Hello,
In case the migration assistant fails to pick the private key associated with a certificate, the reason might be that this certificate expired. Is the certificate date still valid ?
If it's the case, you may still migrate that signing identity yourself. Open your Mac's Keychain app, and select the view that displays certificates and private keys associated altogether (normally it's the default view). Then you can create a .p12 file from the Keychain app, with the certificate and private key that you choose. The private key for your Xcode certificate, if it exists, will necessarily be there.
As for your provisioning profile, you'll perhaps need to locate it on your Mac's disk (use the search utility a.k.a magnifier tool) and copy it by hand, its extension should be .mobileprovision (for iOS) or .provisionprofile (the latter one being more used for macOS apps). I'd be interested in knowing in which location it was found, perhaps my script needs to be adjusted here.
Once you get these 3 (provisioning profile, certificate and private key) you have a valid signing identity. Actually the provisioning profile already embeds a copy of your certificate, so if you have just the provisioning profile and the private key, the builder will figure out the certificate from the profile.
But really, you are choosing a massively hindered path here with these so-called "free" Apple certificates. Each and every week, and each time you change anything in your signing identity or provisioning profile, you'll need to do this all over again. I advise you to go the mainstream road here and enroll in the iOS developer program to break free from these crippling limitations.
Best regards,
Tip: a more "brute-force" but totally efficient method to find a file on macOS with a particular extension is to open Terminal.app and type something like this:
%
find / 2>/dev/null | grep "\.mobileprovision$"
%
The above command will list *all* files on your Mac that have the .mobileprovision extension, wherever they may be. Replace "mobileprovision" with the file extension that you're looking for.
Extra note: the lone character in the middle of the command is not the letter L but a vertical bar a.k.a. "pipe" (|).
