This figure uses a series of labeled and shaded boxes to illustrate message authentication.

At the left, we see a light blue box labeled "m," representing the Message. Below this we see a freestanding "s," representing the Shared Secret. Blue arrows extend down from the message and up from the secret and point to the right to a white box labeled "H(.)." A blue arrow extends from this box to the right, to a dark blue box identified as "H(m+s)," then points upwards to a white circle containing a plus sign. A blue arrow also points from the original message to this circle. A blue arrow extends rightward from the white circle and to a cloud representing the Internet. Above this arrow is the light blue box labeled m with a dark blue box attached to it. The blue arrow points rightward from the Internet and splits; one end, containing the original message, points upwards and then rightwards at a white box labeled "H(.)." The freestanding s also appears here and a blue label extends from it to the white box as well. A blue arrow containing the dark blue box labeled "H(m+s)" points rightward from the white box and then downward to a light blue diamond shape, labeled "Compare." The other end of the blue arrow, containing the dark blue box, points downward and then to the right and then upwards to the light blue diamond.