1. A security document comprising: a substrate; a first graphic carried by the substrate, the first graphic conveying a photographic image to human viewers thereof, the first graphic being steganographically encoded to convey first plural bits of data recoverable by computer analysis of said first graphic; and a second graphic carried by the substrate, the second graphic conveying a visual image to human viewers thereof, wherein the second graphic is steganographically encoded to convey second plural bits of data recoverable by computer analysis of said second graphic; and wherein the steganographically encoded first plural bits of data and the steganographically encoded second plural bits of data cooperate to evidence authenticity of the security document.