aes <-- rijndael (rijndael was the original name of aes)withdrawn because only one variant of rijndael became AES
- random-number-generator <-- prng (rng is a synonym already)
- ssl <-- https (in all the questions https is just a specific example for general SSL concerns. securiy.SE has the synonym, too.)
Thanks for your suggestion.
As I see now, the two questions tagged rijndael are about specifics in the non-AES variants of Rijndael (or the differences of these to AES), and also tagged aes. I don't see much to gain from the tag synonym here. (Actually, there might be some more questions which could benefit from the rijndael tag.)
In the case of https, you are right - HTTPS is just a layer of HTTP over SSL/TLS, and the non-SSL specific parts would be off topic here. We could even totally delete the tag, but it might help someone who doesn't actually now that HTTPS is based on SSL/TLS. (An addition to the ssl tag wiki might be useful here.)
I will create synonyms for the latter two cases, and look if I can find some more questions for the rijndael tag in the next some days, if there are no arguments against this.