Crypto Twitter, what's a good way to generate nonces if random is disallowed (chance of collision) and you don't want to leak information about order (which sequential would)?
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I'd say if there's a chance of collision you should make it larger if possible.
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It's chacha20 so 64 or 96 bits. That's only 48 bit safety which doesn't cut it (thus the mandate in standard not to use random w/o collision check).
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XSalsa20 / XChaCha20 have a 192-bit nonce and a separate dedicated 64-bit counter so you can simply generate the nonce with a CSPRNG.
libsodium uses X25519 + XSalsa20 + Poly1305 for their high-level authenticated encryption. It'd presumably use XChaCha20 now that it's defined.
If you have to use ChaCha20, you can use your key to derive a subkey for each message with a KDF. It's essentially what XChaCha20 does for you though. Might as well use it.
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High level libsodium secretbox (symmetric) and crypto_box (asymmetric) APIs are really well made and worth following as an approach even if you reimplement it. XChaCha20 vs XSalsa20 doesn't really matter. Worth caring about it with 8 rounds but 20 is extreme overkill regardless.