- https://cartilha.cert.br/fasciculos/verificacao-duas-etapas/fasciculo-verificacao-duas-etapas.pdfhttps://cartilha.cert.br/….
- https://twofactorauth.org/
- Google 2-step verification
- http://idgnow.uol.com.br/internet/2012/08/16/veja-como-aumentar-a-seguranca-de-sua-conta-no-google/http://idgnow.uol.com.br/….
- http://www.mattcutts.com/blog/google-two-step-authentication/http://www.mattcutts.com/….
- https://accounts.google.com/b/0/SmsAuthConfig
- http://spod.cx/blog/two-factor-ssh-auth-with-pam_oath-google-authenticator.shtmlhttp://spod.cx/….
- OTP
- http://www.openauthentication.org/
- TOTP / Time Based OTP
- http://lwn.net/Articles/470764/
- Yubico
- http://demo.yubico.com/php-yubico/Modhex_Calculator.phphttp://demo.yubico.com/…./Modhex_Calculator.php
- https://groups.google.com/forum/?fromgroups#%21forum/yubico-develhttps://groups.google.com/…./yubico-devel
- http://www.linuxjournal.com/magazine/yubikey-one-time-password-authenticationhttp://www.linuxjournal.com/….
- suporte para LastPass
- pam_yubico