This is a simple setup guide to enabling Two Factor Authentication (2FA) on Linux SSH login. I this article I wont go deep into setup and issues that I have faced when implementing this. First thing is first Update your system first. I have used Ubuntu 20.04 and it is always up to date. To enable 2FA you need to install google authenticator modules sudo apt install libpam-google-authenticator Configuration for PAM and SSHD Add the the following line to /etc/pam.d/sshd and After adding this line please restart the sshd services. auth required pam_google_authenticator.so Go to /etc/ssh/sshd_config and check if the following line exist. Default value will be "no" so change it to "yes" to activate. ChallengeResponseAuthentication yes Configuration for Authenticator In the terminal run google authenticator command It will ask few things to acknowledge by user. Details you can see from the below video. Once this part is done you are ready to use the 2FA in ubuntu. T
When you use microcontrollers and you are out of PIN's to connect some LED's for status or when you want to drive a binary clock with few pins left in microcontroller, there is a way that can solve this issue.
Its called charlieplexing and if you know multiplexing you will have some idea of how it works. I will do an example using Arduino for demonstration.
Charlieplexing is a method to drive multiple LED's with few pins available in your microcontroller.
Here is the formula to calculate how many LEDs can be drive from N number of pins.
Number of LED's = No. of PINS available ( No. of PINS available - 1)
For example you have only 3 pins left in Arduino and you want to know how many LED's can be drive.
Number of LED's = 3(3-1)
Number of LED's = 3(2)
Number of LED's = 6
So from 3 PINS you can drive 6 LED's.
Since arduino have tri-state output (INPUT, HIGH and LOW) its easy to drive the LED's using this method.
L = LOW (make out pin as LOW)
H = HIGH (make out pin as HIGH)
I = INPUT (make pin as INPUT) so this pin will be an open circuit
To drive LED 1
You have to make pin 2 and 3 as output pins. Pin 4 as an input.
So when you sent pin 2 = LOW and pin 3 = HIGH and leave pin 4 as an INPUT, the LED 1 will light up.
To drive LED 3
You have to make pin 3 and 4 as output pins. Pin 2 as an input.
So when you sent pin 3 = LOW and pin 4 = HIGH and leave pin 2 as an INPUT, the LED 3 will light up.
Here is an arduino test code that i have written for test charlieplexing.
#define L1 2
#define L2 3
#define L3 4
int delays = 400;
void setup() {
}
void loop() {
pinMode(L1,OUTPUT);
pinMode(L2,OUTPUT);
pinMode(L3,INPUT);
digitalWrite(L1,LOW);
digitalWrite(L2,HIGH);
delay(delays);
digitalWrite(L1,HIGH);
digitalWrite(L2,LOW);
delay(delays);
pinMode(L1,INPUT);
pinMode(L2,OUTPUT);
pinMode(L3,OUTPUT);
digitalWrite(L2,HIGH);
digitalWrite(L3,LOW);
delay(delays);
digitalWrite(L2,LOW);
digitalWrite(L3,HIGH);
delay(delays);
pinMode(L1,OUTPUT);
pinMode(L2,INPUT);
pinMode(L3,OUTPUT);
digitalWrite(L1,LOW);
digitalWrite(L3,HIGH);
delay(delays);
digitalWrite(L1,HIGH);
digitalWrite(L3,LOW);
delay(delays);
}
Its called charlieplexing and if you know multiplexing you will have some idea of how it works. I will do an example using Arduino for demonstration.
Charlieplexing is a method to drive multiple LED's with few pins available in your microcontroller.
Here is the formula to calculate how many LEDs can be drive from N number of pins.
Number of LED's = No. of PINS available ( No. of PINS available - 1)
For example you have only 3 pins left in Arduino and you want to know how many LED's can be drive.
Number of LED's = 3(3-1)
Number of LED's = 3(2)
Number of LED's = 6
So from 3 PINS you can drive 6 LED's.
Since arduino have tri-state output (INPUT, HIGH and LOW) its easy to drive the LED's using this method.
 |
| how leds are connected to 3 pins of an arduino |
 |
| Truth table of how its drive |
H = HIGH (make out pin as HIGH)
I = INPUT (make pin as INPUT) so this pin will be an open circuit
To drive LED 1
You have to make pin 2 and 3 as output pins. Pin 4 as an input.
So when you sent pin 2 = LOW and pin 3 = HIGH and leave pin 4 as an INPUT, the LED 1 will light up.
To drive LED 3
You have to make pin 3 and 4 as output pins. Pin 2 as an input.
So when you sent pin 3 = LOW and pin 4 = HIGH and leave pin 2 as an INPUT, the LED 3 will light up.
Here is an arduino test code that i have written for test charlieplexing.
#define L1 2
#define L2 3
#define L3 4
int delays = 400;
void setup() {
}
void loop() {
pinMode(L1,OUTPUT);
pinMode(L2,OUTPUT);
pinMode(L3,INPUT);
digitalWrite(L1,LOW);
digitalWrite(L2,HIGH);
delay(delays);
digitalWrite(L1,HIGH);
digitalWrite(L2,LOW);
delay(delays);
pinMode(L1,INPUT);
pinMode(L2,OUTPUT);
pinMode(L3,OUTPUT);
digitalWrite(L2,HIGH);
digitalWrite(L3,LOW);
delay(delays);
digitalWrite(L2,LOW);
digitalWrite(L3,HIGH);
delay(delays);
pinMode(L1,OUTPUT);
pinMode(L2,INPUT);
pinMode(L3,OUTPUT);
digitalWrite(L1,LOW);
digitalWrite(L3,HIGH);
delay(delays);
digitalWrite(L1,HIGH);
digitalWrite(L3,LOW);
delay(delays);
}
hope this will help you.
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