This is a really cool and interesting question. I really like this.
So, I think we should start by breaking down what you’re trying to do in the most abstract, information theory-y way possible. Here’s my understanding:
N > 3 nodes in a group are communicating with one another.
They’re transmitting either a 0 or a 1, a yes or a no.
We’re going to take all inputs, then AND them. In other words, we don’t care about outcome unless everything is a 1; if it’s not all 1, it’s 0. (If you’re not super familiar with logic gates, this might be interesting.)
Each node’s transmission must be unknown by all other nodes.
The question then becomes how technical the solution should be. A more technical solution, with a single low-tech technique, makes this pretty simple:
All nodes attest to their identities in-person, and each node produces a public-private key-pair and gives their public keys to a centralized server.
They then transmit their votes to the centralized server, which decrypts them using their public keys.
The server performs an AND and returns the result.
If we wanted to try and make it lower tech, I think we would want to go with some weirder solutions. Here’s one that comes to mind:
Some dissolvable material is placed in a container, with a small of water. Each person takes turn pouring a chemical into it, blindfolded and holding their breath, with significant background noise.
They select either water for no or a colorless, odorless acid for yes. The acid is strong enough as to dissolve materials in a considerable amount of solvent, over time.
After pouring their votes, the setup then stands undisturbed for some amount of time. This amount is determined by finding the amount of time required to visibly dissolve the object, given a vote result of N - 1 negative votes.
After the determined amount of time passes, all members view the results, separately (in order to avoid expressing reactions), then converge to discuss the results.
Ultimately, regardless of how much computers play into the choice, the answer is about preserving confidentiality and integrity in a transmission environment that drops confidentiality if the transmissions pass an AND gate. The water-acid solution is one among many possibles, but in my opinion it still gets the job done.
Great question! This was really fun to think about. If I missed any constraints you already mentioned, stick it in a comment and I’ll revise.
A firewall is a security tool that blocks network traffic, with many different configuration options.For example, you could configure your firewall to block all traffic except attempts to connect to port 80 and 443 (for a web server) as well as your ssh port.This is an example of the best practice of “blocking everything except what is explicitly allowed”.
However, even with this sort of policy in place, the security gains for a basic firewall are limited.If you only have a web server and sshd server running, then there is nothing to block because connecting to a different port will fail anyway.Chiefly the benefit from this kind of simple firewall is to prevent inadvertent opening of ports.For example, let’s say you install a piece of software that has a dependency that starts up a service you hadn’t intended to run, or perhaps you change configurations an accidentally set MySQL to expose itself on your public IP. With a basic firewall, traffic to those port will still be blocked so you benefit from this kind of safety net.
Read more to continue…
However, the real power of firewalls becomes evident when we deploy an active firewall that intelligently monitors traffic and blocks traffic to/from specific clients. For example:
If you are running an IMAP service, you have no choice but to have it publicly-facing where it may become a magnet for people trying to brute-force (guess passwords).If you deploy an active firewall, the firewall software will watch the IMAP logs and after a configured number of failed logins, the client IP trying to connect will be temporarily blocked.
Likewise, you could have someone trying passwords on your ssh ports all day but they’ll be quickly stopped by an active firewall after a few failed logins.
Although an active firewall cannot stop a distributed denial of service, it can end some limited denial of service attacks that are coming from a small number of attacking hosts.
Active firewalls are critical when you have less sophisticated users on your system, such as if you’re running a web host or mail server. While you may conscientiously pick good passwords, use SSH keys, etc., your more naive users might not. An active firewall helps protect you by eliminating brute-forcing.
There are number of different products but one of the best known is ConfigServer Firewall.
To set it up, first install some required perl modules.On Debian-based systems:
cd /usr/srcwget https://download.configserver.com/csf.tgz tar xzf csf.tgzcd csfsh install.sh
Next make sure you have all the required kernel modules:
# perl /usr/local/csf/bin/csftest.pl Testing ip_tables/iptable_filter...OKTesting ipt_LOG...OKTesting ipt_multiport/xt_multiport...OKTesting ipt_REJECT...OKTesting ipt_state/xt_state...OKTesting ipt_limit/xt_limit...OKTesting ipt_recent...OKTesting xt_connlimit...OKTesting ipt_owner/xt_owner...OKTesting iptable_nat/ipt_REDIRECT...OKTesting iptable_nat/ipt_DNAT...OKRESULT: csf should function on this server
In this case, all required kernel modules were present. On your VPS you might see an error or missing module but as long as you see the result that “csf should function on this server,” you’re good to go.
CSF starts in “TESTING” mode.This is so you do not accidentally lock yourself out.
Take a look at /etc/csf/csf.conf.You probably want to adjust the following:
TCP_IN has a list of ports you allow.Remove any services you’re not using.For example, if you’re not running an FTP server, you can remove ports 20 and 21.If you’ve changed your SSH port, make sure it is in this list and remove port 22.
TCP_OUT should match TCP_IN
You can probably pare down UDP_IN and UDP_OUT, perhaps just to port 53 (DNS)
If you’re not using IPv6, set IPV6 to 0.Otherwise, adjust TCP6_IN, TCP6_OUT, UDP6_IN, and UDP6_OUT to match the ipv4 versions.
You will be mailed for each blocked IP.You can modify the templates in /etc/csf/alerts to set an appropriate To: address, or you can set LF_ALERT_TO to one master email address.
There are many, many more ways to customize CSF.You should take a look through the documentation on configserver.com to see the plethora of CSF capabilities.
By default, CSF is in TESTING mode.Before you start it for real, it’s handy to pre-set a time to turn it off.You could use a crontab entry like this:
Don’t forget to remove this crontab entry when you are ready to use CSF for real!
This will disable CSF entirely every 10 minutes on the 10s (so 1:00, 1:10, etc.)You can then fire up CSF with the knowledge that if you lock yourself out, you will have to wait a maximum of 10 minutes to get back in.Alternatively, you can login from your VPS provider’s console.
When you’re ready to go, modify /etc/csf/csf.conf and set TESTING to 0.Then:
systemctl restart csfsystemctl restart lfd
Here’s a test of CSF so you can see how it works. I’ve replaced the IP of the server with ‘s.s.s.s’ and the IP of the client with ‘c.c.c.c’.
CSF (specifically the lfd daemon) detected someone trying to brute-force a login by watching the system logs. In /var/log/lfd.log this entry appeared:
Apr7 16:31:23 debian10 lfd(6780): (sshd) Failed SSH login from c.c.c.c (US/United States/some.example.com): 5 in the last 3600 secs - *Blocked in csf* (LF_SSHD)
CSF then created a firewall rule to block the IP. It also made a notation in /etc/csf/csf.deny (so that if the system restarts, the firewall rule is recreated):
c.c.c.c # lfd: (sshd) Failed SSH login from c.c.c.c (US/United States/some.example.com): 5 in the last 3600 secs - Tue Apr7 16:31:23 2020
Everything after the # is a comment so you know why this IP was blocked. CSF also looks up the location of the IP address (this is not 100% perfect) and notes it.
Behind the scenes, CSF is manipulating the kernel’s firewall rules to create blocks. You can see the entire CSF ruleset at any time by issuing this command:
You may also see them in dmesg and /var/log/messages, depending on your syslog config.
If you configured an email address as mentioned above, you’ll get an email like this one:
Time: Tue Apr7 16:31:23 2020 -0700IP: s.s.s.s (US/United States/some.example.com)Failures: 5 (sshd)Interval: 3600 secondsBlocked:Permanent Block (LF_SSHD)Log entries:Apr7 16:31:07 debian10 sshd(6722): Invalid user nonexistant from c.c.c.c port 42858Apr7 16:31:08 debian10 sshd(6722): pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=c.c.c.cApr7 16:31:10 debian10 sshd(6722): Failed password for invalid user nonexistant from c.c.c.c port 42858 ssh2Apr7 16:31:13 debian10 sshd(6722): Failed password for invalid user nonexistant from c.c.c.c port 42858 ssh2Apr7 16:31:18 debian10 sshd(6722): Failed password for invalid user nonexistant from c.c.c.c port 42858 ssh2
After receiving this email I tried sshing from the same client:
$ ssh firstname.lastname@example.org
…and there was an infinite pause. Here I am blocked not just from SSH but at the IP level, so nothing from my client would be able to connect to this server (web, FTP, etc.)
To enable the product, issue these commands:
systemctl enable csfsystemctl enable lfd
Again, if you’ve put any kind of temporary disabling command in cron during testing, be sure to remove it before relying on the firewall.
An active firewall like CSF is a wonderful security tool but as always, security is the inverse of convenience. There are two headaches that are common: a flood of block notifications and accidental blocks.
Reducing Block Notifications
At first, you may be surprised at how many emails you get about blocked IPs. Welcome to the reality of the public Internet! Previously you were in blissful ignorance – now you can see how many attempts to attack your box are routine every day.
If you’re running a web host, then there are many ports that you are kind of stuck with because users expect FTP on port 20/21, email on 25/587, etc. However, you might consider changing your SSH port.
This advice is mildly controversial because changing your SSH port does not prevent someone from finding it and trying to login on that port.However, it will radically cut down how many automated attacks you get.Many attackers will mass-scan IPs and hone in on those that answer on port 22.If your system doesn’t respond on port 22, they will move on to another host.
Some sysadmins take the point of view that once they’ve secured the box by allowing logins only via ssh key and disabling root logins, there is little to fear from script kiddies wasting their time banging on the SSH port. While this is true, the numerous emails you’ll receive daily are tedious and may drown out alerts you truly do want to review.
You can change sshd to run on any port, but ideally an unused high port (above 1024).
For example, let’s say you wanted to use port 32222.To do this, modify /etc/ssh/sshd_config:
# Port 22Port 32222
In this case, we’ve commented out the default and added our port.
Next, be sure to update TCP_IN and TCP6_IN in /etc/csf/csf.conf to both remove port 22 and add your custom port.
Then restart sshd:
systemctl restart sshd
Your ssh command from your client will now look something like this if your account is ‘joe’:
ssh -p 32222 -i my_ssh_key email@example.com
Handling Mistaken Blocks
When CSF blocks a client IP, it adds a firewall rule and also writes it to/etc/csf/csf.deny. If a client is accidentally are blocked by CSF, you can immediately unblock them with this CSF command (here I’m using ‘c.c.c.c’ again for the client’s IP). This will both remove the kernel’s firewall rule and clear it from /etc/csf/csf.deny:
# csf --denyrm c.c.c.c
Removing rule...DROPall opt -- in !lo out *c.c.c.c-> 0.0.0.0/0 LOGDROPOUTall opt -- in * out !lo0.0.0.0/0-> c.c.c.c
You can also whitelist any IP by adding it to /etc/csf/csf.allow. Note that CSF whitelists the IP you’re connected from when you first set the product up. You’ll see an entry like this in /etc/csf/csf.allow:
Consider whitelisting your home IP and/or any VPN IPs you use to connect.
I’m Andrew, techno polymath and long-time LowEndTalk community Moderator. My technical interests include all things Unix, perl, python, shell scripting, and relational database systems. I enjoy writing technical articles here on LowEndBox to help people get more out of their VPSes.
On 2009 the brazilian governament created a new standard that’s different from all other countries in the world:
This image shows the brazillian standard at the bottom and the number of countries (países, in red) that use each other standard.
That said, it is still very common to find the american standard (second one) and mainly the “universal” plug (last one). Actually the most common is to find a plug using the universal without the ground port, so you’d need an adapter anyway.
Post 2009 buildings are required to use the new standard, while older buildings were not required to change existing plugs, so it doesn’t depend on where you go (São Paulo/Rio or the rest), but when the hotel was created/renovated.
That is such a hassle that every hotel must be prepared to have adapters for their guests, I wouldn’t worry if I my things use the american standard, but I guess I would buy one adapter to the american or universal or brazilian standard if using the other standards.
I have to help someone plug a European built electronic device into a 220 volt wall outlet in Taiwan. Right now they have a cord with a two round pin type-C plug.
We’re in northern Taiwan, Hsinchu and Taipei are both easily accessible. Is there any chance that there is a “weird plug store” in northern Taiwan, or some other way we can do this without cutting cables or using an ugly series of adapters? Something that is safe and safety people won’t frown upon? I’ve combed local 3C† stores and can’t find anything reasonable.
FYI, Taiwan plugs and codes generally look similar if not identical to those in the US.
†3C an abbreviation often used in Taiwan for “computer, communication, and consumer electronics”
I have to help someone connect an electronic device built in Europe to a 220 volt outlet in Taiwan. At the moment you have a cable with a 2-pin Type-C connector.
We are in northern Taiwan, Hsinchu and Taipei are both easily accessible. Is there a chance that there is a "weird plug-in" in northern Taiwan, or otherwise can we do this without cutting cables or using an ugly array of adapters? Something that is safe and secure will not disapprove of people? I combed local 3C stores† and cannot find anything reasonable.
fyi Taiwan plugs and codes generally look similar, if not identical, to those in the United States.
†3C is an acronym commonly used in Taiwan for "computers, communications, and consumer electronics".
I want to start a YouTube channel to explain electronics and circuitry, ideally with a quality like this video. I've watched over 50 videos on YouTube and tried to understand their settings but couldn't find the details I need.
I have a budget of around $ 600-700 (in the US) and the most important things I need are:
The wiring must be very clear
The part numbers shown on the chips are important
The lighting should be the same quality as in the video above and not be boring.
What kind of equipment would I need to achieve this and should I spend money on cameras, lenses, lighting equipment, or anything else?
I am a Russian citizen and am traveling in London from Italy to Bridgetown (Barbados).
I will arrive in London (LHR) Terminal 1 with Alitalia Airlines at 11:05 p.m. and take the Atlantic Atlantic from London (LHR) Terminal 3 to the final destination at 11:30 a.m. the next day.
Do I need a transit visa for this route?
For the transit visa I have:
You come and fly by plane.
You have a confirmed onward flight that starts on the day of your arrival or before midnight on the day after your arrival.
You have the right documents for your destination (e.g. a visa for this country).
You have a residence permit in a common format issued by a country in the European Economic Area (EEA) I DONT KNOW because I have a residence permit in Italy, but I am not sure whether it is an electronic residence permit (this is not acceptable for the passage through immigration control without a visa).
In Italian, my residence permit is called "Permesso di Soggiorno Elettronico". Please help me find the difference between the e-residence permit and others.
The reason for the phenomenon is that with large apertures in good light that require a fast shutter speed, most of the exposure comes from a very narrow movement gap between the first and second curtains.
However, the curtains before and after the slit are not the same height.
The electronic curtain moves where the photo pages are.
The mechanical curtain moves a considerable distance from the sensor. In particular, there is an anti-aliasing filter and possibly a protective glass on top of the sensor and only then the shutter curtain.
Since the curtains are at different heights from the sensor, the parallax effect becomes significant. The shorter the shutter speed, the smaller the travel gap and the problem gets worse.
To fix the problem, the shutter speed needs to be slowed down. First, the ISO should be lowered to the lowest ISO if the camera does not yet have the lowest ISO setting. Second, an ND filter could be used if the shutter speed is still too short, causing the effect.
However, the best solution is to use shutter curtains that are level with the photo pages. So if the second curtain is mechanical, the first should also be mechanical. Unfortunately, this is not possible with all cameras.
According to what I've read so far, the electronic shutter is a kind of device that controls whether the image sensor is not mechanically switched on or off.
What exactly does it control and how does it turn the sensor on or off (when charge accumulates, when the sensor is exposed to light from a CCD / CMOS light)?
The reset line is activated at the beginning for all pixel lines. Then the reset line is slowly released so that the first curtain matches the second curtain. The first curtain could be very fast, unless … it must match the slow second curtain!
The second curtain is implemented by simply reading the sensor. There is no way to turn off the charge accumulation without destroying the existing charge except (a) reading the sensor or (b) mechanically covering the sensor. The second curtain is slow, so the first curtain must also be artificially slowed down to maintain constant exposure across the frame.
If it is relatively fast, why is the flash synchronization slower than with a mechanical shutter?
This is a big "if". Usually it's not quick; it's slow. Slowness is due to the slow reading speed.
Canon with EOS R5 will apparently have "new approaches to sensor stabilization". I think the new approaches do not work by mechanically moving the sensor, but by electronically numerous images in a very short time interval. So the EOS R5 could have a faster electronic shutter. Or couldn't. We do not know yet.