加密顺序是EAP报文里的Identifier字段，password，以及EAP报文里的Challenge字段。
The Response Value is the one-way hash calculated over a stream of octets consisting of the Identifier, followed by (concatenated with) the "secret", followed by (concatenated with) the Challenge Value. The length of the Response Value depends upon the hash algorithm used (16 octets for MD5).
参考:
1. RFC-3748 EAP协议
2. RFC-1994 PPP协议
