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For an encryption scheme to satisfy the standard notion of security (IND-CPA), its encryption algorithm must be randomised. Therefore $\textsf{Encrypt}$ has access to random coins, denoted here by $\theta$. It is usually implicit in the syntax of $\textsf{Encrypt}$ $$c\leftarrow\textsf{Encrypt}(pk,m),$$ but it can be made explicit as c=\textsf{Encrypt}(pk,...

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The entropy will never go higher than the amount you put in. So if you require 128 - 4 = 124 bits and you input 32 bits, you can rest assured that the amount left is at most 32 bits. It's really that simple. And in that case, there is a high likelihood of collisions due to the birthday bound. Now generally you will likely not find dupes during regular use, ...

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While SHA2 (SHA-256, SHA-512) original design goals are limited to collision-resistance and (first and second) preimage resistance, it is not known that its computationally distinguishable from a random oracle for messages of fixed length¹. Thus $\text{SHA2}(\text{seed}+n)$ for incremental $n$ is a CSPRNG as far as we know, for a wide-enough random secret ...

2

You can compute an $f(g(m))$ in which $g(m)$ is your encryption method and $f$ is a meaningful mapping function. As an example of $f$ I can mention a dictionary codebook mapping. For example if your output is 11101000 you can see it as 1110 and 1000 then map 1110 to "apple" and 1000 to "grape". Your codebook here must consist of all ...

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