Flipper Authenticator is a software-based authenticator that implements two-step verification services using the Time-based One-time Password (TOTP; specified in RFC 6238) and HMAC-based One-time Password algorithm. It is like Google Authenticator, but for Flipper Zero device.
If you don't want to build the app yourself then just pull FAP file from latest release and put it to your Flipper Zero device by copying it to /ext/apps/Misc. After that you should be able to run it on your Flipper.
Just pull the repo with recursive submodule initialization and then run fbt firmware_totp command to build the app. Once done FAP file will be available here flipperzero-firmware\build\latest\totp.fap
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