Keyboard Interactive Authentication

Keyboard interactive authentication is, in general, a series of questions asked by the server with responses provided by the client. This authentication method is typically used for multi-factor authentication. There are no restrictions on the number of questions asked on a particular authentication stage; there are also no restrictions on the number of stages involving different sets of questions.

To enable keyboard interactive authentication, you must set the absolute path of your authentication program using the keyboard_interactive_auth_program key in your configuration file.

The external program can read the following environment variables to get info about the user trying to authenticate:

SFTPGO_AUTHD_USERNAME
SFTPGO_AUTHD_PASSWORD, this is the hashed password as stored inside the data provider

Previous global environment variables aren't cleared when the script is called. The content of these variables is not quoted. They may contain special characters.

The program must write the questions on its standard output, in a single line, using the following struct JSON serialized:

instruction, string. A short description to show to the user that is trying to authenticate. Can be empty or omitted
questions, list of questions to be asked to the user
echos list of boolean flags corresponding to the questions (so the lengths of both lists must be the same) and indicating whether user's reply for a particular question should be echoed on the screen while they are typing: true if it should be echoed, or false if it should be hidden.
check_password optional integer. Ask exactly one question and set this field to 1 if the expected answer is the user password and you want SFTPGo to check it for you. If the password is correct, the returned response to the program is OK. If the password is wrong, the program will be terminated and an authentication error will be returned to the user that is trying to authenticate.
auth_result, integer. Set this field to 1 to indicate successful authentication. 0 is ignored. Any other value means authentication error. If this field is found and it is different from 0 then SFTPGo will not read any other questions from the external program, and it will finalize the authentication.

SFTPGo writes the user answers to the program standard input, one per line, in the same order as the questions. Please be sure that your program receives the answers for all the issued questions before asking for the next ones.

Keyboard interactive authentication can be chained to the external authentication. The authentication must finish within 60 seconds.

Let's see a very basic example. Our sample keyboard interactive authentication program will ask for 2 sets of questions and accept the user if the answer to the last question is answer3.

#!/bin/sh

echo '{"questions":["Question1: ","Question2: "],"instruction":"This is a sample for keyboard interactive authentication","echos":[true,false]}'

read ANSWER1
read ANSWER2

echo '{"questions":["Question3: "],"instruction":"","echos":[true]}'

read ANSWER3

if test "$ANSWER3" = "answer3"; then
	echo '{"auth_result":1}'
else
	echo '{"auth_result":-1}'
fi

and here is an example where SFTPGo checks the user password for you:

#!/bin/sh

echo '{"questions":["Password: "],"instruction":"This is a sample for keyboard interactive authentication","echos":[false],"check_password":1}'

read ANSWER1

if test "$ANSWER1" != "OK"; then
  exit 1
fi

echo '{"questions":["One time token: "],"instruction":"","echos":[false]}'

read ANSWER2

if test "$ANSWER2" = "token"; then
	echo '{"auth_result":1}'
else
	echo '{"auth_result":-1}'
fi

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Keyboard Interactive Authentication

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