If you need to perform code signing as a part of your build we have several pieces of documentation that can help you get started.
Note: On CircleCI 2.0 the only supported way to do CodeSigning is to use Fastlane Match as described in the above documentation.
If you see errors like "Code Signing Error: No profile" or "Provisioning Profile Not Found" it's likely that you don’t have the keychain unlocked. You can run
security find-identity -v -p codesigning from inside the build to list the current status of your code signing credentials. Please follow the docs here to use Fastlane Match: https://circleci.com/docs/2.0/ios-codesigning/