Potential Service Life of ChaCha20? I have some very sensitive family files that I want to store in the cloud on multiple providers for maximal reliability and redundancy. It's essentially a large archive of all our family photos, documents, and confidential photos. I'll be needing them in about ~20 years, but I won't access them or update them during this interval. These files must not be accessed by anyone but myself.
I am skeptical about AES being able to last for that long, because there could be advances in cryptanalysis. Instead, I'm considering ChaCha20, because it's a modern cipher and was sought for by security engineers at Google.
Supposing that I encrypt all files offline, use a secure key (along with Argon2), and ensure integrity (Poly1305), will my data theoretically be able to last for that long (the best attacks on ChaCha20 only break 7 rounds)? If not, is there a possible cipher that could? There will only be one copy, preventing chosen-plaintext attacks.
There's simply no way to know. I find it very unlikely that AES will be vulnerable to a ciphertext-only attack in the next 20 years (remember, it has been around for over 20 years already and attacks haven't gotten very far). There's no reason to believe the same won't be true for ChaCha20. If you use a good cipher with 256-bit keys (to avoid potential speedups from quantum computers running Grover's algorithm which halves the effective key size of a symmetric cipher), you should be fine.
If you really, really want, you could use something like VeraCrypt which provides you with the option to use three ciphers in a row: AES, Serpent, and Twofish, all of which are extremely secure.
We can say with some level of confidence that it will remain secure for your purposes.