In Gibson’s Johnny Mnemonic, a man stored secrets in a chip in his head. He eventually teams up with a dolphin, modified by the Navy to detect magnetic traces. The Dolphin uses it’s SQUID (superconducting quantum interference devices) technology to read the secrets and blackmail the Yakuza. Johnny is later murdered in Neuromancer. Why wasn’t the information encrypted? Well, Gibson isn’t all that technical.
This idea of storing information in a human is from the Hitchcock movie The 39 Steps, where a mentalist is given a secret plan to memorize and doesn’t even know it.
A Japanese researcher has been able to encode a message in the DNA of bacteria. As much as 90% of DNA is garbage fill between the useful parts. This garbage is bits and pieces of old genes or random noise. It has to be there, though because, at least some of it, is required to get the significant code to work and there is kind of error correcting checksum that relies on parts of the garbage sections to turn out right. (At least this is my understanding.) These garbage sections of code can be rewritten to contain information encoded in base 4.
I am sure that this has been done, but I think an interesting MacGuffin would be to have an innocent person’s DNA encoded with some top secret information and pursued in a novel.