RADIUS attribute from rfc2865

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CHAP-Challenge

This Attribute contains the CHAP Challenge sent by the NAS to a
      PPP Challenge-Handshake Authentication Protocol (CHAP) user.  It
      is only used in Access-Request packets.

      If the CHAP challenge value is 16 octets long it MAY be placed in
      the Request Authenticator field instead of using this attribute.

   A summary of the CHAP-Challenge Attribute format is shown below.  The
   fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |    String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-Type

      60 for CHAP-Challenge.

   Length

      >= 7

   String

      The String field contains the CHAP Challenge.

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