Message digest functions distill the information contained in a file (small or
large) into
a single
large number, typically between 128 and 256 bits in length
.
The best message digest functions combine these
mathematical properties:
◦
Every bit of the message digest function is potentially influenced by every bit
of the
function’s input.
◦
If
any given bit of the function’s input is changed, every output bit has a 50
percent chance
of changing.
◦
Given
an input file and its corresponding message digest, it should be
computationally infeasible
to find
another file with the same message digest value
.
Message digests are also called
one
-
way hash functions
because they produce
values that
are
difficult to invert, resistant to attack, effectively unique, and widely distributed.