We thank an anonymous member of the program committee for the suggestion to use file checksums. We thank the program committee, especially David Rosenthal and Matt Blaze, for valuable advice about presentation and emphasis.
This work was supported in part by the New York State Science and
Technology Foundation's Center for Advanced Technology in Computers
and Information Systems; by a National Science Foundation
CISE Institutional Infrastructure grant, number CDA-90-24735; and by
the Center for Telecommunications Research, an NSF Engineering
Research Center supported by grant number ECD-88-11111.