[WIP] Fix assumption on int size in ecmult_wnaf function #1770

1. 
   Copilot commented at 10:23 am on November 7, 2025: none

   Thanks for assigning this issue to me. I’m starting to work on it and will keep this PR’s description up to date as I form a plan and make progress.

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   This section details on the original issue you should resolve

   <issue_title>_ecmult_wnaf relies on int having at least 32 value bits</issue_title> <issue_description>The current secp256k1_ecmult_wnaf needs the unstated and unchecked assumption that int has at least 32 value bits when it VERIFY_CHECKs that w <= 31. In practice, we call it only with WINDOW_A == 5 and WINDOW_G == ECMULT_WINDOW_SIZE where the latter is configurable in the range 2..24.

   A consequence of this “bug” is that the code fails on a 16-bit platform if you set ECMULT_WINDOW_SIZE > 16. I don’t think we need to support this, but code without unchecked assumptions is bad. So I suggest that we rewrite the function to use int32_t instead of int even if we don’t use my macro approach. (We can keep the “bit position” arguments and variables int.)

   Alternatively, we could add the assumption that INT_MAX >= INT32_MAX but this forbids 16-bit platforms, and the code seems to work on them in principle; see #792 (comment).

   Originally posted by @real-or-random in #1761#pullrequestreview-3432695127 </issue_description>

   <agent_instructions>Use the suggestion to use int32_t instead of int. Keep the “bit positions” variables and arguments int.</agent_instructions>

   Comments on the Issue (you are @copilot in this section)

   • Fixes bitcoin-core/secp256k1#1769

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2. Initial plan 4bfe72b826
3. Copilot assigned Copilot on Nov 7, 2025
4. Copilot assigned real-or-random on Nov 7, 2025
5. 
   real-or-random commented at 4:27 pm on November 10, 2025: contributor

   @copilot Are you still alive?
6. real-or-random closed this on Nov 11, 2025

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7. 
   real-or-random commented at 9:03 pm on November 11, 2025: contributor

   This was a GitHub Actions permissions issue: https://github.com/orgs/community/discussions/179396

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