Captcha is non-functional

[RDS5]

Background information

Dendrite 0.13.7+46902e5 with Postgres database running on Debian 12 compiled with go 1.22.1

Description

Seemingly any hCaptcha (or even reCaptcha, as seen on matrix's official dendrite instance) will give "invalid key type" errors when users attempt to sign up.

Reproducing this is pretty easy, since matrix themselves have already reproduced it.

[Zorg64]

I also cannot get hcaptcha to work, I get the same error.
I was able to get recaptcha to work but would prefer hcaptcha.

For recaptcha, I selected V2 as well as the "I am not a robot" option. In the site settings disable (uncheck) "Verify the origin of reCAPTCHA solutions"

[DevSH01]

Having similar problem here with Hcaptcha, for some reason, its showing up as google Recaptcha instead of Hcaptcha

[BVollmerhaus]

I'm relatively sure this is a problem with Element rather than Dendrite. The current Matrix specification does not accommodate captcha providers other than Google, so Element – and presumably most other clients – always use reCAPTCHA's URL, even if Dendrite advertises a different one (although I haven't confirmed that it actually does that correctly).

The necessary specification change is tracked at matrix-org/matrix-spec#295.