I use Google Authenticator on my SSH servers with Andoid app for generating codes.
After I run $ google-authenticator
application create set of emergyncy keys:
https://www.google.com/chart?chs=200x200&chld=M|0&cht=qr&chl=otpauth://totp/user@machine%3Fsecret%ABCDEFGHIJKLMNOP
Your new secret key is: ABCDEFGHIJKLMNOP
Your verification code is 123456
Your emergency scratch codes are:
12345678
90123456
78901234
56789012
34567890
What can I do if used all 5 codes? How can I generate new set? If I run $ google-authenticator
it will generate new key ABCDEFGHIJKLMNOP
and Android app stop working until I setup new key.