I have it on some authority that Daredevil used to use one. Bear in mind that this isn't the red or yellow and red Daredevil from the 60s recently portrayed in the movies by Ben Affleck, but an earlier character from a different company. His comic, ``Daredevil Comics'' ran from '41 to '56, although he didn't appear in it apparently except for two issues (#79 and 80) after issue #69, which probably appeared in the early '50s.
There's a not-bad picture of him
here. He's the guy in the red and blue suit. You can also see his weapon, the boomerang, and his arch-nemesis, the Claw (or maybe the Yellow Claw) hovering over the text in the bottom right.
I don't honestly know much about him, and I don't
really know that he used a secret decoder ring, but a knowledgeable friend claims that he did. (He also mentioned that there might be some Green Hornet comics out there where a decoder ring is used, but he's not really known as a comic book character.)
Here's a summary better than any I could manage.