I think the graphical representation of certificates may be a research
project for a cryptography person.  Even if we start from a good hash,
are we sure that the graphical representation can't been manipulated so
that to different hashes get very similar representations?

Etc.

So I don't know about vizhash.

Anybody got an opinion?

Meanwhile, I think the svg.el library is good to go.  :-)

(setq svg (svg-create 256 256 :stroke "orange" :stroke-width 5))
(svg-gradient svg "gradient" 'linear '(0 . "red") '(100 . "blue"))
(svg-rectangle svg 100 100 150 150 :gradient "gradient")
(svg-circle svg 150 200 20)
(svg-ellipse svg 100 100 50 70 :stroke "red")
(svg-line svg 100 100 50 70)
(svg-polyline svg '((100 . 100) (200 . 150) (150 . 90)) :stroke "green")
(insert-image (svg-image svg))

Gives this pretty image: