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As I studied, Salsa20/Chacha20 is basically a hash function that accepts a 64-byte input and returns a output of the same size of input.

128-bits of the input are filled with four "Nothing-up-my-sleeve numbers", 64-bits with the nonce and the other 64-bits field with counter. The rest, 256-bits, are filled with the key.

My question is:

Being Salsa20/Chacha20 basically a hash function, can its security be expanded to 448-bits if I fill the "Nothing-up-my-sleeve numbers" and nonce fields with key material?

Would this impact the security of the cipher?

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    $\begingroup$ Why do you need the 448-bit key? 256-bit enough for all. There is XChaCha20 that has 192-bit nonces. You may look that $\endgroup$
    – kelalaka
    Dec 30, 2021 at 19:20
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    $\begingroup$ @kelalaka I told you, I have secrets I want to keep secret forever. I don't trust only 256-bit. Also, I wanna write a disk encryption program accepting keys up to 32768-bits, that's why I'm seeking for a encipherment scheme that allows this key sizes. I'm thinking on creating a modified Chacha20 with larger state sizes for that. $\endgroup$ Dec 31, 2021 at 1:00
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    $\begingroup$ "I don't trust only 256-bit." Why not? Why isn't several billion years enough? $\endgroup$ Jan 1, 2022 at 14:04
  • $\begingroup$ Because you can't mathematically prove it's a billion years can you? Not through civilian mathematics. We don't attack ciphers via brute force. We go around them and through the middle. That's why we use one time pads. $\endgroup$
    – Paul Uszak
    Jan 1, 2022 at 17:45

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No, it cannot. A nonce is necessary to have any security at all if a key is ever reused. The "nothing up my sleeve" number is necessary to keep the input from all being attacker controlled, it prevents the all-zero block, and its asymmetry improves the confusion and diffusion of the function.

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