Successfully passed first two test vectors on ECDSA signing and verification using secp256k1 curve implemented in pure POSIX bc and driven by POSIX sh.

Yes, it is quite slow even on my nettop and even with GNU bc in the POSIX mode (bc -s): every sign/verify operation takes up to 40 seconds. I thought to maybe create an alternative path to include "native" non-POSIX modexp on the platforms that allow it, but I cannot find any explicit modexp in any bc manual except on FreeBSD.

An alternative path, of course, would be using OpenBSD's bc to compile this library into dc.

And the interesting part is, on OpenBSD's bc, these operations take 3 or 4 seconds at most... even on my slow ARM64 VPS.