you can try this to pass the challenge in varnish auth-session (for testing purposes)

varnish-pass.pl

#!/usr/bin/env perl
#
#  
# If you don't have access to:  
#    * shell
#    * varnishadm                                    
# you can try this to pass the challenge in varnish auth-session console
#
# Vicente
 
 
use strict;
use warnings;
use Digest::SHA qw (sha256 sha256_hex);
 
my $challenge = $ARGV[0];
my $key = $ARGV[1];
my $result= sha256_hex($challenge . "\n" . $key . "\n" .  $challenge . "\n");
print $result ."\n";