Answers to frequently asked questions about two-step verification for Apple ID.

Security feature that adds an extra layer of protection to a Dropbox account, by requiring a code from a cellphone or from any app that supports the Time-based One-Time Password (TOTP) protocol. Backup codes also available.

Instructions on how to require a special login code, sent to a mobile phone, each time a user try to access a Facebook account, from a new computer or mobile phone.

Sign in to a Google Account with both a password and a code that the system will send to a landline phone or cellphone, via SMS or call. Backup codes, and offline code generators for Android, iPhone, or BlackBerry also available.

Instructions on how to require an extra security code before authenticating to a Microsoft account.

Introduces "Security key" and "Mobile phone security key", to protect accounts from unauthorized access by requiring a code from a device that generates codes, or a mobile phone.

Feature that helps to keep an account more secure. Instead of relying on just a password, login verification introduces a second check to make sure that you and only you can access your Twitter account.