My Tutorial on Security Lecture 1 and 2

8/8/2019 My Tutorial on Security Lecture 1 and 2

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9/17/2010 Tutorial on Network Security: Sep2003 1

Network SecurityNetwork Security

Bijendra Jain([email protected])


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Lecture 1: IntroductionLecture 1: Introduction


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TopTop--level issueslevel issues

Safety, security and privacy

Security policy threats, both external and internal

economic gains

cost of securing resources cryptographic methods vs. physical security

Information security: nature of resources (HW, SW, information)

during storage, access and communication

limited to a single computer vs. network security various layers (physical through application layers)


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Security threatsSecurity threats

Intentional vs. accidental

Various forms of violations: Non-destructive

Destructive

Repudiation Denial of service

Threat techniques: crypt-analysis

snooping

masquerading replay attacks

virus, worms

etc.


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Security servicesSecurity services

Services (or functions) vs. mechanisms

Security functions: confidentiality

authentication

integrity

non-repudiation

access control

availability


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Security mechanismsSecurity mechanisms

Physical controls

Audit trails

Fraud detection (data mining)

Steganography

Encryption: private-key vs. public-key encryption

key generation, exchange, and management

certification

Firewalls etc.


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Lecture 2: SymmetricLecture 2: Symmetric--keykeyencryptionencryption


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Cryptographic systemsCryptographic systems Symmetric vs. asymmetric encryption

Number of keys used

Key lengths

Block vs. stream cipher

Crypt-analysis (assume algorithm is known) ciphertext (only)

plaintext + ciphertext

chosen plaintext + ciphertext

chosen ciphertext + plaintext

Key size Possible

no. of keys

Time to crack

(1 encryption/microsec)

Time to crack (106

encryptions/microsec)

32 109

36 min 2. msec56 1016 1100 years 10 hrs

128 1038 5 x 1024 years 5 x 1018 years

26 character

permutation

1026 6 x 1012 years 6 x 106 years


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Symmetric cryptographic systemSymmetric cryptographic system Symmetric encryption

Plaintext, X

Ciphertext, Y

Secret keys for encryption, decryption, K

Secret

key, K

Encrypt

EK(X)

Decrypt

DK(X)

Crypt-

analysis

X Y X

K K

Secure channel

Insecure

channel


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Asymmetric cryptographic systemAsymmetric cryptographic system Asymmetric encryption

Plaintext, X

Ciphertext, Y

Two keys K1, and K2. One is secret, other is public

One of them (secret or public) is used to encrypt, the other for decryption

Helps with confidentiality, digital signatures

Key generation, management

Encrypt

EK(X)

Decrypt

DK(X)

Crypt-

analysis

X Y X

K1 K2

Insecure

channel


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9/17/2010Tutorial on Network Security: Sep

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Symmetric encryptionSymmetric encryption

Substitution cipher

Transposition cipher

DES

Triple DES Blowfish, RC5, RC4, etc.


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Substitution cipherSubstitution cipher

Ceasar cipher encrypt Cn (p+k) mod n

decrypt pn (C-k) mod n

assumes set of n characters

easily breakable in n-1 steps

Substitute using n x n table encrypt Cin lookup_encrypt(pi)

decrypt pjn lookup_decrypt(Cj)

26! Different keys

may be broken using known relative frequency of each character

To counter: use multiple symbols to substitute

substitute multiple symbols at a time e.g. two letter strings at a time


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2003 13

Transposition cipherTransposition cipher

Transposition example:

To make it more secure: transposition it multiple times

combine it with substitution ciphers

Key 4 3 1 2 5 6 7

Plaint t a t t a c k p

o s t p o n e

d u n t I l t

o a m x y z

iphertext:

TT AAPTMTSU A C I K LYPETZ


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2003 14

DESDES

Combination of several substitution and transposition ops Applied to each block of size 64 bits

Key is 56 bits

ses portions of key at different steps

ses techniques referred to by diffusion and confusion

Developed by IB 1971-73, accepted by NBS ( SA) as astandard in 1977

Primarily a block cipher

Decrypt

DK(X)

P1

K

C1

Encypt

EK(X)

C1

K

P1


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DES encryption algorithmDES encryption algorithm

Initial permutation

Round 1

Round 2

Round 16

32-bit swap

Inverse permute

K1

K2

K16

Permuted key

Permuted key

Permuted key

Left circular shift

Left circular shift

Left circular shift

Permuted key

64-bit plaintext

64-bit ciphertext

56-bit key


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Cipher Block ChainingCipher Block Chaining

Encrypt

EK(X)

C1

IV

K

+

P1

Encrypt

EK(X)

C2

+

P2

K

Decrypt

DK(X)

P1

IV

K

+

C1

P2

C2

Decrypt

DK(X)

K

+

Primarily a block cipher

ay be used in block chaining mode


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Strength of DESStrength of DES

Key size of56 bits appears to be too small

In 1993 Weiner developed HW device for $100K with 5760 searchengines to break it in 35 hours

In 1997, 70,000 systems on Internet discovered the key in less than96 days (part of plaintext is given)

Automating the process is difficult, unless plaintext is known

Perhaps breakable by studying and exploiting weakness

Differential cryptanalysis

Linear cryptanalysis

Trapdoor

S Govt changed the original design

Continues to enjoy wide acceptibility

Particularly with triple-DES (used in PGP)


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2003 18

DoubleDouble--DESDES

Two stages of encryption, using two different keys

Decrypt

EK2(X)

X

K2

Encypt

EK1(X)

CP

K1


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DoubleDouble--DESDES

two stages cannot be reduced to one stage: for given K1, K2, there is no K s.t. EK2(EK1(P)) = EK(P)

eet-in-the-middle attack Let C = EK2(EK1(P)), and X = EK1(P) = DK2(C)

Let known P and C

Search for K1 and K2 such that X = EK1(P) = DK2(C)

Complexity is O(256 + 256), not O(2128)


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TripleTriple--DESDES

Three stages of encryption, using two different keys

Decrypt

EK2(X)

X1

K2

Encypt

EK1(X)

CP

K1

X2

Decrypt

EK3(X)

K3


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IDEAIDEA

International data encryption algorithm (IDEA)

developed in 1991, gaining ground

block cipher

better understood S government has had no role in its design

design principle: block size 64 bits

key length 128 bits

more emphasis on diffusion and confusion

uses three operations:

exclusive-OR, addition, multiplication

some effort to make HW implementation easier


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RC5RC5

developed by Rivest, in 1994

suitable for HW or SW implementation onmicroprocessors simple

different word length

low memory

high level of security simpler determination of strength

variable no. of rounds, key length


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BlowfishBlowfish

Developed in 1993

block cipher

up to 448 bit keys

no known attacks simple, fast and compact

algorithm cycles/"round" No. of rounds cycles/byte encrypted

Blowfish 9 16 18

RC5 12 16 23DES 18 16 45

IDEA 50 8 50

Triple-DES 18 48 108


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2003 24

Summary: symmetric key encryptionSummary: symmetric key encryption

Since the same key is used to encrypt and decrypt,the system is also know as private-key encryption

Symmetric key encryption uses shared secret keys

also known as private-key encryption Primarily used for purpose of confidentiality

but may be used to authenticate as well, but may berepudiated

Key sharing or management is an issue

particularly when the no. of clients sharing the key is large


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Application to confidentialityApplication to confidentiality

Private-key encryption may be used to provideconfidentiality of messages during transfer overLANsand/or WANs

At issue:

what information: ser data vs. headers

Identity of correspondents vs. node/route identity

in what layer, and between what points Link-layer vs. end-to-end vs. application level

Assumption: data over physical network is accessible Wireless links Employee of the network service provider

Your own colleagues


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LinkLink--level vs. endlevel vs. end--toto--endend

confidentialityconfidentiality

Host

A

Host

B

R R

R

Link-levelenrypt/ decrypt

End-to-endenrypt/ decrypt


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LinkLink--level vs. endlevel vs. end--toto--endend

confidentialityconfidentialityLink-level encryption End-to-end encryption

Security within nodes, hosts

Exposed in intermediate nodes

Exposed in end hosts

Encrypted in intermediate nodes

Encrypted/Decrypted by end hosts

Role of end devices, intermediate nodes

Intermediate nodes require encryptionOne key for each link

Done in hardware

Only end hosts need encryption One keyper session/connection

Perhaps done in software


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Traffic confidentialityTraffic confidentiality

Issues: Identity of communicating entities

Identity of hosts, routers

Traffic volumes, patterns

Link-level encryption offers better confidentiality Padding may be used to hide patterns and volumes


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Key distributionKey distribution

Secret key must be distributed between the communicatingentities, say A and B

Link level encryption requires L number of keys to bedistributed, one for each device at the end of a link

Host-to-host encryption requires N*(N-1)/2 keys to be distributed

Two techniques: Physical delivery (works only in a very limited environs)

A delivers it to B

A trusted third party C delivers the key to A and to B

Electronic delivery using an established secure connection orsession A delivers it to B after suitably encrypting it A trusted third party C delivers the key to A and to B using secure

channels to A and to B.


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9/17/2010

Tutorial on Network Security: Sep

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Key distributionKey distribution Electronic distribution by B to A, though process initiated by A

Above: N1 and N2 are nonce,

Km is the master key used by A and B

KS is the new session key

F is a well-known function, such as ADD 1


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Key distributionKey distribution Electronic distribution by trusted third party C to A and to B


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Above:

KA and KB are keys used by A and B, respectively, to communicatewith C

IDA identifies entity A


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Secure operation of these schemes, against: asquerade

replay attacks

Other issues:

Hierarchy of keys Lifetime of a session key

Generation of Nonce or Random numbers


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9/17/2010


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My Tutorial on Security Lecture 1 and 2

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