RADIUS_Attribute - Chap Challenge

As per RFC 2865

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.

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