Guido Casati 5c601b7d62 Fix RRCReestablishment key derivation to use horizontal derivation
Per TS 33.501 6.9.2.3.4 and Fig 6.9.2.1.1-1, when sending
RRCReestablishment with nextHopChainingCount, the gNB is staying at
the same NCC level (not advancing). Therefore, horizontal derivation
from the currently active KgNB must be used, not vertical derivation
from NH.

Vertical derivation (from NH) is only used when advancing to a new
NCC level, which happens during handover or masterKeyUpdate procedures.

The previous implementation incorrectly used vertical derivation from
NH whenever nh_ncc > 0, which caused key mismatch between gNB and UE
during reestablishment after handover, leading to integrity check
failures.

This fix ensures both gNB and UE use horizontal derivation when
nextHopChainingCount matches the current NCC level, aligning with the
specification's "virtual NH at NCC=0" model where:
- Horizontal derivation = derive KNG-RAN* from currently active KgNB
- Vertical derivation = derive KNG-RAN* from NH (only when advancing NCC)
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OAI

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OpenAirInterface License

It is distributed under OAI Public License V1.1.

The license information is distributed under LICENSE file in the same directory.

Please see NOTICE file for third party software that is included in the sources.

Where to Start

Not all information is available in a central place, and information for specific sub-systems might be available in the corresponding sub-directories. To find all READMEs, this command might be handy:

find . -iname "readme*"

RAN repository structure

The OpenAirInterface (OAI) software is composed of the following parts:

openairinterface5g
├── charts
├── ci-scripts        : Meta-scripts used by the OSA CI process. Contains also configuration files used day-to-day by CI.
├── CMakeLists.txt    : Top-level CMakeLists.txt for building
├── cmake_targets     : Build utilities to compile (simulation, emulation and real-time platforms), and generated build files.
├── common            : Some common OAI utilities, some other tools can be found at openair2/UTILS.
├── doc               : Documentation
├── docker            : Dockerfiles to build for Ubuntu and RHEL
├── executables       : Top-level executable source files (gNB, eNB, ...)
├── maketags          : Script to generate emacs tags.
├── nfapi             : (n)FAPI code for MAC-PHY interface
├── openair1          : Layer 1 (3GPP LTE Rel-10/12 PHY, NR Rel-15 PHY)
├── openair2          : Layer 2 (3GPP LTE Rel-10 MAC/RLC/PDCP/RRC/X2AP, LTE Rel-14 M2AP, NR Rel-15+ MAC/RLC/PDCP/SDAP/RRC/X2AP/F1AP/E1AP), E2AP
├── openair3          : Layer 3 (3GPP LTE Rel-10 S1AP/GTP, NR Rel-15 NGAP/GTP)
├── openshift         : OpenShift helm charts for some deployment options of OAI
├── radio             : Drivers for various radios such as USRP, AW2S, RFsim, 7.2 FHI, ...
├── targets           : Some configuration files; only historical relevance, and might be deleted in the future
└── tools             : Tools for use by the developers/ci machines: code analysis and formatting

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