The code signing process uses the public and private key associated with your development certificate as your digital identity. The development certificate in Developer Certificate Utility contains the public key, and the development certificate in your login keychain contains the private key. The development certificate in your login keychain begins with the text "Mac Developer:" followed by your name.
Private and public keys are requested to be generated while creating your Certificate Signing Request (CSR). Your public and private key pair establishes your iOS Developer identity. Your private key is stored in the login Keychain by default and can be viewed in the Keychain Access application under the "Keys" category.