How to get "alg" from JWK?

[chulkilee]

How can I get alg to use in JWT from JOSE.JWK? Such as RSA256.

I figured out how to do it by checking byte sizes in n - but curious is there better way (e.g. avoid Base64 decoding)

# from jwt.io
public_key_pem = """
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAnzyis1ZjfNB0bBgKFMSv
vkTtwlvBsaJq7S5wA+kzeVOVpVWwkWdVha4s38XM/pa/yr47av7+z3VTmvDRyAHc
aT92whREFpLv9cj5lTeJSibyr/Mrm/YtjCZVWgaOYIhwrXwKLqPr/11inWsAkfIy
tvHWTxZYEcXLgAXFuUuaS3uF9gEiNQwzGTU1v0FqkqTBr4B8nW3HCN47XUu0t8Y0
e+lf4s4OxQawWD79J9/5d3Ry0vbV3Am1FtGJiJvOwRsIfVChDpYStTcHTCMqtvWb
V6L11BWkpzGXSW4Hv43qa+GSYOD2QU68Mb59oSk2OB+BtOLpJofmbGEGgvmwyCI9
MwIDAQAB
-----END PUBLIC KEY-----
"""

{_, jwk_map} =
  public_key_pem
  |> JOSE.JWK.from_pem()
  |> JOSE.JWK.to_map()

alg =
  case jwk_map do
    %{"kty" => "RSA", "n" => n} ->
      "RS" <> (n |> Base.url_decode64!(padding: false) |> byte_size() |> Integer.to_string())
  end

Is there easier way to get the rsa key size at least?