Something I've been is: Why can't one of your enemies just get a piece of your hair or fingernails or something, and make their own DNA ink our of your DNA? They could probably even send it to this DNA ink company, and they'd make your enemy ink out of your own DNA without even noticing. Doesn't seem very secure to me.
[Yeah, it's not crypto related, but I've been wondering about this since the topic was first brought up, and am somewhat surprised that of the few messages there were making fun of the DNA ink, none mentioned this fact. Is that because it's too obvious to mention, or is there something I'm not thinking about which makes DNA ink useful after all?]
This is the same problem that arose with the original idea of seals once the skills of metalworking became commen enough. By the 1500's it was nearly impossible to keep a seal confidential more than a few weeks until somebody got a impression and built a copy.