how do I create an p12 and mobileprovision certificate

A .p12 typically holds a digital identity, that is, the combination of a certificate and the private key that matches the public key in that certificate. If you’re goal is to sign code, the certificate must be issued by the Apple Developer website. My Certificate Signing Requests Explained post explains the background to this.

Once you have both of these credentials, you can convert them to a .p12. If you’re using a Mac, the standard process is to import the certificate into the keychain that contains your private key and then use Keychain Access to export the digital identity as a .p12.

A .mobileprovision file is a provisioning profile. These can only be created using the Developer website (and tools, like Xcode, that act as a front end to that website). See the relevant sections of Developer Account Help.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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