Refactor docs

docs/keyboard-interactive.md

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+# 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
+```
+