sjl appnote 009 - ciphering and secure rrc decoding

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SJL AppNote 009 - Ciphering and Secure RRC Decoding

June 16, 2011

SJL AppNote 009 - Ciphering and Secure RRC Decoding www.sanjole.com 1

Ciphering and Secure RRC DecodingRevision 2.0

Introduction

This document, and its examples provide a preliminary introduction to the topic of ciphering in

WaveJudge, focusing on Secure RRC Decoding.

The examples are based on the following files: IJ Example.RTPHYSAP, IJ Example.FXL, and IJ Example K-

asme.TXT. The files are available athttp://support.sanjole.com/WaveJudge Captures Trial/.

This application note and the associated application are not in their final form and will continue to

evolve. They are currently based on version 4.2.14 of the WaveJudge software.
http://support.sanjole.com/WaveJudge%20Captures%20Trial/http://support.sanjole.com/WaveJudge%20Captures%20Trial/http://support.sanjole.com/WaveJudge%20Captures%20Trial/http://support.sanjole.com/WaveJudge%20Captures%20Trial/


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Table of Contents

Revision 1.0 ............................................................................................................................................... 1Introduction .................................................................................................................................................. 1Table of Contents .......................................................................................................................................... 2Introduction to Keys and Entities used in Ciphering ..................................................................................... 3Configuring Ciphering Support within WaveJudge ....................................................................................... 6

LTE UE Manager ........................................................................................................................................ 7LTE MAC Settings Tree (Settings hierarchy display) ................................................................................. 7

Ciphering process and logic ........................................................................................................................ 13Programming the UE Encryption Keys .................................................................................................... 16


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Introduction to Keys and Entities used in CipheringIMSIInternational Mobil Subscriber Identity

GUTIGlobally Unique Temporary Identifier

GUMMEIGlobally Unique MME Identifier

MMEIMME Identifier

MCCMobile Country Code

MNCMobile Network Code

MMEG(I)-MME Group Identifier

MMECMME Code

M-TMSIMME Assigned Temporary Unique Mobile Subscriber Id

KASME-Key Access Security Management Entity

ASME is defined in TS 33.401[1] as the entity in an access network that receives the top level

keys from the Home Subscriber Server (HSS). For E-UTRAN access, the MME assumes the role of the

Access Security Management Entity (ASME).

SRBSignaling Radio Bearer

DRBData Radio Bearer


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Below are the definitions of the different Keys


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The following is the key hierarchy in which they are derived:


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Configuring Ciphering Support within WaveJudge

To support Ciphering well review two tools in addition to the message list. The LTE UE Manager, andthe LTE MAC Settings.

Both of the tools are automatically populated, however once they are initally populated a cipher key

must be manually provided by the user. Once done the decoder can be rerun and secure messages may

be decoded.

Both tools are located on the tools drop down menu pictured below


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Based on the test configuration users of the WaveJudge 4900 will use LTE MAC Settings for Cell 0 or 1

and LTE UE Manager Cell 0 or 1. IntelliJudge users will use the INtelliJudge LTE MAC Settings and LTE UE

Manager IntelliJudge.

LTE UE Manager

The following is the LTE UE Manager Screen. Other than colors its not user configurable.

The fields are as follows

1. Indexindex of the entry

2. TypeClassifies the mobile element for a particular index entry, Broadcast, Unknown, Merged

and UE

3. IMSIInternational Mobil Subscriber Identity which the unique and permanent identity of a UE

4. GUTIGlobally Unique Temporary ID which identifies the UE and the MME involved in the

connection

5. ColorThe color applied to the RNTI value that corresponds to the associated index entry

LTE MAC Sett ings Tree (Settings hierarchy d isplay)

Below is the LTE MAC Settings Tree. Most of its fields are also preconfigured for convenience.

The user only needs to populate the proper KeySet with a K_ASME encryption key. However the user

may also add any or all entries to the tree if desired. Therefore the information on the controls and


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fields are presented below. This information will then be followed by a section covering UE discovery

and K_ASME configuration.

The LTE MAC Settings Tree appears as follows:

The following are controls available to the LTE MAC Settings configuration window.

The LTE Settings window consists of the LTE Settings Tree. The Tree consists of one leaf or attribute

off of the trunk RA-RNTIs and three branches or nodes identified as Decoders, UEs and MAC-Layers.

Under each branch there are other branches and leafs. Each branch may have one or all of the

following functions

1. Append Childwhich represents a brand

2. Attributesleafs on the branch


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3. Delete

4. Validate

5. Export

6. Import

The menu is accessed with a right button click while the mouse is hovering over the item to update

The Append Child function will identify all branches which have additional branches or attributes

associated with it. Attributes or branches of that child which are mandatory will automatically be

created. Optional as well as mandatory attributes may be added by the user.


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Adding Attributes:

Once attributes have been added the optional entries may be deleted, or their contents may becleared, or set from either the parents menu or the attributes menu as displayed below

Parent clearing an Attribute content:

Parent deleting and Attribute:


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Parent setting an Attribute content:

Export/Import Option

The Export/Import Option is available to every node in the LTE Mac Settings Tree.

Export/Import appends an .sxl extension to the file names.

Export will export everything under a node. For example if Export is performed at the Keysets Level

then the attributes CurrentKSI and NextKSI of KeySets and each KeySet child and their attributes will

be exported.

Exported files must be imported from the level at which they were exported. The following will be

displayed


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The LTE Settings tree may be updated with child branches containing UE data based on decoded from

the LTE Attach process.

The UE data is a super set of the information contained in the UE Manager. UEs are contained in the

UEs object. Each UE identified by a User Equipment object. The objects included are the IMSI, the GUTI

subdivided into its components parts and the Key Set information that are discovered based on

messages sent in the clear.

These are User Equipment Attributes

1. IMSI

2. K_UEEncryption Key for UE Data

The following are the GUTI Component Parts:

1. MCCMobile Country Code

2. MNCMobile Network Code

3. GUMMEIGlobally Unique MME Identifier Component

a. MMEG - MME Group Identifier

b. MMECMME Code

4. M-TMSIMME Assigned Temporary Mobil Subscriber Identifier

The following are the attributes of the Key Sets Child

1. CurrentKSI - Current Key Set Identifier

2. NextKSI - Next Key Set Identifier

3. Key Set Child

a. KSI = Key Set Identifier

b. K_ASMEBase Encryption Key

c. NASContextSecurity setting for NAS traffic

d. eNBContextSecurity settings for eNobeB traffic

e. SRBContextSecurity settings for signal channel traffic

f. DRBContextSecurity settings for data channel traffic

Each of the different security contexts contain the following attributes

1. Cipher Algorithm

2. Integrity Algorithm3. Cipher Key OverrideCypher Key to use when communicating in this context

4. Integrity Key OverrideIntegrity Key to use when communicating in this context

The selectable Cipher Algorithms are:

1. EEA0

2. EEA1

3. EEA2


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4. EEA3

5. EEA4

6. EEA5

7. EEA68. EEA7

The selectable Integrity Algorithms are:

1. EIA0

2. EIA1

3. EIA2

4. EIA3

5. EIA4

6. EIA5

7. EIA6

8. EIA7

Ciphering process and logic

The following is the process of discovering the UEs, and configuration of their K_ASME field.

During the first pass of the captured data through the decoder the RRCConnection Request and

RRCConnection Reestablishment Request messages on different RNTI are monitored. Their receipt

creates UE entries in the UE manager with an associated color and key code.

Next the RRConnection Request is checked for the ue-Identity field which is one of the following

1. 40 bit Random Value2. S-TMSI

If the message includes an S-TMSI field its utilized to fill in the MMEC and the M-TMSI fields of the

GUTI. Otherwise a blank entry is created. From this point on all messages received on the

associated RNTI will display the RNTI using the color configured. Upon receipt of the next

RRCConnection request the color associated with the RNTI may change.

3. Attach Request.

Following the RRCConnection request an Attach Request may be received. If a UE doesnt exist for

the Attach Request messages RNTI a UE entry is created. Next the messages Old GUTI/ IMSI field ischecked. If an IMSI value is present its used to populate the IMSI field, otherwise the GUTI field is

populated if it doesnt already exist.

Below is a decoded Attach Request. Contained within it is an expansion of the Old GUTI/IMSI field.

In it is the IMSI identity field.


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Once this information is obtained the decoder populates the LTE Mac Configuration tree with UEsand the attributes available

This may include

IMSI

GUTI

Key Sets


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The Keys Sets created are empty containers with place holders for the attributes required. The Key

Set is identified in the NAS Security Mode Command pictured below. Different Security Mode

Commands may identify Key Sets, Security Algorithms, and UE security capabilities. The user mustprovide the Authentication key for the key set used. Typically the Key would be the K_asme since all

other keys would be derived from the K_asme. If there are problems using the Kasme, the user has

the option to configure the Cipher Key Override, and or Integrity Key Override for each of the

following: NASContext , ENBContext, SRBContext, or DRBContext.

Below the Security Mode Command decode identifies Ciphering Algorithm EEA1, Integrity Algorithm

EIA1 and NAS Key Set 5 to be used


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Programm ing the UE Encryp t ion Keys

Once the UE Managers UE table and the LTE MAC settings tree has been populated the encryption

key identified by the user may be programmed into the K_ASME field for the associated UE and itskey set.

To identify the UE to configure look through the UE manager list for an entry with a valid ISMI or

valid GUTI.

Item 1 on index 4 represents a valid IMSI. Item 2 represents a valid GUTI. Both were received

through the Attach Requests. The first case represents a UE that has no knowledge of a GUTI, the

second case represents a UE re-entering a network or roaming to another network therefore it has a

previously received GUTI which may be replaced by the MME.

Heres an entry in the LTE Settings tree identifying a UE and the fields that should be matched whenprogramming the K_ASME of a UE

1. The IMSI code corresponding to item 1 above

2. The id of the next Key Set of 5

3. Key Set 5


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Once found Clear and Set the K_ASME attribute.


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After configuring the K_ASME field and clicking the OK button the decoder will run again, and

decode the encrypted data.

Below, are the before and after results of configuring the Cipher Key.


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The following is a message decode prior to adding the key; in the validated field the message No Key is

displayed:


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After adding the key, the UE manager is updated with the GUTI associated with the UE we configured.

The GUTI was obtained from the Attach Accept Message.

In addition the UE in the LTE MAC Settings has been updated with a GUTI and various context

entries have been created, although empty.

Now theres an updated message list. Among messages included, are the various encrypted Security

Protected NAS Messages which includes the Attach Accept message which became visible and

allows the decoder to extract the GUTI. Also visible is the Attach Complete and EMM Information

message. Additionally, the Validated field indicates a key is associated with the Service Request

message.


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The updated message list:

sjl appnote 009 - ciphering and secure rrc decoding

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