I was able to import a few different private/public keys to my ibook (running 10.3.2) and it works like a champ. However, when I imported them to my desktop (also running 10.3.2), it would not recognise them. Mail would not give me the icons for sign or encrypt. Bummer.
What is interesting is how I can get the desktop to recognize the keys. I can mail myself a signed email from my ibook. I receive it on my desk top. Mail tells me, "Unable to verify message signature. If I click details, I get an error, "Unable to verify message signature." Strange.
What comes next is stranger. I click "Show Details," and it tells me about the certificate. Now, using the desktop I can write emails that allow me to sign and encrypt. It works great. However, when I quit the program, it forgets.