Description

I am encountering a sporadic digest mismatch issue when using SIPp for IMS registration with AKAv1-MD5 authentication. The problem is not deterministic — the same scenario sometimes results in a successful REGISTER, and other times fails due to an incorrect RES value being computed by SIPp.

Observed Behavior

• SIPp sends the initial REGISTER and receives a 401 Unauthorized from the S-CSCF (Kamailio).

• SIPp responds with a second REGISTER, including the Authorization header.

• Kamailio logs show:

  UE said: 90152bd36e8dc72e1580cbf4c5fe3ee1, but we expect: d7ec73ce4f61683c2c7b544e9fabd2b9
  

• However, in previous runs with the exact same scenario, SIPp produced the correct RES, and Kamailio accepted the registration.

Expected Behavior

Given the same:

• aka_K = 0x8baf473f2f8fd09487cccbd7097c6862
• aka_OP = 0x8E27B6AF0E692E750F32667A3B14605D
• aka_AMF = 0x8000
• nonce and challenge from the 401

The RES value should always be the same and match what Kamailio expects, but SIPp intermittently calculates a wrong one.

Scenario Snippet

<send retrans="500">
<![CDATA[
REGISTER sip:ims.mnc001.mcc001.3gppnetwork.org SIP/2.0
Via: SIP/2.0/[transport] [field2]:[local_port];branch=z9hG4bK-[call_number];rport;transport=[transport]
From: <sip:[field1]@ims.mnc001.mcc001.3gppnetwork.org>;tag=[call_number]
To: <sip:[field1]@ims.mnc001.mcc001.3gppnetwork.org>
Contact: <sip:[field2]:[local_port]>;
Call-ID: [call_id]
CSeq: 1 REGISTER
Expires: 60000
[authentication
  username="[field1]@ims.mnc001.mcc001.3gppnetwork.org"
  aka_K=0x8baf473f2f8fd09487cccbd7097c6862
  aka_OP=0x8E27B6AF0E692E750F32667A3B14605D
  aka_AMF=0x8000]
Content-Length: 0
]]>
</send>

Environment

• SIPp version: v3.7 (also seen in v3.5.3)
• OS: Ubuntu 20.04
• Scenario: SIP REGISTER using IMS-AKA (AKAv1-MD5) Digest Auth
• S-CSCF: Kamailio with ims_auth module

Reproducability

6 out of 100 times