Time Stamping: comments on nonce field

Hi PKIX-ers,

I have some comments on Time Stamping protocols regarding "nonce" field:

First of all, the declaration of this field in the request and token is somehow incoherent, while the nonce is mandatory in TimeStapReq (it is not declared OPTIONAL), the TimeStampToken´s nonce is OPTIONAL but it is stated that "...must be present if the similar field in TimeStampReq is present..." and hence the clause OPTIONAL is meaningless and confusing for this field shall always be present. I would suggest this field to be declared OPTIONAL both in the request and token or better deleted if we take into account the following:

As far as I understand, a nonce value is present in the request and token in order for the requester to be able to match the responses from the TSA with her requests when using an asynchronous transport. It can be argued that such functionality should be left to the transport layer when required, but furthermore I must say that the nonce is not necessary since matching can be performed using the "messageImprint" field.

The only justification at hand to defend the "nonce" is when asking a TSA "What time it is" (not including messageImprints field), and in order to defeat replay attacks in this case... Anyway, this use of a TSA steps aside from "providing a proof-of-existence for a particular message in an instant in time" and personally don't like it; I would call that a STS (secure time source) (i.e.: authenticated NTP) and would not force TS protocols modifications for that use.

Regards,

Juan.