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The once part inside of the nonce in CTR mode means effectively "once for this particular key". If you use a fresh key for each message (e.g. by encrypting it using public-key crypto or similar), you can use the same nonce for all the messages (or a size-zero nonce). The important part is that the combination of nonce and ctr-value (i.e. what is input into ...


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This will not be secure in general, and is not recommended. The reason is that a PRG is guaranteed to produce a pseudorandom output only if its input is uniformly random (or pseudorandom). Moreover, several PRG outputs are jointly pseudorandom only if the PRG is run on independent seeds. But here you are invoking the PRG on "structured" inputs, i.e., the ...



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