Which are popular two-factor authentication strategies used by online services like cloud storage or online banking?
Security tokens are small devices that typically generate random-looking numbers (one-time passwords or OTPs) every 30 seconds. Alternatively, software programs such as Google Authenticator do the same as mobile apps. How do they synchronize the OTPs with the server?
Hint: Refer to RFC 6238 available at the following link:
M’Raihi, D., Machani, S., Pei, M., & Rydell, J. (May, 2011). TOTP: Time-based one-time password algorithm (Links to an external site.). IETF RFC 6238