RADIUS attribute from rfc5090

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Digest-Opaque

This attribute holds the opaque parameter that is passed to the
         HTTP-style client.  The HTTP-style client will pass this value
         back to the server (i.e., the RADIUS client) without
         modification.  This attribute MUST only be used in Access-
         Request and Access-Challenge packets.
   Type
         116 for Digest-Opaque
   Length
         >= 3
   Text
         In Access-Requests, the RADIUS client takes the value of the
         opaque directive (opaque-value according to ) without
         surrounding quotes from the HTTP-style request it wants to
         authenticate and puts it into this attribute.  In Access-
         Challenge packets, the RADIUS server MAY include this
         attribute.

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