality results for two classes of solutions. We generalize the remaining protocols into a single third class and provide a theoretical analysis, which proves the bandwidth optimality of a protocol that has been independently suggested for the authenticated fragmentation problem. We extend the setting of the protocol by considering a network scenario where neighboring nodes can communicate reliably and show theoretically that in this scenario the amortized bandwidth overhead converges to the minimum possible. Finally, we review a number of approaches presented in the literature on the authenticated fragmentation problem and outline their inadequacies.