tl;dr Versions of Wii IOS before IOS37 (which was also when Trucha was fixed) will ignore certain errors in SSL certs, treating them as valid. This is used by Wiimmfi as of May 20th, 2021.
As a basic guide to create a compatible cert, you:
- Generate a cert and key with OpenSSL, using a 1024 bit key
- Convert the cert .pem to .der (raw) form
- Towards the end (the signature portion) find and change
05 00 03 81 81to04 00 03 81 81 - Convert back to .pem (base64) form
- Install on your webserver