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libngu PR #63: reject two-word entropy cycles

libngu-pr-63

https://github.com/switck/libngu/pull/63

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 Summary
 Adds a small entropy-source health check to ngu.random.
 Each source word is rejected when it is zero or matches either of the two most recently accepted words. Zero rejection stays in random.c, where the platform policy belongs. The helper tracks initialization explicitly, so zero is treated as real history when the helper is tested on its own.
 This was referenced Aug 1, 2026

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switck
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Second of the three-PR split of #59, open at first capture on 7 Aug 2026 with one commit. Registered from its own page rather than from the title shown on #59 and #61.

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  1. source content difference between and source content +3 -1

    The pull request state changed from Open to Merged, and the page rendered additional contributor labels.

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    What changed from the previous capture 4 lines
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    +Contributor
     Summary
     Adds a small entropy-source health check to ngu.random.
     Each source word is rejected when it is zero or matches either of the two most recently accepted words. Zero rejection stays in random.c, where the platform policy belongs. The helper tracks initialization explicitly, so zero is treated as real history when the helper is tested on its own.
     This was referenced Aug 1, 2026
     random: harden entropy backends and state handling
     #64
    -Open
    +Merged
     random: fail-closed entropy backends; replace Yasmarang with CAVP-verified HMAC_DRBG
     #59
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     Aug 7, 2026
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     You’re right. I rechecked SP 800-90B and the STM32L4 documentation.
     SP 800-90B defines the repetition-count test for consecutive equal samples and the adaptive-proportion test. It permits extra tests when they address a documented source-specific failure mode with a stated false-alarm policy.
    
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  2. source content difference between and source content +20 -2

    The author closed the pull request and posted a comment explaining that the extra A, B, A rejection is not justified by STM32 documentation or SP 800-90B and will be dropped from the rebase of #64.

    seen · Captured here 9,131 chars
    What changed from the previous capture 22 lines
     Insights
     random: reject two-word entropy cycles - #63
     #63
    -Open
    +Closed
     jgmontoya wants to merge 1 commit into
     switck:masterswitck/libngu:masterfrom
     jgmontoya:rng-2-entropy-healthjgmontoya/libngu:rng-2-entropy-healthCopy head branch name to clipboard
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     random: reject two-word entropy cycles#63
     jgmontoya wants to merge 1 commit into
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    +jgmontoya
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    +Aug 7, 2026
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    +You’re right. I rechecked SP 800-90B and the STM32L4 documentation.
    +SP 800-90B defines the repetition-count test for consecutive equal samples and the adaptive-proportion test. It permits extra tests when they address a documented source-specific failure mode with a stated false-alarm policy.
    +STM32L4 detects long constant or alternating sequences in the raw seed bits internally. I found no documented failure where one valid 32-bit A, B, A sequence indicates a failing RNG. This software check also runs on conditioned outputs such as getrandom() and arc4random(), so the STM32 raw-bit detector does not justify it.
    +Your probability estimate is correct for the new condition. Under uniform independent 32-bit output, the extra A, B, A rejection occurs about once per 2^32 checked words. I don’t have evidence that this false-alarm cost catches a real source fault.
    +I’ll close #63 and rebase #64 without this check, preserving the existing zero and adjacent-repeat behavior. Thanks for catching it.
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    +Aug 7, 2026
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