Would using the keys (as in two passes with a key each) increase security for Salsa20 or ChaCha stream ciphers increase security?
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Would using the keys (as in two passes with a key each) increase security for Salsa20 or ChaCha stream ciphers increase security?
More or less.
It wouldn't increase the difficulty of brute-force by a lot (one bit to be precise) due to the infamous meat-in-the-middle-attack if you would use the same keylength, if you used different ones the effective keylength would be the higher of the two lengths.
What it would do though, is to increase the cryptanalytic security of the construction. Ie an attacker would have to successfully break both ciphers with cryptanalysis which is indeed more unlikely than him breaking one.
Is it worth it? No. The extra keybit is not that valueable and it's already a matter of "meh, can't break" anyways even with a single cipher. Regarding the cryptanalytic defense, neither ChaCha nor Salsa have been broken, but these ciphers are quite similar so you don't get the advantage you would get from two ciphers with radically different designs.
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$\begingroup$ It's "meet-in-the-middle". "meat-in-the-middle" is a sandwich. $\endgroup$ Commented Mar 24, 2017 at 19:14
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1$\begingroup$ @SAIPeregrinus It wasn't a mistake but rather a joke attempt. $\endgroup$– SEJPMCommented Mar 24, 2017 at 19:17