Verify non-null data arg in ellswift xdh_hash_function_prefix #1806

1. 
   furszy commented at 8:14 pm on January 23, 2026: member

   The secp256k1_ellswift_xdh API has a data arg that is optional for one of the hash functions (unused) but required for the other.

   This adds a simple check to provide a clear error instead of crashing due to an out-of-bounds memory access when the user mistakenly provides data=NULL and selects the xdh_hash_function_prefix() function.

   Note: Open to improving the arg description in the API as well. I’m just adding this check because a better error would have saved me a few minutes.

2. Verify non-null data arg in ellswift xdh_hash_function_prefix

   The secp256k1_ellswift_xdh API has a `data` arg that is optional
   for one of the hash functions (unused) but required for the other.
   
   This adds a simple check to provide a clear error instead of crashing
   due to an out-of-bounds memory access when the user mistakenly provides
   data=NULL and selects the `xdh_hash_function_prefix()` function.

   f4708d24d2
3. 
   sipa commented at 8:19 pm on January 23, 2026: contributor

   ACK f4708d24d21c252279cd865d0e64799ab2159f90
4. real-or-random added the label assurance on Jan 23, 2026
5. real-or-random added the label tweak/refactor on Jan 23, 2026
6. 
   real-or-random commented at 8:23 pm on January 23, 2026: contributor

   when the user mistakenly provides data=NULL

   Note that VERIFY_CHECKs are only active in the tests.

7. 
   furszy commented at 8:26 pm on January 23, 2026: member

   when the user mistakenly provides data=NULL

   Note that VERIFY_CHECKs are only active in the tests.

   We can’t use ARG_CHECK because ellswift_xdh_hash_function_prefix() doesn’t receive the context (for now). #1777 will solve that and let us add the proper arg check.

   Update: We probably could leave this open until the other PR gets merged.

Contributors
furszy sipa real-or-random

Labels
assurance tweak/refactor