How do I import a JKS where I do not have the private key

Question

The public cert is issued by the claims provider. It is for custom apex JWT verification. I get an error when uploading:

"Error: No certificate was imported because the keystore file doesn't contain a PrivateKey entry. Check the file and try again."

This works if I load the public key manually:

Blob publicKey = EncodingUtil.base64Decode('MIIBCg......');

//returns true    
System.debug(Crypto.verify('RSA-SHA256', Blob.valueOf(payload), bsignature, publicKey));

I want to be able to use this way instead:

Crypto.verify('RSA-SHA256', Blob.valueOf(payload), bsignature, '<cert name>'))

Folloing is the coomand to create the keystore:

keytool -importcert -file cert.cer -keystore cert.jks -alias stgcert -storetype jks