Sometimes, when you upload your photos to certain social networks, they change your photos' filenames from something like pchelka_mia.jpg to ze58dadc2.jpg.
I tried to delete a photo and software magically (but only once) decoded filename ze58dadc2.jpg as pchelka_mia.jpg.
My question is: In general, is it possible to decode these filenames to the original filenames? What algorithm/scheme encodes it? What software can decode it?
If the information is anywhere, it's in the EXIF data - there's quite a few tools that can read this, such as exiftool. Without knowing what exactly the social network is, it's hard to test to make sure, but that's the most likely explanation.
ze58dadc2 is an randomly generated string, they can use a database (they fill in themselves) to look up what filename it belongs to, which would return pchelka_mia. There is no magic algorithm going on here, just basic lookup, which is why you can't decode them.
They keep the extension intact because it's always four characters and they don't want to be storing (even though compressed) the extra four characters in the database. Furthermore, the reason they usually do this is to make URLs shorter and anonymize the picture for when you share it, such that people don't know the name of the person / album when they get their hands to it.
It's not stored in exif simply because there is no field for it (unless you misuse another field), and for the sake of simplicity they aren't going to manipulate the image when you upload it (apart from maybe stripping the exif data so they can't track your location).
This is more or less an educated guess, but I'm going to say that, in most cases, it isn't possible to decode the original. The new filename you see (in your example, ze58dadc2.jpg) is probably some form of arbitrary ID. Once you uploaded your file to said social network, it was added to a database, and the counter (probably in base64, hence the letters) was likely incremented by one unit. In other words, the end file name almost definitely had no correlation to the initial filename. This is in order to make sure that every file has a unique name, so if someone else uploaded a file called pchelka_mia.jpg, both files could coexist.
As Journeyman has said, the original name could have been in the metadata of the photo, but I can't say for certain without knowing what social network it is.
