[RADIATOR] CHAP flow

[M P]

Hello all,
 
In CHAP, how does Radiator verifies the password submitted by the end user matches the password in the database? Please correct my understanding on the following process flow:
 
[1] End user submits the username and password via CHAP.
[2] Upon hitting the Radiator with the CHAP-Password attribute, it will respond with Access-Challenge (exit 3).
[3] Perform challenge-response and decide whether the Radiator will Access-Accept or Access-Reject.
 
My question is, between items [2] and [3], how does Radiator checks and verifies the password of the username from its database? Isn't it that Radiator should check first its database for the username's password during step [2] or before step [3]?
 
Please advice as I am confused. I am actually using AuthBy EXTERNAL and executing an external script to check an external API for the user's password.
 
Thank you in advance.
 		 	   		  
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