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I have gigabites of files, which I have to backup. I have some servers for storing this files. But these servers are untrusted. So I need to encrypt files, then backup files. When my PC breaks down, I will dowload files from unstrucsted servers and restore them.

So, basically, I think about AES-256. Is it OK for me, or there is more suitable symmetric algorithms?

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    $\begingroup$ There is much more you have to worry about than a choice of pseudorandom permutation family, which is what AES-256 is—that's about the least interesting question. At the very least, start one level up: What authneticated cipher should I use? Standard answers are NaCl crypto_secretbox_xsalsa20poly1305, AES-GCM, AES-SIV. (These are all somewhat different: slightly different security contracts. If you want details on that, ask another question!) Study the design of Tarsnap or borgbackup if you're interested in actually pursuing this. $\endgroup$ – Squeamish Ossifrage Jun 12 '18 at 0:46
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Just a note: AES is not a encryption scheme, it is technically just a block cipher; though it is a fine one.

To create a cipher you need to use it in a mode like counter mode.

To obtain chosen ciphetext security (which is the usual aim) you should use an authenticated encryption scheme such as Galois/Counter Mode.

As usual, don't try make it yourself, use a premade secure library.

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