docs: def pem2der(pem_string):
      """Convert PEM string to DER format"""
      # Encode all lines between the first '-----\n' and the 2nd-to-last '-----'.
      pem_string = pem_string.replace(b"\r", b"")
      first_idx = pem_string.find(b"-----\n") + 6
      if pem_string.find(b"-----BEGIN", first_idx) != -1:
          raise Exception("pem2der() expects only one PEM-encoded object")
      last_idx = pem_string.rfind(b"-----", 0, pem_string.rfind(b"-----"))
      base64_string = pem_string[first_idx:last_idx]
      base64_string.replace(b"\n", b"")
      der_string = base64.b64decode(base64_string)
      return der_string