How to add a digital signature to my program?

Which reverse web proxy can rewrite URLs to add hashes to the parameters?

• I am looking for a reverse proxy that can compute a hash across the existing query parameters and add it as a new parameter. For example: with parameters a=1&b=2, compute a hash and rewrite the URL to have a=1&b=2&signature=0123456789abcdef I'm trying to avoid writing a new server-side program to do this, if possible. Writing a script or helper program is acceptable.

• Answer:

  Varnish Cache can do this. If you don't know Varnish it might take you an hour or so to make sense of it all. Varnish has modules, Vmods. Use the Digest VMOD (google: "digest vmod"). Then in vcl_recv you need to extract the parameters with a regex and store it in a header or a variable (if you are using the variable vmod). Then calculate the hash with the digest vmod. Add the hash to req.url and you're done.