The Signalling Programme
Fjernbane Infrastructure East/West
BAFO Tender Document / Appendix 3.1
Att 11 Online Key Management System Concept
Banedanmark
The Signalling Programme
Amerika Plads 15
DK-2100 Copenhagen E
Denmark
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Tender Document / Appendix 3.1
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Author: Fjernbane
Signalling System Project
Mail: Fjernbane@bane.dk
Phone: +45 8234 0000
www.banedanmark.dk
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Att 11 OKMS Concept
Table of Contents
1
Change Log
2
2.1
2.1.1
2.1.2
Introduction
Goals and prerequisites
Concept Description
KMS Delivery
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1
Change Log
Made by
Version
SODE/
2.0
Commented
Approved by
Status
by
Issued to Tenderers for BAFO
27.06.2011
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2
Introduction
Introduction
The security in the message flow in ERTMS Level2 signalling systems is based on the
use of a secret key that must be known by both the train and the infrastructure. The
distribution of these secret keys (KMACs) is a problem due to the fact that KMACs -as
per current practise described in the UNISIG subset - are to be installed “manually”
during service periods in trains, and infrastructure.
This has lead to praxis in specific Level2 networks to simplify the KMAC usage by
having only one KMAC common for many trains and large infrastructure areas. This has
both Security and Safety implications because a secret shared by many parties is
effectively not a secret. A mismanaged KMAC can easily be compromised and thereby
potentially compromise the security of the railway network which in turn compromises
safety of operations.1
The large number of ETCS elements in the Signalling Programme that is supposed to
enclose the whole country makes it even more demanding to create an enhanced system
for handling and distributing KMACs.
In the following the “Online Key Management System Concept” (OKMS) for the Danish
infrastructure describes a Key Management System as a combination of a Key
Managemnet Centre and an online Key Distribution System. The aim is to keep the basic
functionalities of the current specifications as they are while enhancing the security of
using secret key pairs by enhancing the possibility to renew KMACs for entities in the
Danish domain.
It is our belief that OKMS will solve the security as well as the safety issues mentioned
and that the established system will be able to manage a large ERTMS Level2
infrastructure cost effectively.
2.1
Goals and prerequisites
The main goals for the OKMS concept are the following:
1. To implement a Key Management Centre that is:
o Interoperable
The secret key – the KMAC - is regarded as an obvious target for hackers and others
that might want to introduce threats, denial of service attacks or catastrophic events on the
rail traffic. As the railway infrastructure turns more and more towards IP based solutions
threats are to be found in new types of communities.
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o
provides the management interface for:
 Creating
 Renewing
 Disabling of KMACs
2. To implement a secure online service for the Danish domain entities based on
“Commercial Off The Shelf” products for IP based systems.
3. To implement an online distribution system for KMACs for the Danish domain
entities2 covering as both visiting trains and roaming trains.
Prerequisites
The prerequisites of the requirements are the following strategic decisions taken by the
Signalling Programme:
1. The Danish ERTMS infrastructure and OBUs must be enabled for packet
switched mobile data access (GPRS/EDGE) and thus consists of IP enabled
entities.
2. The security of transmission based on the Euroradio protocol must be enhanced
without impacting the protocol by allowing changing the security keys on a
regular basis or on request.
3. The KMAC distribution concept should be based on “One Unique KMAC per
OBU” in order to make it easy to revoke compromised KMACs.
4. Use of an asymmetric key (Private- and Public- Key) based online distribution
system for secured online data exchange instead of symmetric KTRANS keys, to
provide authenticity, integrity and confidentiality on distribution of files
containing KMACs.
5.
The Danish enhancements to the ERTMS standard must follow or be compatible
with ERA specifications. That applies particularly for the usage of packet
switching in ETCS (CR741) and the evolving specifications and
recommandations for off-line Key Management (SUBSET 114 and SUBSET
038).
2
Danish domain includes Danish Radio Block Centres (RBCs) and Danish OnBoard Units
(OBUs)
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2.2
Concept Description
The goals and prerequisites lead to the following subdivision of the OKMS concept:




2.2.1
Key Management Centre (KMC).3
Infrastructure for secure online distribution (PKI).
Key Distribution Centre functionality (KDC).
OnBoard enhancements for Online Key Distribution (OBU+).
Key Management Centre
The concept for the Management of KMACs in the Signalling Programme domain is
basically the same as in any other KMC:
 It must be interoperable
 it must provide the security consistent with the domain
 it must provide access management for trains visiting the domain and for own
OBUs that need to roam to foreign ERTMS domains
“Neighbouring KMC” domains EAST and WEST is and option in the Danish infrastructure. If
OPT 10 is chosen ther will be only one KMC in Denmark.
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The basic ERTMS KMC functionality of the KMC is to manage the secret key usage
(KMAC) in the domain of the KMC and to provide interface for KMACs that are to be
exchanged with foreign KMCs.
The KMC controls the use KMAC keys in the domain for as well domain OBUs and
RBCs as for visitors in the domain. The KMC is also managing foreign KMACs to be
used by national trains that need to visit foreign domains (roamers)4.
The user interface of the KMC should support creation of “secret key racks” for OBUs
and RBCs in the domain and be able to issue “immediate” and coordinated renewal of
“secret key racks” in security/safety critical situations. According to the evolving
SUBSET 114 it may also be required to handle lifetime confined KMACs in RBCs –
these and other details should be defined during the “Joint Design” process.
The KMC should be able to establish and update a “Domain” view of the KMAC usage.
The figure below is an example of how such a “domain view” may look5:
4
5
Roaming needs to be prearranged and secret kys exchanged in advance by the KMCs
KMACs to be used in RBC-RBC handovers should be specified seperately.
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DOMAIN TRAFFIC
NATIONAL TRAFFIC
RBCy.se
RBCx.de
RBCy.de
Remarks
KM1.dk
KM1.dk
KM1.dk
KM1.dk
KM1.dk
KM1.dk KM1.dk
KMx.de
KMy.de
DSB example
KM2.dk
KM2.dk
KM2.dk
KM3.dk
KM2.dk
KM3.dk
KM2.dk
KM2.dk KM2.dk
KMx.de
KMy.de
KMx.dk
KM2.dk
...
KMx.dk
KMx.dk
KMx.dk
KMx.se
KMy.se
KMx.dk
KMy.dk
KMx.nl
KMx.nl
KMx.se
KMy.se
KMx.dk
KMy.dk
KMx.nl
KMx.nl
KMx.se
KMy.se
KMx.dk
KMæ.dk
KMz.de
KMø.dk
KMæ.dk
KMå.dk
KMø.dk
RBCz.se
RBCx.se
RBC-RBC KMACK Left
RBC-RBC KMACK Rigth
RBCx.dk
VISITOR TRAFFIC
OBU x.se
OBU y.se
OBU x.de
OBU y.de
OBU x.nl
OBU y.nl
..
..
RBC3.dk
OBU 2.dk
OBU 3.dk
.
.
OBU x.dk
National OBU´s in foreign Domains
RBC2.dk
2)
1)
RBC1.dk
OBUid/RBCid
KMAC
OBU 1.dk
FOREIGN DOMAIN (ROAMING)
TOC example
KM2.dk KM2.dk
KMx.de
KMy.de
TOC example
1) RBC secret key rack
2) OBU secret key rack
KMx.dk
KMx.nl
KMx.nl
KMx.se
KMå.dk
RBCs uses KMAC Euroradio over IP during domain RBC-RBC HO
RBCs uses KMAC Euroradio over a joint design connection during borderline RBC-RBC HO
KMC-KMC KTRANS SE
KMC-KMC KTRANS DE
KMC-KMC KTRANS NL
KMC-KMC KTRANS BE
"
"
Figur 1 Domain view
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The table shows “OBU secret key rack”s in rows and “RBC secret key rack”s in
columns. The yellow and the green area shows the KMAC references to be used by
domain OBUs inside (yellow) the home domain and outside (green) in foreign domains.
The extensions of the KMAC references KMx.dk or .se or .de are denoting the origin or
the KMC that have issued the KMAC. The example reflects the possibility to have “one
KMAC per train” within the home domain and either having exported or imported
KMACs when roaming.
The blue area shows foreign OBUs that need to have access to the domain either using
imported keys in the KMC or keys issued by BDK to be used foreign trains.
2.2.2
PKI infrastructure for secure online distribution
As online distribution of KMACs and other online communications e.g. between Hand
Held Terminals and TMS, are containing safety critical data, special measures need to be
taken to communicate data securely.
It is envisaged that there is a need to set up a Public key Infrastructure server and to take
advantage of the TCP/IP protocol suite in order to secure authentication, integrity and
confidentiality on online communications.


2.2.3
The PKI should be using a de facto standardised hybrid-cryptosystem
including mutual authentication.
It is recommended that the system should be based on TLS6 as it provides a
high level of security and is well-known IP technology.
Key Distribution Centre (KDC) functionality.
The functionality of the KDC is to establish a distribution server for “secret key rack”
/KMAC data produced in the KMC to be delivered to domain OBUs and RBCs.
The KDC shall have access to KMC data and utilize the secure PKI infrastructure to
connect domain OBUs and RBCs and manage a download processes of the “secret key
rack”s.
The aim of current draft UNISIG SUBSET 114 is to specify an offline distribution
standard for KMAC data between KMC and OBU. Therefore the joint design process
should take SUBSET 114 into consideration.
6
Transport Layer Security (TLS) standardised by IETF in RFC 5246
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The KDC shall be able to send a “semaphore” to let an OBU/RBC know that an
immediate “secret key rack” update is necessary. The semaphore might be sent via SMS
for triggering the OBUs to look for new keys. The fact that the KDC and the RBCs are IP
connected makes it fast and easy to revoke a compromised key from the infrastructure
and thereby mitigate the risk of masquerading MAs. As the OBUs are not aways online
seen form the KDC a “network first” migration strategy should be preferred also in case
of a normal frequent key renewal procedure. RBCs should accept both “current and new”
KMACs over a migration period large enough for all OBUs to be updated.
2.2.4
OnBoard Online Key Management enhanced functionality
In order to provide support for Online Key Management a Danish domain OBU must
include special software to support the following key management process:

The EVC should activate an ”update secret key rack” procedure before the
Euroradio protocol is activated during the “open desk procedure”.7
or

The EVC should activate at any time the ”update secret key rack” procedure
caused be “a semaphore request” received e.g. via SMS .

The procedure8 should establish a TLS based session to the KDC and download
new versions of “secret key rack” tables. When the procedure has taken place the
normal procedure should be continued.
Note: When OBUs are travelling in GSM-R networks that do not support
GPRS/EDGE it will not be possible for the OBU to contact the home domain KDC.
As a consequence the OBU sw should not activate the update process if the ETCS
variable for GPRS support is not “true” (i.e. last ETCS session balise read).
2.3
OKMS Delivery
For the Fjernbane Infrastructure and On-board Contracts the Suppliers will be asked to
deliver an online Key Management System encompassing both ETCS Infrastructure, Onboard Units and other relevant parts of the Signalling System like TMS and Handheld
Terminals.
On-line secured solutions as mentioned above are commonly used in client server
solutions IP based solutions. The Customer will work for the timely specification of On7
The key update procedure must not conflict with interoperability ie be finalised before SoM is
initiated.
8
The protocol vher the OBU present its EVC-id and current ”key rack version” and eventually
downloads a new version (joint design issue).
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line Key Management for ERTMS in the framework of the ERTMS Users Group and the
Fjernbane Infrastructure and On-board Suppliers will be expected to jointly drive the
work on UNISIG side.
Requirements not covered by the evolving ERTMS specifications for Key Management
will be defined during the “Joint Design Phase” of the Signalling Programme.
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