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This question is related to this (but it is not the same).

Let's suppose I have a seed with an entropy of 1024-bits and hash it with a counter using a hash function with one-quarter of the seed size in bits as BLAKE2s (256-bits digest size).

I hash the seed with counters and XOR the result to plaintext.

As said in this answer, some options are (the third I proposed by myself):

  1. H(00∥F) ∥ H(01∥F) ∥ H(02∥F) ∥ H(03∥F)...
  2. H(H(F)∥00) ∥ H(H(F)∥01) ∥ H(H(F)∥02) ∥ H(H(F)∥03)...
  3. H(00∥H(F)) ∥ H(01∥H(F)) ∥ H(02∥H(F)) ∥ H(03∥H(F))...

/\ H is the hash, F is the file and 00, 01, 02, 03 the counters.

PS: For H(F∥00)∥H(F∥01)∥H(F∥02)∥H(F∥03), it was answered here.

Assuming that the hash function is not vulnerable to length-extension attacks and not caring about optimizations, what of these it is the most secure scheme in practice? And why?

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  • $\begingroup$ Are you assuming the hash function is not vulnerable to length-extension attacks? And do you care about optimizations? Finally, do you care about which is slightly more secure in theory, or only whether or not one of them is more secure than the others in practice? $\endgroup$
    – forest
    Nov 24, 2022 at 1:38
  • $\begingroup$ I modified the question. What I want to know is what scheme is more secure (in practice). And yes, hash function should be immune to length-extension attacks, not counting optimizations (just what is more secure) and of course, what is more secure in practice (I don't like theories very much). $\endgroup$ Nov 24, 2022 at 3:26

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I’m unsure of your exact definition of practical, but the first scheme is more secure than the other two. From an entropy point of view, the second and third schemes throw away roughly three-quarters of the key entropy. Put another way, they have very large families of equivalent keys: every pair of values $F$ and $F’$ with $H(F)=H(F’)$ produce the same stream under schemes 2 and 3.

In particular, if $2^{128}$ instances of the cipher are used, there are likely to be two instances with the same key stream. This may not count as practical, but is very undesirable.

On a slightly related picky note, you should require that the seed have a large min-entropy and not just a large Shannon entropy. It is possible to construct sources of random numbers that have over, say, 1024-bits of entropy but which output one particular number, say, half of the time.

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  • $\begingroup$ Are you saying 2^128 instances because it's half of Blake2s digest size (256-bits)? $\endgroup$ Nov 24, 2022 at 17:29
  • $\begingroup$ @phantomcraft Yes, in particular that is the birthday bound for a collision. $\endgroup$
    – Daniel S
    Nov 24, 2022 at 19:21
  • $\begingroup$ You said about birthday bound for a collision in another paragraph of your answer, did you mean that this is also applicable to H(00∥F) ∥ H(01∥F) ∥ H(02∥F) ∥ H(03∥F)... ? $\endgroup$ Nov 24, 2022 at 23:10
  • $\begingroup$ The birthday issue does not apply as strongly in the first case, if $H$ is a well-designed hash function, then whole stream should only collide if $F=F'$. With cases 2 and 3 the streams will collide if $H(F)=H(F')$. For uniformly distributed $F$ and $F'$ the first is $2^{-1024}$ event and is likely to occur in a sample of $2^{512}$ streams; the second and third is a $2^{-256}$ event and is likely to occur in a sample of $2^{128}$ streams. $\endgroup$
    – Daniel S
    Nov 25, 2022 at 7:53

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