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I'm currently reading "Serious Cryptography - A Practical Introduction to Modern Encryption" written by "Jean-Philippe Aumasson" published by NoStarch in 2018.

In "Chapter 5 Stream Ciphers", under the heading "Attacking Salsa20/8" on Page 100, there is one paragraph (see screenshot below) which I do not quite understand.

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The author mentions the need for $2^{31}$ pairs of key-stream blocks. How does index 31 come about or how do we arrive at this index 31?

Any kind folks who can help to explain this clearly greatly appreciated!

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