Hi Nicolas,
TL;DR: robust encryption/decryption in javascript is definitely possible but not straightforward, but maybe you don’t need it anyway
As Florian said, if your goal is simply to keep track of your participants and your recruiting platform already provides unique IDs, I strongly recommend you just use that instead.
If you still need to use an encryption method, most browsers now implement the SubtleCrypto API. There are some limitations though: some browsers like Edge do not support some algorithms, and most importantly, the API will only work on secure domains (the PCIbex Farm uses a secure domain).
If your goal is to store encrypted email addresses in your results file that only you can later decrypt, you will need an algorithm that takes a public encryption key and a private decryption key. There are a few of those, one of them is the ECDSA, illustrated here. You would also need to export your public key, as illustrated here.
Jeremy