decoder — JWT and Base64, decoded in your browser

Open https://decoder.utils.epiccloud-dev.io/ in a browser and paste. What you paste is
decoded by JavaScript in that tab: it never reaches this server, which
has no endpoint that accepts input at all.

That is also why there is nothing here for curl to call. To decode in a
terminal instead:

  # Base64, standard alphabet
  printf '%s' "$s" | base64 -d

  # Base64URL (JWT segments): -_ alphabet, padding omitted
  b64url() {
    local s=${1//-/+}; s=${s//_//}
    case $((${#s} % 4)) in 2) s+='==';; 3) s+='=';; esac
    printf '%s' "$s" | base64 -d
  }

  # JWT header and payload
  b64url "$(cut -d. -f1 <<<"$jwt")" | jq .
  b64url "$(cut -d. -f2 <<<"$jwt")" | jq .

  # PowerShell
  [Text.Encoding]::UTF8.GetString([Convert]::FromBase64String($s))

None of these verify a signature, and neither does the page.