decode_base64_field doesn't allow to decode unpadded base64 strings

[michaelarnauts]

It seems that decode_base64_field is using base64.StdEncoding.DecodeString to decode the string, but that function only supports unpadded strings. There also is a base64.RawStdEncoding.DecodeString function, but this one only supports unpadded strings.

#25817 tried to solve this by appending =-signs until the string was fully padded, but another solution could be to strip all the =-signs, and using base64.RawStdEncoding.DecodeString.

The use-case for this is to decode a JWT Authorisation header to extract the user_id field, like has been documented here. This example is using a script processor to add the padding however, and this isn't available in all *beat projects.

Version: packetbeat-7.13.4 (but actually all the latest libbeat-projects)
Operating System: Ubuntu 20.04

[elasticmachine]

Pinging @elastic/agent (Team:Agent)