Doesn't look very hand rolled to me. It's standard HMAC. The only unusual thing is the timing-unsafe comparison[1], which probably needs fixing. It looks like an attempt was made at a constant-time comparison (|= ^ pattern sure looks like it), but the early return breaks it again. I'm not sure if much can be gained from a timing attack in this particular instance though, since the key fully depends on user input in the first place.

(By the way, even Microsoft's own documentation doesn't use constant-time comparisons for HMAC[2]!)

[1] https://github.com/nrosvall/ylva/blob/2a4afcfb3727151fa09fdd...

[2] See the example on https://docs.microsoft.com/en-us/dotnet/api/system.security....