MacOS – I have several hundred expired certificates starting with com.apple.idms.appleid.prd. What are they and are they safe to delete

If I understand your question correctly, you're attempting to reset the permissions on a user's home directory. This is easily accomplished by resetting ACLs on the desired home directly by using the Reset Password Utility in the Recovery Partition:

1. Restart your computer from the recovery partition (if running Lion) or the gray disk (if not running Lion).

2. Open Disk Utility and run a permissions repair on your startup volume. After this is complete, close Disk Utility. We're doing this just for good measure.

3. Open Terminal from the Utilities menu. Type in resetpassword and select your user account (NOT System Administrator/root) from the drop down menu.

4. Click the Reset button at the bottom of the window in the Reset home folder permissions and ACLs section.

5. Quit the Password Utility and go back to the main recovery screen.

6. On your keyboard, hit ⌘ + Q and restart your computer. It's very important that you don't hold down the power button to exit the recovery session, or the ACL reset won't occur.

Good luck!

Take a look at OTATrustUtilities, which is Apple's implementation of extracting data from the system keychain. Specifically the InitializeAnchorTable function looks at extracting the data from those two files. The certsIndex.data file is a Database Index file that contains an array which can be read using NSData, this contains a list of sha1 hashes and offsets. These offsets correspond to the start of the certificate data found in the certsTable.data file, so you can start reading from the offset to the end of the certificate. It is worth noting that an application which accesses these private files will not be accepted onto the AppStore and this should only really be used for personal use or jailbreak development.