Data Network Security : Part I Problem Survey and Model

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Rolf Blom Viiveke Fåk Ingemar Ingemarsson INTERNSKRIFT LiTH- ISY-I-0176

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l ' l CONTENTS l. Introduction

2. A Communication Scenario; Proteetian Problems

3. Threats

4. A Communication Model

5. Encryption

6. Authenticators

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l. INTRODUCTION

Data encryption and related methods may be used to pre-serve information security in a data network. Here information security is defined as the degree to which the destruction, change or loss of information is pre-vented. Information is defined as the content of the message represented by the data. The information in a block of data is unchanqed if the intended result of the transmi ssion of the block i s obtained. This means for example that the original message reaches the correct destination where i t is interpreted as intended. Un

-disturbed information does not, in general, requi re un -disturbed data.

The network is s~pposed to be a public network, accessed by many different users. We are interested in a wel l -defined group of users who are cornrnunicating mainly among themselves. Different groups, however, are also allowed to cornrnunicate in a well defined manner. The logical structure of the communication within a group is star-shaped. The information cornrnunicated within the group shal l be protected against threats from other users of the network, from i l legitimate users (wi retappers etc) and from mernbers in the group. The structure of the

threats is described in section 3 of this paper.

The network itself and the requirement i t imposes are supposed to be unchanged. Encryption and decryption are taking place outside the network. The encrypted data shall comply with the requirements of the network. The cornrnunication process in the group consists of time -l imited messages which are essentially transmitted from one point to another in the network. This is the basis for the model of the communication which is described in seetian 4. The model, although simple, enables us to struct ure the problems in connection with encryption/ decryption. This is done in seetian 5 and 6.

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1. 2

The purpose of the paper is to form a basis for syn -thesis of security rneasures by rneans on cryptological rnethods. The analysis is general enough to be applied

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2. A COMNUNICATION SCENARIO; PROTECTION PROBLEHS

When you first look at all the details, that are involved in protection of data communications, you will probably find i t hard to make heads and tails of it. Differ ent systern architectures, arnbigous use of the nornenclature and other di fficulties add to the general confusion.

As an exarnple we can look at an irnaginary syst ern with a hos t cornputer, a front-end communication cornputer, a package switching network with concentrators, a small local cornputer at a branch office and an intelligent ter-minal. An application in the host cornputer generates a rnessage, which will be displayed on a terminal in the branch office. The appl ication takes the original string of text characters and adds a check-sum to i t . This

longer str ing of characters, i s passed to one operating systern, where a general block surn is added to the rnessage. The rnessage i s then passed to an I/0-handler, which hap-pens to be a rernote cornrnunications handler. This routine attaches the pararnetric information about destination

and sender directly to the rnessage, adds a sequence nurnber, and sends the bunch of characters to the front-end corn -puter. There the actdressing information is rernoved and transforrned, the rnessage i s divided into packages and each package is given additional protocol information with addresses, sequence number within message, check surns a.s.o. The packages are sent to the nearest con-centrator, where some checks are made, erroneous packages are signal led to be retransmitted, and the whole bunch is finally one by one passed to the local computer. There the checks in the concentrator are performed again as well as some additional ones, which are peculiar to the specific front end - local computer comrnunication. The packages are stripped of their protocol and merged into a single rnessage, whi ch i s provided wi th a new protocol and passed to the intel l igent terminal. There all the

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2.2

remaining protocols with their controls are pealed off one by one and the message is finally displayed on t he terminal.

This tiny novel about the life and adventures of a message in a complicated system simply serves to show how diffi -cult i t can be to analyze such a situat ion, if i t is

viewed in its entirely. The important lesson to be learned is that communication eecurs at different levels. What is a mere message at one level is a message plus de -tailed protocol information at the level above. The link level protocol is common t o everyone using that network, but not necessarily to anyone else . The front end computer level protocol is common to everyone com -municating with that computer, but not necessarily t o users of other cbmputers. The application program's formats and controls are common to every terminal com -municating with that application, but not necessarily to other terminals and applications. Thus, as one goes up -wards in the levels, the "message" shrinks and more and more parts are found to be higher level protocols. But each level will add, control, and remove only its own protocol information. Lower level protocols are already pealed off or not yet added, and higher level protocols are just a part of the message. This makes i t possible at each level to identify sources, where messages are received from higher levels or generated and protocols are added, nodes, where the protocols are just used and receivers, where the protocol is used and removed from the message.

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Message

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\ Applica tion Terminal

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\ Host general routines {:} Terminal

Front end Local computer

Modulated signals

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3.1

3. THREATS

Implementation of security measures in a data network

aims at protection against and/or discovers of illegi-timate manipulation of the data flow in the network. The threats that occur can generally be classified into the following categories:

a) Passive wiretapping

b) Substitution of messages

c) Insertion of messages d) Detection of messages

By passive wiretapping we mean that a record of trans-mitted messages is obtained. Such a record of messages and protocol info'rmation can give away sensitive infor-mation. For example, cleartext messages can go public,

traffic-analysis may reveal a company s modus operandi

and hints of how to make an intrusion into the network can be obtained. Passive wiretapping is also the basic

threat because i t is a necessary tool in effectuating

the threats of substitution, insertian and detection

of message, i t is necessary to know if there is a message or not.

A common collective term for the last three threats b, c and d is active wire-tapping. It is called active,

because the threat is that the stream of data is changed

in some way. And the purpose of this change of data is

to con the intended reciever into doing something diffe-rent from the right thing. In a data network handling bank transactions we can exemplify the threats by:

b) When a customer makes a deposit, a change of amount of money or of account number in the message is a sub-stitution threat. c) If the message of the deposit is

fed into the network another time this is an insertion

threat. d) If a withdrawal is made and the message is detected i t is a detection threat. The objective of

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the wire-tapper is evident in t he exaroples above .

One should not interprete wire-tapper l iteral ly in the

terms active and passive wire-tapping. The threats are

just as actual in any computer or concentrator that is

used in the network. Instead of making a "simple" con -nection to a transmission l ine the mysterious world

of trapdoors and trojan horses in computer programming

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IT (l NIT 4.1 4. A COMMUNICATION HODEL

A typical structure of the conununication system used by the group of users is shown in figure 4.1.

1 Main compute Main computer of a different group Data network

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/ comput e r i l

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intelligent t erminal

(cabable of data pro-cessing)

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non-intelligent

t erminal IT Lo c al computer

l:J----1

NIT

'---~ Other peripheral uni ts

Figure 4.1 Communication system

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Typically the communication consists of tirne- l irnited

rnessages transrnitted one-way between two points in t he

network. These points rnay be for exarnple a terminal

and the local cornputer, a terminal and the rnain cornputer

or a point in the local cornputer and the rnain cornputer.

Tirne-lirni ted cornrnunication bebtleen two different rna in

cornputers is also all owed, as indicated by the cotled

line in figure 4.1.

The sirnplest way to describe the transmission of each

rnessage is done in the frarnework of the rnodel in figure 4. 2.

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Node

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Receiver

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Figure 4.2 Cornrnunication rnodel

Here the node irnposes certain restriction on the

corn-rnunication between the source and t he receiver. The

node rnay for exarnple include a local cornputer where the

rnessages are processed. The processing requires certain

portions of the rnessage to be clear text (i.e. non

-encrypted). Or the node includes the public data network

with its requirernents on formats, address information etc.

The source output rnay represent rnany different points in

the cornrnunication systern. One of the most extreme cases

i s when the source output is the input data to the ter

-minal. The source output rnay also be the outpLt from the

terminal or the result of a part of the processing in

the local cornputer.

The information in'the rnessage from the source is divided

into two parts: the node-sensitive and the node- insensi

-t ive information. The node-sensitive information is

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4. 3

is not. This distinction enables us to distinguish

be-tween three types of information protection methods:

l. Line encryption, used only between source and node

or between node and receiver in figure 4.2.

2. Message-encryption, that is encryption of node- insen -sitive information, can be used all the way through

the node.

3. Verification data. This can be various forms of data

used for example for error detection, verification

of message origin etc.

All these methods can be used at each of the levels

men-tioned in seetian 2. It should be noted that line encryp -tion, i.e. encryption of every character leaving the source, at one level is equivalent to encryption of only

node-insensitive information on the level below. The

difference is that in the former case encryption is per -formed before the information is passed across the inter -face to the lower level, and thus the responsibility is

with the high-level. In the latter case encryption occurs

after the interface and the lower level has the responsi

-bility for that protection.

Thus i t is possible to find a way through the problems

initially mentioned simply by performing the following steps:

l) Identify the levels in the system.

2) Apply the communication model to each of the l evels

established in step l . Find their sources, where a "message", i.e. node- insensitive information is given

protocol information, i.e. node-sensitive information,

their nodes, where node-sensi t ive information is used,

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is rernoved.

3) Study the possible rnethods of protection and list

carefully what threats t hat counter at each level.

4) Study the lists made in step 3. Tick off every rnethod,

that is indispensable at some level, because i t

offers protection, which can~t be obtained by any other

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rnethod at any level. Also tick off this threat and

any other threats that are met by the rnethod. Take a look at the rest of the threats, and pick out, if po

s-sible, a cornbination of rnethods at different levels

that is optimal in the sence that i t covers the re

-maining threats at a minimum cost of investments,

com-puting t ime and inconvenienc

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e to t he users and rnain

-taining staff of the systern.

5) Take the lists from step 2 and use thern to identify the points in the system, where the methods found in

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5.1

5. ENCRYPTION

The distinction between line- and message encryption is

not important in this section, where we discuss different

requirements on the encryption algorithm and its use.

Encrypt ion algorithms can be divided into two different

classes, namely: blockciphers and running key ciphers.

A blockcipher takes fixed size blocks of symbols and

performs a transformation on the block. The

transfor-mation does not differ between blocks, that is the key

is the same for all blocks. A running key cipher also

works on fixed size blocks of symbols. The blocks may

contain l or rnore symbols. On each block a transformat ion

is performed but the transformations differ between blocks.

This change of transformation is goverened by a sequence

of keys. Thus the first data block is enciphered with the

first key and so on. We observe that strict syneranism

must be kept between the key and the data sequence, when

encryption and decryption is done. This is not the case

for block ciphers.

In general a running key cipher is concidered to be stronger

than a block cipher. This is partly due to the fact that

a block cipher will transform a typical message the same

way every t ime i t is sent, while a running key cipher

will not show this property. Some specific counter mea

-sures such as block chaining exist, but they have a cost

in that additional data must be added to the message.

To be a good block cipher the block size should allow at least 260 different keys. That is the block should

con-tain at least 20 bits. (A tacit assumption has been made

that we work on binary data). Due to difficulty in con

-strueting practical enciphering algorithms the blocks will

contain substantially more than 20 bits when the number

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of a block cipher contains a large number of bits.

This can be good, when the block size or multiples of

i t approxirnately matehes the length of rnessages to be encrypted. But when a smal l nurnber of bits, for exarnple

a character of 8 bits, needs to be encrypted we get an u

n-wanted expansion of the rnessage, which degrades systern

perforrnance. On the other hand a running key cipher can

work on very small blocks without leosing cryptographic

strength, but then we have a syncronisation problem.

Thus there are pros and eons for both rnethods and which

rnethod to use must be answered for each specific situa

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6.1 6. AUTHENTICATORS

Authenticators should detect any attempt to alter the

scquence of messages. Alteration by removal of a message

can be detected only if the messages are held tagether

either by counters or by repetition of a part of a message

in the next message. Both these methods should rather be

regarded as a kind of protocol than direct authentica

-tors. But both of them also adds information, which must

be protected from alteration. Thus, once one of these

methods has been applied, authenticators proteet against

any subsequent, undetected alteration of the message

stream.

As was noted in seetian 4, every message leaving a

source consists ef two parts. The first part comes from

the level above (or from the outside world) . It is just a sequence of bits to the source. To that sequence the

source adds information, which will be used by nodes and the receiver on the same level. This latter part consists

of different data items, where the rneaning and purpose is

cornpletely clear to the source. One of these iterns may

be an authenticator of the rest or only a part of the

message. With disregard of the actual physical placing

of the pars, we can picture the rnessage as in figure 6.1.

Node-insensitive information Figure 6.1

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Node- sensltive information

Authenticators can be used for

a) the node-insensitive information only

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c) the whole rnessage as i t is about to leave the source~

The node-insensitive information can;t be further sub

-divided, and hence authenticators for i t should give

the same arnount of protection to every bit of it. The

node-sensitive information consists of pieces of known

value, and hence only parts of i t rnay be picked out as

worthy of protection. If the whole rnessage is to be

authenticated, i t is not very l ikely that any part of

i t should be left out. In all three cases a certain nurn

-ber of bits will be delivered to a procedure which fabri

-cates the authenticator. This can be regarded as delive

-ring an input x to a funktion f in order to get the out

-put y= f(x). If sameene wants to insert a rnessage x

inta the strearn of valid data, or if he wants to change

a rnessage x into'x1, he also has to find the correct

autenticator y 1 = f(x 1 ). Hence f can;t be a publicly

known function of any sort, since that would allow any

intruder to campute y1 and thus get his rnessage authenti

-cated and accepted. f can then be assurned to be a function

of two variables, f(k,x). x is then the rnessage, and k is

a secret key, which is known only to the source and re

-ceiver and perhaps also the node.

If rnessages can be inserted and altered, they can als o be

intercepted and analyzed. Just as y is a function of k

and x, all possible pairs of x and the i r y are a func

-tio n of k. If this function is invertible, we can c om

-pute k from known pairs (x,y) . The ideal is if (x,y) =

= f 1 (k) is a one-way function, which rneans that i t can;t

be inverted no matter how rnany valid (x,y)-pairs that

are known. If this ideal can;t be achieved, we have to

resort to holding the bastions as lang as possible. This

rneans that the cornputation of k = f-1(x,y) should be

as time-consurning as possible. Once i t is done i t should

turn out to have rnany possible solutions. Every pair

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6.3

If the latter didn; t hold, we could use any of the keys

in the first solution and be sure to get the correct y1 = f(k,x1) . All this can be

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rized

thus:

The authenticated data x are sent through a function

to get y = f(k,x)

k is a secret key, which is ehosen from so many alter -natives, that noone is likely to guess t he correct value.

f is so constructed that i t is highly unlikely that f(k

1,x1)

=

f(k2,x2) if k1

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k2 or x2

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x2

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k

=

f (x,y) should preferably not be computable.

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If k

=

f (x,y) is computable, i t should have so many solutions for each pair (x,y) that i t still is unlikely to pick the right key from the solut ions. Also, in order to weed out one remaining correct key from different

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f (x.,y.), a great many pairs (x.,y.) should be needed.

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7. CONCLUDING REM~RKS

To summarize the ideas presented previous1y in the paper, consicter figure 7.1. It shows in a schematic way the

threats and counter measures that have been discussed.

Source 1ine encryption message encryp-tion authenticators Nod e

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assive wire-tapping Substitution of message Injection of rnessages Dete4tion of messages

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Figure 7.1 Threats and counter measures in a data network

Let us take the counter measures one by one and discuss

the effect i t has on the possibility to carry out the

different threats. Line encryption gives good protection

against passive wire-tapping a1though i t probab1y can~t

hide information about whether or not a message i s sent on the transmission 1ine. However, which message that

is sent, is not revea1ed. In spite of this a chance attach

of active wire-tapping may succeed, if no other counter measures exist. For examp1e, injection of previous1y re

-corded messages or detection of messages can remain un

-noticed. It wi11 certain1y remain unnoticed if no mes

-sage sequence information exists. A1so substitution of a

part of the encrypted message that does not contain node -sensit ive information may not be discovered. Thus

line-(

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7.2

encryption and for the same reason, rnessage- encryption

should be cornbined with use of authenticators. Another

reason for this is that normal transmission errors also will be de tected by the authentici ty control.

Message-encryption will give the node-insensitive infor

-mation protection against passive wire-tapping, but as is said above i t should be cornbined with use of

authen-ticators. If no line-encryption exists the node-sensi -t ive information is revealed to a wire-tapper, which

will give hirn an opportunity to learn how the network

operates. Even if the node-sensit ive information is

pro-tected in a way that makes i t irnpossible to carry out any

active wire-tapping threats without discovery, the fact that node-sensitive information is in clear text rnay make i t easy to jam the systern into a deadloch. This is a threat that must be seriously considered when the ad-ministrative routines of the network are designed.

As have becorne obvious from what is said above,

authen-ticators is a fundamental counter rneasure. It can~t pro

-teet against passive wire-tapping, but i t is basis for protection against all active wire- tapping threats.

Up to now, we have talked about encryption without a single reference to how keys for enciphering and deciphe -ring should be rnaintained and distributed in the network. The same holds true for pararneters in authenticator func -tions. This is quite obviously a very irnportant problem, but its solution can~t be given in a general form. The

di stribution and handling of, let us cal l i t, security parameters, in the network, must be considered in