It turns out that my base64 encoded signature was not url-safe. Solved by replacing + and / with - and _.

Hi, I'm currently trying to get the r and s from my ANS.1 DER signature, signed by my ATECC608B. I need these to build a 64-byte signature and base64 encode it to use in my jwt. I have attempted this, but the base-64 signature I'm getting back contains (invalid?) characters/symbols such as / and + ....