[cap-talk] use of hashcodes?

[Ben Laurie]

On 22 February 2010 16:54, David Wagner <daw at cs.berkeley.edu> wrote:

> David, I feel like we're talking past each other.  I want to check if
> you're familiar with the ETags mechanism in particular, because it is a
> very specific mechanism.  It's important to understand how ETags work,
> as specified in the HTTP protocol, before selecting a cryptographic
> primitive to use for computing ETags.
>
> ETags is part of HTTP.  The server can optionally send an ETag together
> with its HTTP response.  The ETag is an opaque value that's much shorter
> than the response itself (say, 256 bits or so).  The client then uses
> this to detect whether the page has changed or not.  If the ETag is
> the same, then the client can use its cached copy of the page, without
> downloading the page a second time.  The ETag is an opaque value that
> is not interpreted by clients.  The semantic requirement on servers is
> that the ETag should be the same for two requests only if the content
> (the response) to both would be identical.
>
>  http://en.wikipedia.org/wiki/HTTP_ETag


Quibble: you have the protocol backwards. The client sends the cached ETag
with its request. If it matches the current ETag the server doesn't bother
to send the page.
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