function md5ff(a, b, c, d, x, s, t) {
        return md5cmn((b & c) | (~b & d), a, b, x, s, t);
      }
Basic operation the algorithm uses.

@param {number} a a
@param {number} b b
@param {number} c c
@param {number} d d
@param {number} x x
@param {number} s s
@param {number} t t
@returns {number} Result