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Cryptography

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    $\begingroup$ I agree with the conclusion, but not with "statically sized" as the sole argument (I get it as: length extension does not apply). It is necessary to invoke that SHA-2 is a good enough hash that $X\mapsto H(S\mathbin\parallel X)$ is a good PRF keyed by $S$ for constant-size $X$. It would be possible to define a weak $H$ such that observing $X\mapsto H(S\mathbin\parallel X)$ leaks $S$, when that would not apply to $\text{HMAC-H}(S,X)$. That is, not using HMAC puts unduly pressure on the hash, and that pressure is not part of its explicit design criteria. HMAC is advisable for unspecified $H$. $\endgroup$ Commented Apr 14, 2020 at 19:34
  • $\begingroup$ @fgrieu once can use SHA256/224 or SHA512/256 etc. to mitigate from length extension attacks. $\endgroup$ Commented Apr 14, 2020 at 20:21
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    $\begingroup$ @fgrieu Sure, I put in a stronger hash requirement. You're theoretically correct of course, but the hash needs to be really weak for short, static, random content to become a problem in this scenario. $\endgroup$ Commented Apr 14, 2020 at 20:39