Backup codes. I print mine on paper and keep it in a folder with about a dozen other important papers that I rarely access. Kinda archaic, but it is scalable if

Backup codes. I print mine on paper and keep it in a folder with about a dozen other important papers that I rarely access. Kinda archaic, but it is scalable if you consider the fallback mechanism for account access to be user domain. I haven't used a backup code for Google in a while but I think it causes a notification email to be sent to both the account in question and the email account setup for security notices.