Short of you copying an encrypted file from the server onto a local trusted Linux distro (with no Intel ME on the machine), airgapping yourself, entering the decryption passphrase from a piece of paper (written by hand, never printed), with no cameras in the room, accessing what you need, and then securely wiping the machine without un-airgapping, you will most likely be tripping through several CIA-backdoored things.
Basically, the extreme level of digital OPSEC maintained by OBL is probably the bare minimum if your adversary is the state machinery of the United States or China.
SK should have no shortage of motive or much trouble (it's an extremely wealthy country with a very well-funded, sophisticated government apparatus) implementing its own version of hardcore data security for backups.
DES. Almost all pre-2014 standards-based cryptosystems due to NIST SP 800-90A. Probably all other popular ones too (like, if the NSA doesn't have backdoors to all the popular hardware random number generators then I don't know what they're even doing all day), but we only ever find out about that kind of thing 30 years down the line.
As for backdoors, they may exist if you rely on a third party but it's pretty hard to backdoor the relatively simple algorithms used in cryptography