Alan (Miburi-san) Wexelblat <wex@media.mit.edu> said:

The problem with bit-rot is a more significant one. In this case you might want to compute your hash not over every bit of the image, but over the "significant" ones. That way if you lose low-order bits that no one cares about your signature is still valid.

If your signature does not include the low order bits of your image, then someone could embed a secret message in those low order bits (via Romana Machado's "Stego," for example) and your signature still would be valid. I wouldn't want my signature over someone else's steganized message. I'm sure there's a simple fix for that, such as ensuring enough bit rot to blow away any but the most error-tolerant steganography or including a disclaimer of responsibility for the low-order bits, but I couldn't resist pointing it out. Kevin Q. Brown INTERNET kqb@whscad1.att.com or kevin_q_brown@att.com