Got it. This has been some keychain misconfiguration issue. User A's digital identity was spread over different keychains: The certificate was located in the system keychain. The related private key was located in the login keychain. That's why keychain access did not show any digital identity under 'my certificates'. codesign however was able to use these scattered items. Additionally, other unrelated private keys were located in the system keychain. This led me export a digital ID with an invalid combination of key and certificate. This digital ID did not work for codesigning with user B. Exporting the digital ID of User A with a valid combination of private key and certificate and importing this for User B has solved the problem. By the way, the following article has been very enlightened: Certificate Signing Requests Explained: https://developer.apple.com/forums/thread/699268 Thanks!

Hi again, We have now imported the p12 file into user B's login keychain via security import IDENTITY_FILE.p12 -k login.keychain After this, still the same behaviour: no identities listed from security find-identity -p codesigning and codesign still throws "The specified item could not be found in the keychain". Reading your reply, I begin to wonder why user A stores it's identity within the system keychain instead of the login keychain. I can only state that codesigning works this way for user A. Following https://developer.apple.com/forums/thread/699268 , digital identities are listed in keyring within login / my certificates. This ist not the case for both users A and B, not in system and not in login. Mh ...