Have there been any decentralized 2fa service built into the blockchain? it can be bitcoin or any altcoin.

  • Nope, there's none. – MCCCS Oct 4 '17 at 14:48
  • This makes no sense. You have to prove the validity of your transaction to the chain - the chain can't send you an SMS to verify. – Pieter Wuille Oct 4 '17 at 18:26