Monday, October 22, 2018

How to set up Raspberry Pis without a keyboard, mouse, or monitor

There are plenty of guides out there about how to set up headless Raspberry Pis, but they get out of date quickly, and I do this often enough that I'm constantly searching for up to date ones.  So for my own benefit here's my documentation of the process.

Download Raspbian Lite.  This is the version without the GUI components.

Put your SD card in your computer and use lsblk to identify which drive your SD card is. Be careful, if you use the wrong drive below you will overwrite your main hard drive.

Use dd to copy the date over.  They constantly recommend you use the program Etcher, but I've never had it work successfully.  The command is sudo dd bs=4M if=2018-10-09-raspbian-stretch-lite.img of=/dev/sde conv=fsync status=progress

Your card should have 2 partitions, open the boot partition and add an empty file called ssh to enable ssh, and create a file called wpa_supllicant.conf to configure wifi.  The contents of the file are this:

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev


You can either put your actual password in there as the psk, or use the tool wpa_passphrase to convert your password into a hash that will also work.

Put the card in the Pi, boot it up, and it should connect to your network and you should be able to ssh in with username pi and password raspberry.  Note that you need to boot once for it to expand the filesystem.

You should put your public key in ~/.ssh/authorized_keys and turn off password ssh access.  You should also run sudo raspi-config once you ssh in, and update with sudo apt update && sudo apt upgrade

Saturday, October 6, 2018

Blockchain Technology Overview

NIST just published a good overview of blockchain technologies.  Very thorough, yet digestible for non-technical readers.
Blockchains are tamper evident and tamper resistant digital ledgers implemented in a distributed fashion (i.e., without a central repository) and usually without a central authority (i.e., a bank, company, or government). At their basic level, they enable a community of users to record transactions in a shared ledger within that community, such that under normal operation of the blockchain network no transaction can be changed once published. This document provides a high-level technical overview of blockchain technology. The purpose is to help readers understand how blockchain technology works.

GoogleMeetRoulette: Joining random meetings
Let’s see… I generated a meeting, got a Google Meet phone number, and all subsequent phone numbers are also from the same carrier. Let me call and see if I get the Google Meet greeting to confirm. Bingo! For countries like Australia and Spain, Google Meet phone numbers are assigned in batches that are sequential. I can generate a meeting myself and just check the subsequent phone numbers to obtain more Google Meet numbers. You can use them to join/find meetings in the US as the phone numbers from other countries are not specific to meetings in that country, they are global.

10 seconds per call, 3 PINs at a time, 10,000 PINs to try. It would take about 9 hours to cover all PIN combinations making one call at a time. Because Twilio is designed to make calls at scale, we can make hundreds of calls at the same time making the process much faster. The script fires so many calls that the line will be busy sometimes. Not a problem! The script will detect failed calls and simply retry. Actually, Twilio notifies of failed calls immediately using webhooks making the script very efficient handling calls that did not go through.  
I did some benchmarks and on average it takes 25 minutes to try all 10k PINs and find 15 different valid PINs for 15 different meetings for a cost of $16. Not bad!