Custom close vote sounds reasonable to me if nothing else fits!

Compare ‘What code does GCC compile c == 0 into on powerpc64 systems with AIX ABI?’ and ‘Is there a danger that this isolated string comparison routine in C might leak secrets via a timing side channel?’. The first one is obviously just a programming question. The second one, although clearly a C-related question, is something that essentially only crypto people are likely to have a good handle on, and is clearly about cryptography.

The example question you gave seems to me to fall under the category of programming help with a specific library.