Exporting and re-importing ECC keys with file-based keychain

I'm trying to export and re-import a P-256 private key that was originally generated via SecKeyCreateRandomKey(), but I keep running into roadblocks. The key is simply exported via SecItemExport() with format formatWrappedPKCS8, and I did set a password just to be sure.

Do note that I must use the file-based keychain, as the data protection keychain requires a restricted entitlement and I'm not going to pay a yearly fee just to securely store some private keys for a personal project. The 7-day limit for unsigned/self-signed binaries isn't feasible either.

Here's pretty much everything I could think of trying:

1. Simply using SecItemImport() does import the key, but I cannot set kSecAttrLabel and more importantly: kSecAttrApplicationTag. There just isn't any way to pass these attributes upfront, so it's always imported as Imported Private Key with an empty comment. Keys don't support many attributes to begin with and I need something that's unique to my program but shared across all the relevant key entries, otherwise it's impossible to query for only my program's keys. kSecAttrLabel is already used for something else and is always unique, which really only leaves kSecAttrApplicationTag. I've already accepted that this can be changed via Keychain Access, as this attribute should end up as the entry's comment. At least, that's how it works with SecKeyCreateRandomKey() and SecItemCopyMatching(). I'm trying to get that same behaviour for imports.

2. Running SecItemUpdate() afterwards to set these 2 attributes doesn't work either, as now the kSecAttrApplicationTag is suddenly used for the entry's label instead of the comment. Even setting kSecAttrComment (just to be certain) doesn't change the comment. I think kSecAttrApplicationTag might be a creation-time attribute only, and since SecItemImport() already created a SecKey I will never be able to set this. It likely falls back to updating the label because it needs to target something that is still mutable?

3. Using SecItemImport() with a nil keychain (i.e. create a transient key), then persisting that with SecItemAdd() via kSecValueRef does allow me to set the 2 attributes, but now the ACL is lost. Or more precise: the ACL does seem to exist as any OS prompts do show the label I originally set for the ACL, but in Keychain Access it shows as Allow all applications to access this item. I'm looking to enable Confirm before allowing access and add my own program to the Always allow access by these applications list. Private keys outright being open to all programs is of course not acceptable, and I can indeed access them from other programs without any prompts.

4. Changing the ACL via SecKeychainItemSetAccess() after SecItemAdd() doesn't seem to do anything. It apparently succeeds but nothing changes. I also reopened Keychain Access to make sure it's not a UI "caching" issue.

5. Creating a transient key first, then getting the raw key via SecKeyCopyExternalRepresentation() and passing that to SecItemAdd() via kSecValueData results in The specified attribute does not exist. This error only disappears if I remove almost all of the attributes. I can pass only kSecValueData, kSecClass and kSecAttrApplicationTag, but then I get The specified item already exists in the keychain errors. I found a doc that explains what determines uniqueness, so here are the rest of the attributes I'm using for SecItemAdd():

   • kSecClass: not mentioned as part of the primary key but still required, otherwise you'll get One or more parameters passed to a function were not valid.
   • kSecAttrLabel: needed for my use case and not part of the primary key either, but as I said this results in The specified attribute does not exist.
   • kSecAttrApplicationLabel: The specified attribute does not exist. As I understand it this should be the SHA1 hash of the public key, passed as Data. Just omitting it would certainly be an option if the other attributes actually worked, but right now I'm passing it to try and construct a truly unique primary key.
   • kSecAttrApplicationTag: The specified item already exists in the keychain.
   • kSecAttrKeySizeInBits: The specified attribute does not exist.
   • kSecAttrEffectiveKeySize: The specified attribute does not exist.
   • kSecAttrKeyClass: The specified attribute does not exist.
   • kSecAttrKeyType: The specified attribute does not exist.

   It looks like only kSecAttrApplicationTag is accepted, but still ignored for the primary key. Even entering something that is guaranteed to be unique still results in The specified item already exists in the keychain, so I think might actually be targeting literally any key. I decided to create a completely new keychain and import it there (which does succeed), but the key is completely broken. There's no Kind and Usage at the top of Keychain Access and the table view just below it shows symmetric key instead of private. The kSecAttrApplicationTag I'm passing is still being used as the label instead of the comment and there's no ACL. I can't even delete this key because Keychain Access complains that A missing value was detected. It seems like the key doesn't really contain anything unique for its primary key, so it will always match any existing key.

6. Using SecKeyCreateWithData() and then using that key as the kSecValueRef for SecItemAdd() results in A required entitlement isn't present. I also have to add kSecUseDataProtectionKeychain: false to SecItemAdd() (even though that should already be the default) but then I get The specified item is no longer valid. It may have been deleted from the keychain. This occurs even if I decrypt the PKCS8 manually instead of via SecItemImport(), so it's at least not like it's detecting the transient key somehow. No combination of kSecAttrIsPermanent, kSecUseDataProtectionKeychain and kSecUseKeychain on either SecKeyCreateWithData() or SecItemAdd() changes anything.

7. I also tried PKCS12 despite that it always expects an "identity" (key + cert), while I only have (and need) a private key. Exporting as formatPKCS12 and importing it with itemTypeAggregate (or itemTypeUnknown) does import the key, and now it's only missing the kSecAttrApplicationTag as the original label is automatically included in the PKCS12. The outItems parameter contains an empty list though, which sort of makes sense because I'm not importing a full "identity". I can at least target the key by kSecAttrLabel for SecItemUpdate(), but any attempt to update the comment once again changes the label so it's not really any better than before.

8. SecPKCS12Import() doesn't even import anything at all, even though it does return errSecSuccess while also passing kSecImportExportKeychain explicitly.

Is there literally no way?