How to generate a certificate signature on the iPhone?

If I had a private certificate file and a string on the iPhone开发者_JAVA技巧, how do I use them to generate a signed string that can be verified by a server with the matching public key? What library should I use on the iPhone?

I would take a look at Certificate, Key, and Trust Services Reference on Apple's website. You can import the PKCS #12–formatted blob with SecPKCS12Import and sign the data with SecKeyRawSign.