IAE redirect_to_web clarification, multiple use of PKCE_verifier ok?

[mickrau]

I currently reviewing IAE and creating some sequence diagrams. I stumpled over this passage when using redirect_to_web with PKCE.

In the event that PKCE as defined in [RFC7636] was used in the initial authorization request to the interactive authorization endpoint, the Authorization Server MUST enforce the correct usage of the code_verifier in the follow-up request that follows the completion of the redirect_to_web interaction.

Does this mean that the same PKCE_verifier is used twice? First in a follow-up IAR (6) and second in the token request (10).
Even though I don't see any immediate problem, it seems unusual at first glance. I have always considered PKCE_verifier to be single-use.

+---------------+       +-------------+       +----------------------+                      
|               |       |             |       |                      |                      
|     Wallet    |       |  Browser    |       | Authorization Server |                      
|               |       |             |       |                      |                      
+--------+------+       +------+------+       +-----------+----------+                      
         |                     |                          |                                 
         |                                                |                                 
         |                                                |                                 
         +----------------------------------------------->|(1)Initial                       
         |                                                |   Interactive                   
         |                                                |   Authorization                 
         |                                                |   Request                       
         |                                                |   (PKCE_challenge)              
         |                                                |                                 
         |                                                |                                 
         |<-----------------------------------------------+(2) Interactive                  
         |                                                |    Authorization                
         |                                                |    Response                     
         |                                                     (status=require_interaction, 
         |                                                     type=redirect_to_web,        
         |                                                     request_uri,                 
         |                                                     auth_session1)               
         |                                                                                  
         |                     |                                                            
         +-------------------->| (3) Open Authorization Request                             
         |                     |          (request_uri)   |                                 
         |                     +--------------------------+-+                               
         |                     |                          | | (4) User interaction          
         |                     |                          | |      (multiple rounds         
         |                     |<-------------------------+-+         possible)             
         |                     |                          |                                 
         |                     |                                                            
         |<--------------------+ (5) Redirect to wallet                                     
         |                     |     (auth_session_2)                                       
         |                     |                                                            
         |                                                                                  
         |                                                |  (6) Follow-up                  
         +----------------------------------------------->|      Interactive                
         |                                                |      Authorization              
         |                                                |      Request                    
         |                                                |      (auth_session2,            
         |                                                |       PKCE_verifier)            
         |                                                |                                 
         |<-----------------------------------------------+  (7) IA Response                
         |                                                |      (auth_session3,            
         |                                                |                                 
         |                                                |                                 
         +----------------------------------------------->+- (8) Follow-up                  
         |                                                |      IA Request                 
         |                                                |                                 
         |<-----------------------------------------------+  (9) Authorization Code Response
         |                                                |      (code)                     
         |                                                |                                 
         |                                                |                                 
         +----------------------------------------------->|  (10) Token Request             
         |                                                |       (code,                    
         |                                                |        PKCE_verifier)           
         |                                                |                                 
         |                                                |  (11) Token Response            
         |<-----------------------------------------------+       (access_token)            
         |                                                |                                 
         |                                                |                                 

[jogu]

Discussed on today's WG call:

Yes, a PKCE verifier would be used more than once in a single session in these cases. This doesn't seem to be problematic from a security point of view as long as the re-use is within that same session.

Double check if the PKCE RFC says anything on this topic.

[fkj]

In my opinion this is fine and does not contradict any language in RFC 7636.
The security model of PKCE relies on the attacker not learning the code_verifier. All requests involving the code_verifier in this scenario are back-channel (i.e. POST) requests, so the security model is applicable.