4.3 Limited Use Nonce Values
The Digest scheme uses a server-specified nonce to seed the
generation of the request-digest value (as specified in section
3.2.2.1 above). As shown in the example nonce in section 3.2.1, the
server is free to construct the nonce such that it may only be used
from a particular client, for a particular resource, for a limited
period of time or number of uses, or any other restrictions. Doing
so strengthens the protection provided against, for example, replay
attacks (see 4.5). However, it should be noted that the method
chosen for generating and checking the nonce also has performance and
resource implications. For example, a server may choose to allow
each nonce value to be used only once by maintaining a record of
whether or not each recently issued nonce has been returned and
sending a next-nonce directive in the Authentication-Info header
field of every response. This protects against even an immediate
replay attack, but has a high cost checking nonce values, and perhaps
more important will cause authentication failures for any pipelined
requests (presumably returning a stale nonce indication). Similarly,
incorporating a request-specific element such as the Etag value for a
resource limits the use of the nonce to that version of the resource
and also defeats pipelining. Thus it may be useful to do so for
methods with side effects but have unacceptable performance for those
that do not.
