This question has cropped up a few times in various guises, but I've not seen an answer that satisfies my requirement or fills me with much confidence. Let me set the scene.
We currently have a web application, which allows users to submit responses to pre-set questions where the data ends up in an SQL Server database, we also have a Windows application that does the same thing but works in an offline capacity; i.e. it connects to the SQL Server, downloads the questions, allows the user to complete them offline and when they next have a network connection they can synchronise the data, uploading it to the SQL Server. Great!
As part of our development strategy, given HTML 5's offline capabilities and local storage,it seems perfectly sensible to attempt to consolidate these products into a single web application. This would mean we're able to work on a single code base, and this would also enable the application to run in a browser on most devices; platform independent.
Looking into this I see a couple of potential problems, I'd really appreciate a steer on these:
Users need the ability to login, in offline and on-line modes. This could mean we download the hash's of the all users usernames and passwords, or just those that have logged in whilst in on-line mode. However, even doing this there needs to be a way to check these and given that the Javascript is readable someone could easily reverse engineer their credentials. Yes you can obfuscate the code but this isn't infallible.
The data that needs to be stored locally could be highly sensitive; contain personal information etc. Therefore this also needs encrypting, at minimum AES 256.
Am I hoping for utopia? Is this something that's just not possible at this time? Do I need to be looking at another solution and dismissing this for the time being?
Any help from you lovely people would be much appreciated.
First the easier question 2: that is perfectly possible. You can generate the key on the device and on sync send it via https to your server which can decrypt the data then.
As for question 1 I'd say an offline login is not really feasible BUT do you actually need one? Once the questionaire is downloaded (which requires online mode, so requiring login is fine) you only need to transmit it on sync, where online is again required and you can ask the user for his login there, too. I'd not recommend to download any sensible user data (e.g. hashes) to the device.
What you can do is to cache the current user only after logging in online. This would mitigate the risk of enumerating the users in your local DB.
You then need to encrypt the user's data on the front-end, I'd go with a library that does the job for you (for example, RxDB). RxDB accepts a password (which you can generate on the fly) and based on it, encrypts your DB data. The user then fills in the form (does whatever he wants) and if all of the sudden the internet is gone, the user is still able to continue his work and that work must be added as pending requests in order to do the sync. (which you already have)
When the internet is restored, you're going to check whether the session has expired for the user and if so, prompt the user to log in again and do the sync if it was the same user. If it's still there, perform the sync.
My advice based on my personal experience for the offline part.
You can create a local variable that allows the user to login once using internet for the first time then he will be able to auto login for several times as much as you decided in the local variable and when the value is 0 he will need an internet again to get another offline access for the same value you decided before.
so, in a small words. offline counter that will need an internet Only after many offline logins (when the counter decrease to 0)
Flowcharts
There are already numerous questions about jQuery cookie popups, so no need to go into any detail about that functionality. What I want to know is if this is the only way jQuery can handle popups only once per user or a time-based popup per user. I.e. the popup loads when the user first comes to the site and then doesn't load again for a week.
I know there is IP validation, but can jQuery do IP validation and if so, can you point me to a resource? I've searched here for jQuery popup IP validation and so far I haven't turned up anything.
I've said it countless times before and I'll say it again: IPs are a data delivery mechanism, not an identification or authentication tool. IPs are not guaranteed to be unique or stable in any way and are entirely unsuitable for identifying individual users at one machine.
You need to store an identifier on the user's machine somehow to remember for that particular user what he did or didn't do. A cookie is the most obvious answer, local storage, WebDBs, Flash cookies etc. are alternative fallbacks.
I must agree with deceze, considering a company for example could use a single IP while 10+ computers are connected to the web through this IP-address. So if an employee would visit your website the other 9 won't get to see your pop-up because someone with the same IP already visited your website.
But regardless of this being the right approach or not, to get back at you´re question, I came across this SO answer:
How to get client IP address using jQuery
It looks relative easy to get a vistors IP address, even though you should consider you're dependent on a third party this way.
This has more to do with an understanding of how web browsers work and less to do with JQuery. What you need is a way to identify a user. The easiest and most accurate way is a cookie. Something that only the user will have. Plus, you're trying to trying to do this on the client side. The only way for javascript to persist data across requests is via cookies, and for some browsers an Html 5 data persistance structure like IndexedDB.
If you can program on the server you can try to do IP lookups but you potentially get one identifier for multiple devices which can be multiple users. This is because it is common for multiple people to use the same IP (think about how many devices are connected to your home router). You can get more accurate by using the http header value for user-agent. These values are not accessible via javascript.
It may help you to get a better understanding of how http requests and responses.
This question already has answers here:
Unique IPs in a voting system
(7 answers)
Closed 9 years ago.
I'm building a simple web app with an up-vote option. I plan on offering cash rewards for the most up-voted so I want a relatively secure system. I have a couple questions about conception. I know that my post is similar to a few others but none seem to be specific enough to the platform to put my mind at ease.
My web app is utilizing javascript and firebase for loading all of the objects that are being voted on. I'm going to force a user to be logged in and store IP addresses, user IDs etc.
Questions:
Is this fundamentally flawed from the start for using javascript? I see a large potential for writing a script that just changes values and re-votes. (maybe I can verify the front end data is correct and that the user exists with an ajax call?)
With the off-beat chance my app becomes successful Is this going to be too much front end computing?
Edit:
I'm sorry, but I left out the key fact that I do have a larger back end system(WordPress) that handles authentication. The app I'm working on is largely independent from wordpress. I'm simply pulling some user information for filtering purposes. I chose Firebase as a storage solution for its real-time features.
I'm hoping to combat voter fraud with a few methods:
low rewards $100/month given away.
being logged in isn't a compromise, I actually want users to be registered and verified with human eyes to be eligible to vote. Others can witness the contest but cannot vote.
server-side checks. If my app gains popularity I can write scripts to monitor voting patterns for irregularities? if someone is abusing the system, I disable their ability to win.
It is certainly possible to do this securely client-side. However, as noted by others, it does require users to login. If you're already using Firebase, this is actually pretty easy to implement using the FirebaseAuthClient.
You can then couple this with Firebase security rules to enforce that any logged in user can only upvote once. Check out the screencast on the security page for an example!
Your security rules might look like:
{
"rules": {
"users:" {
"$userid": {
"voted_on": {
"$articleid": {
".write": "!data.exists()"
}
}
}
}
}
}
This ensures that you can write any given value to /users/anant/voted_on/article1 exactly once. You can add a .validate rule to ensure that the value is a boolean (or something else).
This is what you should probably do:
1) As soon as user votes, you should make an ajax call to the server and update the flag in the database and disable the voting using javascript.
2) If the user refreshes the page and tries to vote up again, the server would be knowing that the vote has already been made(as it is saved in database) so the voting system will appear disabled on the page.
3) If the user tries to enable the voting using chrome tools or firebug by modifying the source of page, you can create a check at database end by setting the composite key on userID and "vote" flag which would prevent the duplicate votes.
Hope it helps..
There is no way to prevent duplicate votes but forcing users to sign up (maybe with email confirmation) and authenticate (thus, storing votes somewhere in your DB and checking). Other techniques are flawed:
cookies/session storage can be disabled or cleared whenever the user wants.
IP address tracking prevents usage behind NATs, proxies and gateways, since all clients will share the same public address (or a small number of).
Still, a user could register multiple accounts and there is no way to prevent this.
A thing you can do is detecting rapid sequences of vote actions, which could be generated by scripts.
Taking advantage of OAuth authentication, it's highly unlikely that users cannott find a way to login via either FB/Twitter/Google/OpenID.
It is still cheap and fast to vote, and in case the user does not want to login via third party services, you can provide an email fallback.
As everybody pointed out already, you have to rely on you own server.
I need to figure out a way uniquely identify each computer which visits the web site I am creating. Does anybody have any advice on how to achieve this?
Because i want the solution to work on all machines and all browsers (within reason) I am trying to create a solution using javascript.
Cookies will not do.
I need the ability to basically create a guid which is unique to a computer and repeatable, assuming no hardware changes have happened to the computer. Directions i am thinking of are getting the MAC of the network card and other information of this nature which will id the machine visiting the web site.
Introduction
I don't know if there is or ever will be a way to uniquely identify machines using a browser alone. The main reasons are:
You will need to save data on the users computer. This data can be
deleted by the user any time. Unless you have a way to recreate this
data which is unique for each and every machine then your stuck.
Validation. You need to guard against spoofing, session hijacking, etc.
Even if there are ways to track a computer without using cookies there will always be a way to bypass it and software that will do this automatically. If you really need to track something based on a computer you will have to write a native application (Apple Store / Android Store / Windows Program / etc).
I might not be able to give you an answer to the question you asked but I can show you how to implement session tracking. With session tracking you try to track the browsing session instead of the computer visiting your site. By tracking the session, your database schema will look like this:
sesssion:
sessionID: string
// Global session data goes here
computers: [{
BrowserID: string
ComputerID: string
FingerprintID: string
userID: string
authToken: string
ipAddresses: ["203.525....", "203.525...", ...]
// Computer session data goes here
}, ...]
Advantages of session based tracking:
For logged in users, you can always generate the same session id from the users username / password / email.
You can still track guest users using sessionID.
Even if several people use the same computer (ie cybercafe) you can track them separately if they log in.
Disadvantages of session based tracking:
Sessions are browser based and not computer based. If a user uses 2 different browsers it will result in 2 different sessions. If this is a problem you can stop reading here.
Sessions expire if user is not logged in. If a user is not logged in, then they will use a guest session which will be invalidated if user deletes cookies and browser cache.
Implementation
There are many ways of implementing this. I don't think I can cover them all I'll just list my favorite which would make this an opinionated answer. Bear that in mind.
Basics
I will track the session by using what is known as a forever cookie. This is data which will automagically recreate itself even if the user deletes his cookies or updates his browser. It will not however survive the user deleting both their cookies and their browsing cache.
To implement this I will use the browsers caching mechanism (RFC), WebStorage API (MDN) and browser cookies (RFC, Google Analytics).
Legal
In order to utilize tracking ids you need to add them to both your privacy policy and your terms of use preferably under the sub-heading Tracking. We will use the following keys on both document.cookie and window.localStorage:
_ga: Google Analytics data
__utma: Google Analytics tracking cookie
sid: SessionID
Make sure you include links to your Privacy policy and terms of use on all pages that use tracking.
Where do I store my session data?
You can either store your session data in your website database or on the users computer. Since I normally work on smaller sites (let than 10 thousand continuous connections) that use 3rd party applications (Google Analytics / Clicky / etc) it's best for me to store data on clients computer. This has the following advantages:
No database lookup / overhead / load / latency / space / etc.
User can delete their data whenever they want without the need to write me annoying emails.
and disadvantages:
Data has to be encrypted / decrypted and signed / verified which creates cpu overhead on client (not so bad) and server (bah!).
Data is deleted when user deletes their cookies and cache. (this is what I want really)
Data is unavailable for analytics when users go off-line. (analytics for currently browsing users only)
UUIDS
BrowserID: Unique id generated from the browsers user agent string. Browser|BrowserVersion|OS|OSVersion|Processor|MozzilaMajorVersion|GeckoMajorVersion
ComputerID: Generated from users IP Address and HTTPS session key.
getISP(requestIP)|getHTTPSClientKey()
FingerPrintID: JavaScript based fingerprinting based on a modified fingerprint.js. FingerPrint.get()
SessionID: Random key generated when user 1st visits site. BrowserID|ComputerID|randombytes(256)
GoogleID: Generated from __utma cookie. getCookie(__utma).uniqueid
Mechanism
The other day I was watching the wendy williams show with my girlfriend and was completely horrified when the host advised her viewers to delete their browser history at least once a month. Deleting browser history normally has the following effects:
Deletes history of visited websites.
Deletes cookies and window.localStorage (aww man).
Most modern browsers make this option readily available but fear not friends. For there is a solution. The browser has a caching mechanism to store scripts / images and other things. Usually even if we delete our history, this browser cache still remains. All we need is a way to store our data here. There are 2 methods of doing this. The better one is to use a SVG image and store our data inside its tags. This way data can still be extracted even if JavaScript is disabled using flash. However since that is a bit complicated I will demonstrate the other approach which uses JSONP (Wikipedia)
example.com/assets/js/tracking.js (actually tracking.php)
var now = new Date();
var window.__sid = "SessionID"; // Server generated
setCookie("sid", window.__sid, now.setFullYear(now.getFullYear() + 1, now.getMonth(), now.getDate() - 1));
if( "localStorage" in window ) {
window.localStorage.setItem("sid", window.__sid);
}
Now we can get our session key any time:
window.__sid || window.localStorage.getItem("sid") || getCookie("sid") || ""
How do I make tracking.js stick in browser?
We can achieve this using Cache-Control, Last-Modified and ETag HTTP headers. We can use the SessionID as value for etag header:
setHeaders({
"ETag": SessionID,
"Last-Modified": new Date(0).toUTCString(),
"Cache-Control": "private, max-age=31536000, s-max-age=31536000, must-revalidate"
})
Last-Modified header tells the browser that this file is basically never modified. Cache-Control tells proxies and gateways not to cache the document but tells the browser to cache it for 1 year.
The next time the browser requests the document, it will send If-Modified-Since and If-None-Match headers. We can use these to return a 304 Not Modified response.
example.com/assets/js/tracking.php
$sid = getHeader("If-None-Match") ?: getHeader("if-none-match") ?: getHeader("IF-NONE-MATCH") ?: "";
$ifModifiedSince = hasHeader("If-Modified-Since") ?: hasHeader("if-modified-since") ?: hasHeader("IF-MODIFIED-SINCE");
if( validateSession($sid) ) {
if( sessionExists($sid) ) {
continueSession($sid);
send304();
} else {
startSession($sid);
send304();
}
} else if( $ifModifiedSince ) {
send304();
} else {
startSession();
send200();
}
Now every time the browser requests tracking.js our server will respond with a 304 Not Modified result and force an execute of the local copy of tracking.js.
I still don't understand. Explain it to me
Lets suppose the user clears their browsing history and refreshes the page. The only thing left on the users computer is a copy of tracking.js in browser cache. When the browser requests tracking.js it recieves a 304 Not Modified response which causes it to execute the 1st version of tracking.js it recieved. tracking.js executes and restores the SessionID that was deleted.
Validation
Suppose Haxor X steals our customers cookies while they are still logged in. How do we protect them? Cryptography and Browser fingerprinting to the rescue. Remember our original definition for SessionID was:
BrowserID|ComputerID|randomBytes(256)
We can change this to:
Timestamp|BrowserID|ComputerID|encrypt(randomBytes(256), hk)|sign(Timestamp|BrowserID|ComputerID|randomBytes(256), hk)
Where hk = sign(Timestamp|BrowserID|ComputerID, serverKey).
Now we can validate our SessionID using the following algorithm:
if( getTimestamp($sid) is older than 1 year ) return false;
if( getBrowserID($sid) !== createBrowserID($_Request, $_Server) ) return false;
if( getComputerID($sid) !== createComputerID($_Request, $_Server) return false;
$hk = sign(getTimestamp($sid) + getBrowserID($sid) + getComputerID($sid), $SERVER["key"]);
if( !verify(getTimestamp($sid) + getBrowserID($sid) + getComputerID($sid) + decrypt(getRandomBytes($sid), hk), getSignature($sid), $hk) ) return false;
return true;
Now in order for Haxor's attack to work they must:
Have same ComputerID. That means they have to have the same ISP provider as victim (Tricky). This will give our victim the opportunity to take legal action in their own country. Haxor must also obtain HTTPS session key from victim (Hard).
Have same BrowserID. Anyone can spoof User-Agent string (Annoying).
Be able to create their own fake SessionID (Very Hard). Volume atacks won't work because we use a time-stamp to generate encryption / signing key so basically its like generating a new key for each session. On top of that we encrypt random bytes so a simple dictionary attack is also out of the question.
We can improve validation by forwarding GoogleID and FingerprintID (via ajax or hidden fields) and matching against those.
if( GoogleID != getStoredGoodleID($sid) ) return false;
if( byte_difference(FingerPrintID, getStoredFingerprint($sid) > 10%) return false;
These people have developed a fingerprinting method for recognising a user with a high level of accuracy:
https://panopticlick.eff.org/static/browser-uniqueness.pdf
We investigate the degree to which modern web browsers
are subject to “device fingerprinting” via the version and configuration information that they will transmit to websites upon request. We
implemented one possible fingerprinting algorithm, and collected these
fingerprints from a large sample of browsers that visited our test side,
panopticlick.eff.org. We observe that the distribution of our finger-
print contains at least 18.1 bits of entropy, meaning that if we pick a
browser at random, at best we expect that only one in 286,777 other
browsers will share its fingerprint. Among browsers that support Flash
or Java, the situation is worse, with the average browser carrying at least
18.8 bits of identifying information. 94.2% of browsers with Flash or Java
were unique in our sample.
By observing returning visitors, we estimate how rapidly browser fingerprints might change over time. In our sample, fingerprints changed quite
rapidly, but even a simple heuristic was usually able to guess when a fingerprint was an “upgraded” version of a previously observed browser’s
fingerprint, with 99.1% of guesses correct and a false positive rate of only
0.86%.
We discuss what privacy threat browser fingerprinting poses in practice,
and what countermeasures may be appropriate to prevent it. There is a
tradeoff between protection against fingerprintability and certain kinds of
debuggability, which in current browsers is weighted heavily against privacy. Paradoxically, anti-fingerprinting privacy technologies can be self-
defeating if they are not used by a sufficient number of people; we show
that some privacy measures currently fall victim to this paradox, but
others do not.
It's not possible to identify the computers accessing a web site without the cooperation of their owners. If they let you, however, you can store a cookie to identify the machine when it visits your site again. The key is, the visitor is in control; they can remove the cookie and appear as a new visitor any time they wish.
A possibility is using flash cookies:
Ubiquitous availability (95 percent of visitors will probably have flash)
You can store more data per cookie (up to 100 KB)
Shared across browsers, so more likely to uniquely identify a machine
Clearing the browser cookies does not remove the flash cookies.
You'll need to build a small (hidden) flash movie to read and write them.
Whatever route you pick, make sure your users opt IN to being tracked, otherwise you're invading their privacy and become one of the bad guys.
There is a popular method called canvas fingerprinting, described in this scientific article: The Web Never Forgets:
Persistent Tracking Mechanisms in the Wild. Once you start looking for it, you'll be surprised how frequently it is used. The method creates a unique fingerprint, which is consistent for each browser/hardware combination.
The article also reviews other persistent tracking methods, like evercookies, respawning http and Flash cookies, and cookie syncing.
More info about canvas fingerprinting here:
Pixel Perfect: Fingerprinting Canvas in HTML5
https://en.wikipedia.org/wiki/Canvas_fingerprinting
You may want to try setting a unique ID in an evercookie (it will work cross browser, see their FAQs):
http://samy.pl/evercookie/
There is also a company called ThreatMetrix that is used by a lot of big companies to solve this problem:
http://threatmetrix.com/our-solutions/solutions-by-product/trustdefender-id/
They are quite expensive and some of their other products aren't very good, but their device id works well.
Finally, there is this open source jquery implementation of the panopticlick idea:
https://github.com/carlo/jquery-browser-fingerprint
It looks pretty half baked right now but could be expanded upon.
Hope it helps!
There is only a small amount of information that you can get via an HTTP connection.
IP - But as others have said, this is not fixed for many, if not most Internet users due to their ISP's dynamic allocation policies.
Useragent String - Nearly all browsers send what kind of browser they are with every request. However, this can be set by the user in many browsers today.
Collection of request fields - There are other fields sent with each request, such as supported encodings, etc. These, if used in the aggregate can help to ID a user's machine, but again are browser dependent and can be changed.
Cookies - Setting a cookie is another way to identify a machine, or more specifically a browser on a machine, but as others have said, these can be deleted, or turned off by the users, and are only applicable on a browser, not a machine.
So, the correct response is that you cannot achieve what you would live via the HTTP over IP protocols alone. However, using a combination of cookies, as well as IP, and the fields in the HTTP request, you have a good chance at guessing, sort of, what machine it is. Users tend to use only one browser, and often from one machine, so this may be fairly relieable, but this will vary depending on the audience...techies are more likely to mess with this stuff, and use more machines/browsers. Additionally, this could even be coupled with some attempt to geo-locate the IP, and use that data as well. But in any case, there is no solution that will be correct all of the time.
There are flaws with both cookie and non-cookie approaches. But if you can forgive the shortcomings of the cookie approach, here's an idea.
If you're already using Google Analytics on your site, then you don't need to write code to track unique users yourself. Google Analytics does that for you via the __utma cookie value, as described in Google's documentation. And by reusing this value you're not creating additional cookie payload, which has efficiency benefits with page requests.
And you could write some code easily enough to access that value, or use this script's getUniqueId() function.
As with the previous solutions cookies are a good method, be aware that they identify browsers though. If I visited a website in Firefox and then in Internet Explorer cookies would be stored for both attempts seperately. Some users also disable cookies (but more people disable JavaScript).
Another method to consider would be I.P. and hostname identification (be aware these can vary for dial-up/non-static IP users, AOL also uses blanket IPs). However since this only identifies networks this might not work as well as cookies.
The suggestions to use cookies aside, the only comprehensive set of identifying attributes available to interrogate are contained in the HTTP request header. So it is possible to use some subset of these to create a pseudo-unique identifier for a user agent (i.e., browser). Further, most of this information is possibly already being logged in the so-called "access log" of your web server software by default and, if not, can be easily configured to do so. Then, a utlity could be developed that simply scans the content of this log, creating fingerprints of each request comprised of, say, the IP address and User Agent string, etc. The more data available, even including the contents of specific cookies, adds to the quality of the uniqueness of this fingerprint. Though, as many others have stated already, the HTTP protocol doesn't make this 100% foolproof - at best it can only be a fairly good indicator.
When i use a machine which has never visited my online banking web site i get asked for additional authentification. then, if i go back a second time to the online banking site i dont get asked the additional authentification...i deleted all cookies in IE and relogged onto my online banking site fully expecting to be asked the authentification questions again. to my surprise i was not asked. doesnt this lead one to believe the bank is doing some kind of pc tagging which doesnt involve cookies?
This is a pretty common type of authentication used by banks.
Say you're accessing your bank website via example-isp.com. The first time you're there, you'll be asked for your password, as well as additional authentication. Once you've passed, the bank knows that user "thatisvaliant" is authenticated to access the site via example-isp.com.
In the future, it won't ask for extra authentication (beyond your password) when you're accessing the site via example-isp.com. If you try to access the bank via another-isp.com, the bank will go through the same routine again.
So to summarize, what the bank's identifying is your ISP and/or netblock, based on your IP address. Obviously not every user at your ISP is you, which is why the bank still asks you for your password.
Have you ever had a credit card company call to verify that things are OK when you use a credit card in a different country? Same concept.
Really, what you want to do cannot be done because the protocols do not allow for this. If static IPs were universally used then you might be able to do it. They are not, so you cannot.
If you really want to identify people, have them log in.
Since they will probably be moving around to different pages on your web site, you need a way to keep track of them as they move about.
So long as they are logged in, and you are tracking their session within your site via cookies/link-parameters/beacons/whatever, you can be pretty sure that they are using the same computer during that time.
Ultimately, it is incorrect to say this tells you which computer they are using if your users are not using your own local network and do not have static IP addresses.
If what you want to do is being done with the cooperation of the users and there is only one user per cookie and they use a single web browser, just use a cookie.
You can use fingerprintjs2
new Fingerprint2().get(function(result, components) {
console.log(result) // a hash, representing your device fingerprint
console.log(components) // an array of FP components
//submit hash and JSON object to the server
})
After that you can check all your users against existing and check JSON similarity, so even if their fingerprint mutates, you still can track them
Because i want the solution to work on all machines and all browsers (within reason) I am trying to create a solution using javascript.
Isn't that a really good reason not to use javascript?
As others have said - cookies are probably your best option - just be aware of the limitations.
I guess the verdict is i cannot programmatically uniquely identify a computer which is visiting my web site.
I have the following question. When i use a machine which has never visited my online banking web site i get asked for additional authentification. then, if i go back a second time to the online banking site i dont get asked the additional authentification. reading the answers to my question i decided it must be a cookie involved. therefore, i deleted all cookies in IE and relogged onto my online banking site fully expecting to be asked the authentification questions again. to my surprise i was not asked. doesnt this lead one to believe the bank is doing some kind of pc tagging which doesnt involve cookies?
further, after much googling today i found the following company who claims to sell a solution which does uniquely identify machines which visit a web site. http://www.the41.com/products.asp.
i appreciate all the good information if you could clarify further this conflicting information i found i would greatly appreciate it.
I would do this using a combination of cookies and flash cookies. Create a GUID and store it in a cookie. If the cookie doesn't exist, try to read it from the flash cookie. If it's still not found, create it and write it to the flash cookie. This way you can share the same GUID across browsers.
I think cookies might be what you are looking for; this is how most websites uniquely identify visitors.
Cookies won't be useful for determining unique visitors. A user could clear cookies and refresh the site - he then is classed as a new user again.
I think that the best way to go about doing this is to implement a server side solution (as you will need somewhere to store your data). Depending on the complexity of your needs for such data, you will need to determine what is classed as a unique visit. A sensible method would be to allow an IP address to return the following day and be given a unique visit. Several visits from one IP address in one day shouldn't be counted as uniques.
Using PHP, for example, it is trivial to get the IP address of a visitor, and store it in a text file (or a sql database).
A server side solution will work on all machines, because you are going to track the user when he first loads up your site. Don't use javascript, as that is meant for client side scripting, plus the user may have disabled it in any case.
Hope that helps.
I will give my ideas starting from simpler to more complex.
In all the above you can create sessions and the problem essentialy translates to match session with request.
a) (difficulty: easy) use client hardware to store explicitely a session id/hash of some sort (there are quite some privace/security issues so make sure you hash anything you store ), solutions include:
cookies storage
browser storage/webDB/ (more exotic browser solutions )
extensions with permission to store things in files.
The above suffer from the fact the the user can just empty his cache in case he doesn want.
b) (difficulty: medium) Login based authentication.
Most modern web frameworks provide such solution the core idea is you let the user voluntarily identify himself, quite straghtforward but adds complexity in the architecture.
The above suffer from additional complexity and making essentially non public content.
c)(difficulty: hard -R&D) Identification based on metadata, (browser ip/language /browser / and other privace invasice stuff so make sure you let your users know or you miay get sued )
non perfect solution can get more complicated (a user typing with specific frequency or using mouse with specific patterns ? you even apply ML solutions ).
The claimed solutions
The most powerful since the user even without wanting explicitely he can be identified. It is straight invasion of privacy(see GDPR) and not perfect eg. ip can change .
Assuming you don't want the user to be in control, you can't. The web doesn't work like that, the best you can hope for is some heuristics.
If it is an option to force your visitor to install some software and use TCPA you may be able to pull something off.
My post might not be a solution, but I can provide an example, where this feature has been implemented.
If you visit the signup page of www.supertorrents.org for the first time from you computer, it's fine. But if you refresh the page or open the page again, it identifies you've previously visited the page. The real beauty comes here - it identifies even if you re-install Windows or other OS.
I read somewhere that they store the CPU ID. Although I couldn't find how do they do it, I seriously doubt it, and they might use MAC Address to do it.
I'll definitely share if I find how to do it.
A Trick:
Create 2 Registration Pages:
First Registration Page: without any email or security check (just with username and password)
Second Registration Page: with high security level (email verification request and security image and etc.)
For customer satisfaction, and easy registration, default
registration page should be the (First Registration Page) but in the
(First Registration Page) there is a hidden restriction. It's IP
Restriction. If an IP tried to register for second time, (for example less than 1 hour) instead of
showing the block page. you can show the (Second Registration Page)
automatically.
in the (First Registration Page) you can set (for example: block 2
attempts from 1 ip for just 1 hour or 24 hours) and after (for example) 1 hour, you can open access from that ip automatically
Please note: (First Registration Page) and (Second Registration Page) should not be in separated pages. you make just 1 page. (for example: register.php) and make it smart to switch between First PHP Style and Second PHP Style