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Pedro avatar Crypto

Salsa20 encryption process

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Pedro

I am studying the Salsa20 algorithm and I was wondering if you could help me understanding the expansion function. I don´t understand what the Pos or stream position as I have read in other places does, is it meant to randomize the data, similar to the nonce? Any help would be greatly appreciated!

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benrg avatar Crypto
cn flag
benrg

The intent is that you choose a random key and use it to encrypt messages, whatever "message" may mean to your application. After each 512-bit block you increment the position, and after each message you increment the nonce and reset the position to 0.

There's nothing in Salsa20's design that requires you to use fields of those sizes for those purposes, since they're all hashed in the same way, but following the recommendation minimizes the chance that you'll inadvertently use the same initial state for two different blocks, which would leak plaintext.

Neither the nonce nor the position is meant to add randomness, just uniqueness. They're too short to be safely chosen at random. There are Salsa/ChaCha variants with a longer nonce if you need it to be random.

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