We begin by supposing that we have a b-bit message as input, and that
we wish to find its message digest. Here b is an arbitrary
nonnegative integer; b may be zero, it need not be a multiple of
eight, and it may be arbitrarily large. We imagine the bits of the
message written down as follows:

m_0 m_1 ... m_{b-1}

The following five steps are performed to compute the message digest
of the message.