questions on RFC2631 and DH key agreement

[' =JeffH ']

I think I already know the answer to this question, but I just want to test my 
understanding...

How wise (in a real-world sense) is it, in a protocol specification, to 
specify that one simply obtain an ostensibly random value, and then use that 
value directly as the signature key in, say, an HMAC-based signature, without 
any further stipulated checking and/or massaging of the value before such use?

E.g., here's such a specfication excerpt and is absolutely everything said in 
the spec wrt obtaining said signature keys:

  When generating MAC keys, the recommendations in [RFC1750] SHOULD be 
followed.
  ...
  The quality of the protection provided by the MAC depends on the randomness 
of
  the shared MAC key, so it is important that an unguessable value be used.

How (un)wise is this, in a real-world sense? 


[yes, I'm aware that using a only a SHOULD here leaves a huge door open 
compliance-wise, but that's a different issue]

thanks,

=JeffH


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