From DESCR:
2fa is a two-factor authentication agent.
"2fa -add name" adds a new key to the 2fa keychain with the given name. It
prints a prompt to standard error and reads a two-factor key from standard
input. Two-factor keys are short case-insensitive strings of letters A-Z and
digits 2-7.
"2fa name" prints a two-factor authentication code from the key with the
given name.
With no arguments, "2fa" prints two-factor authentication codes from all
known time-based keys.
The default time-based authentication codes are derived from a hash of the
key and the current time, so it is important that the system clock have at
least one-minute accuracy.
The keychain is stored unencrypted in the text file "$HOME/.2fa".
