> Can you, please, recommend some technologies or services that are now state-of-the-art solutions of this problem? Between U2F, bank-issued eIDs, smartcards,

> Can you, please, recommend some technologies or services that are now state-of-the-art solutions of this problem? Between U2F, bank-issued eIDs, smartcards, SIM applications, blockchain-based solutions (think Metamask), keybase, email links, anything around SAML, NTLM and Kerberos/RADIUS there's quite a lot of options possible. As for sign-on through Whatsapp, a lot of companies were doing it since 2012 or earlier, first checking if SIM is online (HLR -> IMEI) to confirm it was not replaced since last login, followed by trying to find active online session through 3rdparty gateways and using it. AFAIK most of these are gone or changed their products lineup by now, engageSPARK and messagebird seem to be alive though.