COSIC Seminar - Shape And Expand: Two New Generic Hash Constructions (John Kelsey)

Most published hash functions are inherently sequential, so hashing a very long message takes a lot of time. Most published hash functions also have a fixed-length output, which means that if you need more bits of output, you must come up with some nonstandard way to produce them. In this talk, I’ll describe two new generic hashing modes, SHAPE and EXPAND. Both are designed to work with existing sequential, fixed-output-length hash functions. SHAPE constructs a parallel hash function from the underlying sequential one. EXPAND constructs a variable-length hash function from a fixed-length hash function. I’ll discuss the design of these modes, and their (ongoing) security analysis.