How to pass a unique user ID to a page with user-specific, personal data - security

Question

I'm sending a mass email though Emma (3rd party vendor) that will contain a link to a landing page. The landing page will be personalized and display some of the user's identifying info (name, title, email). Additionally, there will be a form collecting a few of the user's preferences that will be saved back to that user's record in Emma's database.
The user ID column in the 3rd party's database is incremental so I obviously can't just append that value through the query string otherwise user 522, for example, would get a link such as www.example.com?landing/?uid=522 allowing him (or anyone with the link)cto take a wild guess at other values for uid (such as 523... or 444) and change other users' preferences as well as view their personal data quite easily.
Bottom line is that I'm trying to find a secure way to pass an ID (or other unique value) that I can look up via API and use to dynamically display and then resubmit personal info/data on this landing page on a user-to-user basis.
I had an idea to add a custom column to my list in Emma for a unique identifier. I would then write a script (accessing Emma's API) to BASE64 Encode the ID (or possibly email address, as that would be unique as well) and add that to the list for each user. In my email, I could then pass that to the landing page in for the form of ?xy=ZGF2ZUBidWRvbmsuY29t, but I know this is encoding and not encrypting so not all that secure... or secure at all for that matter.
To my knowledge, there's no remote risk of anyone receiving the mailing having the ability and/or inclination to know what those extra characters in the link are, BASE64 Decode, BASE64 ENCODE another email address or integer an make a request with the newly BASE64 encoded value in order to manipulate my system in an an unintended way.
BUT for the purpose of this question, I'd like to know the "right" way to do this or what levels of security are currently being taken in similar circumstances. I've read about JWT tokens and some OOth stuff, but I'm not quite sure that's possible given that I've got the Emma API to deal with as well... and/or if that is overkill.
What is appropriate/standard for passing values to a page that are in turn used for a form to be resubmitted along with other user-supplied values when giving the user the ability to submit a "compromised" (intentionally or not) form could, at worst, could cause one of their competitors to have bad preference and opt-in saved data in our Emma mailing list?

Answer 1

Security on the web is all about "acceptable risk". You can reduce risk in various ways, but ultimately there's always some risk exposure you must be willing to accept.
Your very best option would be to force users to be logged-in to view the page, and to avoid using any querystring parameters. That way the backend for the page can pull the ID (or whatever it might need) out of the server's session.
Your next best option still involves forcing the user to be logged in, but leave the uid in the URL -- just be sure to validate that the user has access to the uid (i.e. don't let a user access another user's info).
If you can't do that... then you could create random keys/ids that you store in a database, and use those values (rather than uid or email or real data) in the URL. BUT let's be clear: this isn't secure, as it's technically possible to guess/deduce the scheme.
Absolutely DO NOT try passing the info in the URL as base64 encoded data, that's likely to be the first thing a hacker will figure out.
Keep in mind that any unsecured API that returns PII of any kind will be abused by automated tools... not just a user farting around with your form.
To my knowledge, there's no remote risk of anyone receiving the
mailing having the ability and/or inclination to know
^ That's always always always a bad assumption. Even if the result is at worst something you think is trivial, it opens the door for escalation attacks and literally exposes the company to risks it likely doesn't want to accept.
If you're stuck between bad options, my professional advice is to have a meeting where you record the minutes (either video, or in a document) and have someone with "authority" approve the approach you take.

Answer 2

In case anyone needs a working example, I found this at https://bhoover.com/using-php-openssl_encrypt-openssl_decrypt-encrypt-decrypt-data/. It uses PHP's openssl_encrypt and openssl_decrypt, and it seems to work perfectly for my purposes
<?php
$key = base64_encode(openssl_random_pseudo_bytes(32));
function my_encrypt($data, $key) {
// Remove the base64 encoding from our key
$encryption_key = base64_decode($key);
// Generate an initialization vector
$iv = openssl_random_pseudo_bytes(openssl_cipher_iv_length('aes-256-cbc'));
// Encrypt the data using AES 256 encryption in CBC mode using our encryption key and initialization vector.
$encrypted = openssl_encrypt($data, 'aes-256-cbc', $encryption_key, 0, $iv);
// The $iv is just as important as the key for decrypting, so save it with our encrypted data using a unique separator (::)
return base64_encode($encrypted . '::' . $iv);
}
function my_decrypt($data, $key) {
// Remove the base64 encoding from our key
$encryption_key = base64_decode($key);
// To decrypt, split the encrypted data from our IV - our unique separator used was "::"
list($encrypted_data, $iv) = explode('::', base64_decode($data), 2);
return openssl_decrypt($encrypted_data, 'aes-256-cbc', $encryption_key, 0, $iv);
}
I first ran my_encrypt in a loop to encrypt the uid of each member in the list.
$members[$uid] = array('unique-identifier' => my_encrypt($uid, $key));
Next, through the API, I modified each member's record with the new value.
$ret = update_members_batch($members);
That only had to be done once.
Now in my email, I can pass the uid through the query string like this www.example.com/landing/?UID=<% unique-identifier %>, which will look something like www.example.com/landing/?UID= XXXXX2ovR2xrVmorbjlMMklYd0RNSDNPMUp0dmVLNVBaZmd3TDYyTjBFMjRkejVHRjVkSEhEQmlYaXVIcGxVczo6Dm3HmE3IxGRO1HkLijQTNg==
And in my page, I'll decrypt the query string value and use it via the API to get the email address with something like:
$member_email = get_member(my_decrypt($_GET['UID']))['email'];
and display it in the appropriate location(s) on my page.
I think this covers all my bases, but I am going to have a stakeholder meeting to get sign-off. What potential vulnerabilities does this expose that I should warn them about?