conditionally setting a cookie help

[Vader Mader]

Hi All,

I'm having trouble setting a cookie conditionally based upon
an upstream variable The hope is to cache an auth token in an
encrypted session and only go to the backend auth token generator once.

I have something like this but it seems set-cookie happens no matter what,
so I alternate between 'my_login=1848430=' and 'my_login='.

    location = /auth {
        set_decode_base32 $b32 $cookie_my_login;
        set_decrypt_session $auth_tok $b32;

        if ($auth_tok != '') {
            return 200;
        }

        include        fastcgi_params;
        fastcgi_pass   unix:/tmp/fcgi_auth_tok_gen.sock;
    }

    location / {
        root   /var/www;
        index  index.html index.htm;

        auth_request /auth;
        auth_request_set $new_auth_tok $upstream_http_auth_tok;

        if ($new_auth_tok != false) {
            set_encrypt_session $enc_auth_tok $new_auth_tok;
            set_encode_base32 $b32 $enc_auth_tok;
            add_header Set-Cookie 'my_login=$b32';
        }
    }

Ideas?
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