I have a few TiB of data on machines that I control. For georedundant backup purposes I want to also have the data in the cloud. Obviously I don't want to give the provider (or any intelligence service that snoops around) access to the data, so I have to encrypt it.
But because of the long-term nature of that type of storage, I worry that AES-256 is not going to be enough. While is secure against Grover's algorithm, it may be broken in another way (NSA does research in Tau statistics to break it) in maybe 20 years.
I can use cascading ciphers, but I'm not sure what the security implications are and what cascade to use. There is Serpent/Twofish/AES, but I could also use something like AES/Twofish/Salsa20 because Salsa20 is a different type of cipher. I would obviously choose 3 independant IVs and 256bit keys. Is that a reasonable thing to do? What about the MACs?