Example :

<match test>
    type filter_codec
    add_tag_prefix decoded.
    codec base64-decode
    field key1
    output_field key2
</match>

Example of records :

{
    'key1' => "Tmljb2xhcyBDYWdl",
    'foo' => "bar"
}

... will output (unchanged) :

{
    'key1' => "Nicolas Cage",
    'foo' => "bar"
}

You can omit 'output_field' so 'field' value will be replaced by output value.