RADIUS attribute from rfc2865

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CHAP-Password

This Attribute indicates the response value provided by a PPP
      Challenge-Handshake Authentication Protocol (CHAP) user in
      response to the challenge.  It is only used in Access-Request
      packets.

      The CHAP challenge value is found in the CHAP-Challenge Attribute
      (60) if present in the packet, otherwise in the Request
      Authenticator field.

   A summary of the CHAP-Password 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 4 5 6 7 8 9
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  CHAP Ident   |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-Type

      3 for CHAP-Password.

   Length

      19

   CHAP Ident

      This field is one octet, and contains the CHAP Identifier from the
      user's CHAP Response.

   String

      The String field is 16 octets, and contains the CHAP Response from
      the user.

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