I am using a as an smart cart. I now want to add a new UID for this key and get an error saying "gpg: signing failed: Bad signature". How can I add a new uid onto the smartcard?

UIDs are not stored on the smartcard. Actually only subkeys are stored there. You need primary (master) key from external source to even use smartcard subkeys.

Maybe you don’t have your master key available to complete the UID adding procedure? (Master key with Certify ability can be put on a smartcard).

@wiktor Right. I have 3 subkeys on the smart card and the master key is backup somewhere save. So I need to add the uid to the master key?

Yes, that’s right. You need to add User ID where you have your private master key (offline machine?). Then export the public key and your new User ID will be there.

Internally User ID is a packet that is signed by the master key.

@wiktor alright. Thanks for the clarification 👍

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