computer network security

Computer Network Computer Network SecuritySecurity

Hyun-Sung KimHyun-Sung KimDept. of Computer EngineeringDept. of Computer Engineering

Kyungil UniversityKyungil Universitykim@kiu.ac.krkim@kiu.ac.kr

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Computer Network SecurityComputer Network Security

Hyun-Sung Kim, Kyungil UniversityHyun-Sung Kim, Kyungil University

Necessity of network security Services for network security Security techniques for Internet

service Secure Internet banking

example

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Necessity

Web service, Mail service, Telnet service…

Router

Client B

Mobile Node

Internet

mail server

web serverTelnet server

Router

Client A

Basic concernsBasic concerns

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Necessity

Basic Scenario Request services to a remote server

– Attack : Make the server down by requiring multiple service requests

Router

Client B

Mobile Node

Internet

mail server

Router

Client A

– Defense : Heavy traffic control– Security hole : Other kinds of

attacks that could break down the server

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Defense(Firewall)

Blocking incoming access by potential attackers– IP check, Port check

Router

Client B

Mobile Node

Internet

mail server

Router

Client A

Firewall

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Defense(IDS)

Detecting unauthorized access to a computer network– Packet analysis, Event analysis

Router

Client B

Mobile Node

Internet

server

Router

Client A

Firewall

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Internet

6. Report 5. Database 4. Alert 3. Log 2. Detection

Engine 1. Network

Packet

Defense(N-IDS)

Router

Mobile Node

Client AFirewallIDS

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6. Report

5. Database

4. Alert

3. Log

2. Detection Engine

1. Network Packet

Defense(N-IDS)

Internet

Data linkheader

Internetheader

Transportheader

Applicationheader Data

… …

srcport

dstport

“SYN FIN SCAN Attack”

SYN FIN SCAN Attack was detected from 155.230.90.99to 203.230.91.25 at 23:00 34 June 2004

Storing data for the data forensic

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Defense(Virus)

Detecting instruction sequence for lots of types of virus– Checks all the files on disk and instructions in memory

Router

Client B

Mobile Node

Internet

server

Router

Client A

Firewall

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Is that all about the security ?

InternetInternetRouter

Client B

server

Router

Firewall

Router

Mobile Node

Client AFirewallIDS

Other concernsOther concerns

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Necessity

Scenario 1 Access to a remote server by

Telnet

– Attack : Illegal user try to login a Telnet server

Router

Client B

Mobile Node

Internet

mail server

Router

Client A

– Defense : Three times of login failure check

– Security hole : Un-continuous login attempts

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Necessity

Scenario 2 Access to files which has no permission

– Attack : Unauthorized user try illegal access to files

Router

Client B

Mobile Node

Internet

mail server

Router

Client A

– Defense : Role control– Security hole : Is there any method

to break the defense mechanism?

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Necessity

Scenario 3 Sending a very important

information over Internet

– Attack : Illegal user try to listen the information

Router

Client B

Mobile Node

Internet

mail server

Router

Client A

3510 2211…

– Defense : Encoding & decoding– Security hole : Is there any method to

break the defense mechanism?

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Necessity

Scenario 4 Sending a very important information

over Internet

– Attack : Illegal user try to modify the information

Router

Client B

Mobile Node

Internet

mail server

Router

Client A

3510 2211…

– Defense : Encryption

– Security hole : Is there any method to break the defense mechanism?

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Necessity

Scenario 5 Repudiate what he did

– Attack : User denies what he did

Router

Client B

Mobile Node

Internet

mail server

Router

Client A

3510 2211…

– Defense : Signature

– Security hole : Is there any method to break the defense mechanism?

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Relation of service and mechanism

E-money E-contract Intrusion DetectionE-commerce Biometric Mobile SecurityE-auction Secure Multimedia VPNE-vote Firewall

AlgorithmsDES AES SEEDElGamal RSA ECCHash Function PRG

MechanismsEncryption Digital SignatureAccess Control AuthenticationKey-Exchange

ServicesAuthentication Non-repudiationAccess Control ConfidentialityIntegrity

Applications

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Security services

Authentication -> Scenario 1 Access control -> Scenario 2 Confidentiality -> Scenario 3 Integrity -> Scenario 4 Non-repudiation -> Scenario 5

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Security services

Authentication– An assurance that the identity is not

false– Ensures that the origin is correctly

identified Non-repudiation

– Requires that neither the sender nor the receiver of a message be able to deny the transmission

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Security services

Confidentiality– Ensures that the information are

accessible only by authorized parties Integrity

– Ensures that the only authorized parties are able to modify information

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Mechanisms

Encryption Digital signature Authentication Key-exchange

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Mechanisms

EncryptionEncryption– DES, AES, SEED, ElGamal, RSA, ECC

Digital signatureDigital signature– Public-key cryptosystem

AuthenticationAuthentication– Public-key cryptosystem

Key-exchangeKey-exchange– Diffie-Hellman key-exchange protocol

CryptographyCryptography

ConfidentialityConfidentiality

IntegrityIntegrity

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Cryptography

Internet

Router

Client B

server

Router

Firewall

Router

Mobile Node

Client AFirewallIDS

Un-secure channel => Secure Channel(Symmetric-key and Public-key systemSymmetric-key and Public-key system)

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Cryptography

Symmetric-key cryptosystemSymmetric-key cryptosystem

EncryptionEncryptionAlgorithmAlgorithm

plaintext plaintextciphertext

The same key(K)The same key(K)

DecryptionDecryptionAlgorithmAlgorithm

CC=E(=E(PP,K),K) PP=D(=D(CC,K),K)Sender Receiver

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Cryptography

Symmetric-key cryptosystemSymmetric-key cryptosystem– Caesar CipherCaesar Cipher(Basic scheme)(Basic scheme)

KeyKey => 3

Plain text : meet me after the partyCipher text : phhw ph diwhu wkh sduwb

Encryption algorithm : Addition Decryption algorithm : Subtraction

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Cryptography

Symmetric-key cryptosystemSymmetric-key cryptosystem– ProblemsProblems in Caesar Cipher

• The key size is so small : -25 ~ 25 (about 50 keys)• Weak at the brute force attack

– SolutionsSolutions• Enlarge the key size• Apply more complex operations

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Cryptography

Symmetric-key cryptosystemSymmetric-key cryptosystem– DES(Data Encryption Standard)DES(Data Encryption Standard)

• Key size : 56 bits• Basic operations

– Transposition– Substitution– XOR– Shift

• USA standardUSA standard – DES : from 1977 to 1998– AES : 2001 draft

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DES(Data Encryption DES(Data Encryption Standard)Standard)

Initial permutation

Round 1

Round 2

Round 16

32-bit swap

Inverse IP

Permuted choice 2

Left circular shift

Permuted choice 1

64-bits 64-bits plaintextplaintext

64-bits 64-bits ciphertextciphertext

56-bits 56-bits keykey

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DES(Data Encryption DES(Data Encryption Standard)Standard)

32-bits32-bits 28-bits28-bits

Ri-1 Ci-1 Di-1

28-bits28-bits32-bits32-bits

Li Ri Ci Di

substition/choice(S-box)

permutation/contractionpermuted choice 2

expansion/permutation

left shift left shift

permutation

48484848

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Cryptography

Symmetric-key cryptosystemSymmetric-key cryptosystem– The strength of DES

Key size Number of One Encryption 10Key size Number of One Encryption 1066 Encryption Encryption Alternative Keys per micro sec per micro secAlternative Keys per micro sec per micro sec

32bits 223 = 4.3 * 109 35.8 minutes 2.15ms 56bits 256bits 25656 = 7.2 * 10 = 7.2 * 101616 1142years 10.01h 1142years 10.01h 128bits 2128bits 2128128 = 3.4 * 10 = 3.4 * 103838 10 102424years 5.4 * 10years 5.4 * 101818 years years

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Cryptography

Is DES secure enoughsecure enough?– No!

• There are potential weaknesses• Key size is not secure enough

Is there any alternativeany alternative?– Yes!

• Enlarge key size from 56 to 128 => Triple DES

• AES

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Cryptography

Symmetric-key cryptosystemSymmetric-key cryptosystem

plaintext plaintextciphertextDecryptionDecryptionAlgorithmAlgorithm

C=E(P,K)C=E(P,K) P=D(C,K)P=D(C,K)Sender Receiver

The same keyThe same key

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Cryptography

Public-key cryptosystemPublic-key cryptosystem

plaintext plaintextciphertext

Public-key(PUPublic-key(PURR))

CC=E(=E(PP,,PUPURR)) PP=D(=D(CC,,PRPRRR))Sender Receiver

Private-key (PRPrivate-key (PRRR))PUPURR PUPUSS

PRPRRRPRPRSS

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Cryptography

Public-key cryptosystemPublic-key cryptosystem– RSA(Rivest, Shamir, Adleman)RSA(Rivest, Shamir, Adleman)

Input size : less than or equal to nEncryption : C = Me mod nDecryption : M = Cd mod n = (Me)d mod n

Public-key = {e,n}, Private-key = {d,n}

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Cryptography

Public-key cryptosystemPublic-key cryptosystem– RSA(Rivest, Shamir, Adleman)RSA(Rivest, Shamir, Adleman)

* Key Generation• Select p,q, both prime• Calculate n=p*q• Calculate (n)=(p-1)(q-1)• Select integer e, which gcd((n),e)=1; 1<e<(n)• Calculate d, d=e-1 mod (n)• Public key = {e,n}, Private key = {d,n}

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Cryptography

Key GenerationKey Generation– Select p,q, both prime– Calculate n=p*q– Calculate (n)=(p-1)(q-1)– Select integer e, which gc

d((n),e)=1; 1<e<(n)– Calculate d, d=e-1 mod

(n)– Public key = {e,n}{e,n}, Private

key = {d,n}{d,n}

ExampleExample– p=7, q=17– n=pq=7*17=119 (n)=6*16=96– e=5– Determine dde = 1 mod 96d = 77, 77*5=385 mod 96– Public key = {5,119}{5,119}, Priv

ate key = {77,119}{77,119}

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M=19M=19 6666

Sender Receiver

{5,119}{5,119}EncryptioEncryptionn

191955 mod mod 119119

191955=2476099/119=2476099/119

20807 with a remainder20807 with a remainder

of 66of 66

{77,119}{77,119}

66667777 mod 119 mod 119

DecryptioDecryptionn M=19M=19

RSA(Rivest, Shamir, Adleman)RSA(Rivest, Shamir, Adleman)

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Cryptography

Public-key cryptosystemPublic-key cryptosystem– The security of RSAThe security of RSA

• Brute force• Mathematical attacks

– The factoring problem, finding large two primes from p and q

• Timing attacks

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Cryptography

Is RSA secure enoughsecure enough?– Yes!

• But, requires the large key size, 1024

Is there any alternativeany alternative?– Yes!

• ECC with much less key size, 160 bits

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Cryptography

Symmetric-key Symmetric-key systemsystem– One key is used

– Disadvantage• Difficult to share the

– Advantage• High speed,

substitution and transposition

Public-key systemPublic-key system– Two keys are used

– Advantage• Easy to share the

public key

– Disadvantage• Low speed,

exponentiation

Digital signatureDigital signature

AuthenticationAuthentication

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Cryptography

Public-key cryptosystemPublic-key cryptosystem– Digital signature and Digital signature and

authenticationauthentication

plaintext plaintextSignedtextDecryptionDecryptionAlgorithmAlgorithm

Public-key(PUPublic-key(PUSS))

PP=D(=D(CC,,PUPUSS))Sender ReceiverCC=E(=E(PP,,PRPRSS))

Private-key (PRPrivate-key (PRSS))PUPURR PUPUSS

PRPRRRPRPRSS

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Encryption vs. Digital Encryption vs. Digital signaturesignature EncryptionEncryption

Sender Receiver

PUPURR PRPRRR

Sender Receiver

PRPRSS PUPUSS

Confidentiality withConfidentiality with

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First step for Digital signatureFirst step for Digital signature

Sender

PRPRSS

Receiver

Second step for EncryptionSecond step for Encryption

Sender

PUPURR

PUPUSS

PRPRRR

Non-repudiationNon-repudiation

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Cryptography

Public-key distribution with a Public-key distribution with a trusted third party(Certificate trusted third party(Certificate authority)authority)

User A User B

PUPUAA

CCAA==EEPRPRCACA[Time[Time11,ID,IDAA,PU,PUAA]]

PUPUBB

CCBB==EEPRPRCACA[Time[Time22,ID,IDBB,PU,PUBB]]

PUPUCACA PUPUCACA

Key exchangeKey exchange

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Cryptography

With a certificateWith a certificate

Sender

PUPURR

KeKeyy

Receiver

KeKeyy

Ciphered Ciphered KeyKey

PRPRRR

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Cryptography

Diffie-Hellman key exchange protocolDiffie-Hellman key exchange protocol

Select Select private private XAXACalculate Calculate public YApublic YAYA=YA=XAXA mod qmod q

Select Select private XBprivate XBCalculate Calculate public YBpublic YBYB=YB=XBXB mod qmod q

User AUser A User BUser B

Generate secret keyGenerate secret keyKK=(YB)=(YB)XAXA mod qmod q

Generate secret keyGenerate secret keyKK=(YA)=(YA)XBXB mod qmod q

Secure Internet Secure Internet BankingBanking

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Secure Internet Banking

User authentication Issue a certificate Key-exchange Transaction Additional security with a secret

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User authenticationUser authentication

Banking serverBanking serverAccept or rejectAccept or reject

Login requestLogin request(ID, Password)(ID, Password)

SSLSSL

CACACertificate RequestCertificate Request

Issues a certificateIssues a certificate

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Key exchangeKey exchange

Banking serverBanking server

Exchange a keyExchange a keyfor the sessionfor the session

E(PUE(PUC C ,Key),Key)CertificateCertificate

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TransactionTransaction

Banking serverBanking server

CC=E(M,=E(M,KeyKey))

CC=E(M,=E(M,KeyKey,,Alt.KeyAlt.Key))

Secret CardSecret Card

1:8975 2:1348 3:1796 4:2465 5:2696 6:3147

…28:1323 29:1369 30:1416

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Relation of service and Relation of service and mechanismmechanism

Algorithms

Mechanisms

Services

Applications

E-money E-contract Intrusion DetectionE-commerce Biometric Mobile SecurityE-auction Secure Multimedia VPNE-vote Firewall

Authentication ConfidentialityIntegrity Non-repudiationAccess Control

Encryption Digital SignatureAccess Control AuthenticationKey-Exchange

DES AES SEEDElGamal RSA ECCHash Function PRG

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Relation of service and Relation of service and mechanismmechanism

AlgorithmsAlgorithms

MechanismsMechanisms

ServicesServices

Symmetric-keySymmetric-key&&

Public-keyPublic-keyCryptosystemCryptosystem

DES, AESDES, AESRSA, ECCRSA, ECC

EncryptionEncryptionKey exchangeKey exchange

ConfidentialityConfidentiality

Symmetric-keySymmetric-key&&

DES, AESDES, AESRSA, ECCRSA, ECC

EncryptionEncryptionKey exchangeKey exchange

IntegrityIntegrity

RSA, ECC,RSA, ECC,DSSDSS

DigitalDigitalSignatureSignature

AuthenticationAuthentication

RSA, ECC,RSA, ECC,DSSDSS

DigitalDigitalSignatureSignature

Non-repudiationNon-repudiation

Thank you !Thank you !

Hyun-Sung KimHyun-Sung Kim

kim@kiu.ac.krkim@kiu.ac.kr

computer network security

kyungil universityis

kyungil universitydefensenids

client adefense

client a3510

illegal access

computer engineeringkyungil

illegal user

unauthorized user

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