I have a Samsung Galaxy St duos with stock Android Marshmallow, and I'm considering encrypting my SD card to prevent access to its contents in case I lose my phone. The main reason of not doing this is that I'm not sure if there's a way to mount an encrypted SD card on Windows (or other desktop OS's)
When I encrypt the SD card on my Samsung Galaxy S5, is it using a known implementation of encryption, or is it something that only works on Android (or perhaps Samsung) phones?
I do not want to use USB connection to my phone to access my SD card, it's terrible. Mounting the SD card is much easier and faster and less annoying.
The encrypted SD card cannot be accessed in a PC unless you are connecting via a USB cable with the parent phone.
Android Marshmallow (6.0+) have a unique feature to format SD card as internal storage (I think this carries some encryption along with the formatting process). I have tried this, and after it is formatted to internal storage, the SD card is useless without mounting in the same phone in which it was formatted.(Edit: Unless you have root access.)
However SD card can be accessed from a PC, after formatting the SD card.
Marshmallow has two ways to treat the external SD card :
Portable Storage : In this the entire memory of the ext.SD card is available for use, if it is not encrypted (natively) and you can mount it on your PC to read/ transfer contents
Adoptable-storage: In this the ext. SD is treated as a part of the phone and when you encrypt the phone, the ext. SD card is also encrypted. Once it is encrypted, it can only be read through USB and cannot be read independently by using a card reader to mount on your PC. You can read more on this at adoptable-storage tag and it's wiki has questions to answer the common concerns. So, for all practical purposes encrypted ext. SD card cannot be read on PC independent the device. However, if your device is rooted, you can try the method suggested here How to decrypt Adopted Storage?
Full Disk Encryption details different encryption modes on Android Marshmallow. Whether OEMs implement it with additional security features is not known and surely won't be shared on public domain
Edit: Answering OPs query in comments and edit to question
Samsung implements proprietary encryption as evident from the question and also this Reasons for full disk encryption differences between Nexus & Samsung
Encrypted card cannot be read without the device as on the first question linked
So, it would be not possible to read the card on your PC, by mounting it via card reader on your PC as you propose to do. To clarify further, it can be read if you connect your device using USB
Edit: For a technical understanding, see the excellent answer here How does Marshmallow encryption work technically?