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06/27/22 Tutorial on Network Sec urity: Sep 2003 1 Network Security Network Security Bijendra Jain ([email protected])
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Page 1: Security Lecture 1

04/14/23 Tutorial on Network Security: Sep 2003

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Network SecurityNetwork Security

Bijendra Jain([email protected])

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Lecture 1: IntroductionLecture 1: Introduction

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Top-level issuesTop-level 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: Symmetric-key Lecture 2: Symmetric-key encryptionencryption

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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. msec 56 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 KSecure 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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Symmetric encryptionSymmetric encryption Substitution cipher Transposition cipher DES Triple DES Blowfish, RC5, RC4, etc.

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Substitution cipherSubstitution cipher Ceasar cipher

– encrypt C (p+k) mod n– decrypt p (C-k) mod n– assumes set of n characters– easily breakable in n-1 steps

Substitute using n x n table– encrypt Ci lookup_encrypt(pi)– decrypt pj 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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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 7Plaintext a t t a c k p

o s t p o n ed u n t I l tw o a m x y z

Ciphertext:TTNAAPTMTSUOAODWCOIXKNLYPETZ

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DESDES Combination of several substitution and transposition ops

– Applied to each block of size 64 bits– Key is 56 bits– Uses portions of key at different steps– Uses techniques referred to by “diffusion and confusion”

Developed by IBM 1971-73, accepted by NBS (USA) as a standard 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

–May be used in “block chaining mode”

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Strength of DESStrength of DES Key size of 56 bits appears to be too small

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

– In 1997, 70,000 systems on Internet discovered the key in less than 96 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– US Govt changed the original design

Continues to enjoy wide acceptibility– Particularly with triple-DES (used in PGP)

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Double-DESDouble-DES Two stages of encryption, using two different keys

Decrypt

EK2(X)

X

K2

Encypt

EK1(X)

CP

K1

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Double-DESDouble-DES

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

Meet-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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Triple-DESTriple-DES 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 US 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 on

microprocessors– 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 encryptedBlowfish 9 16 18RC5 12 16 23DES 18 16 45IDEA 50 8 50Triple-DES 18 48 108

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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 be “repudiated”

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 provide confidentiality of messages during transfer over LANs

and/or WANs At issue:

– what information: User 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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Link-level vs. end-to-end Link-level vs. end-to-end confidentialityconfidentiality

Host

A

Host

BRR

R

Link-level enrypt/ decrypt

End-to-end enrypt/ decrypt

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Link-level vs. end-to-end Link-level vs. end-to-end confidentialityconfidentiality

Link-level encryption End-to-end encryption

Security within nodes, hosts

Exposed in intermediate nodesExposed in end hosts

Encrypted in intermediate nodesEncrypted/Decrypted by end hosts

Role of end devices, intermediate nodes

Intermediate nodes require encryption One key for each linkDone in hardware

Only end hosts need encryption One key per session/connectionPerhaps 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 communicating entities, say A and B Link level encryption requires L number of keys to be distributed, 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 or session 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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Key distributionKey distribution Electronic distribution by B to A, though process initiated by A

Above:– N1 and N2 are “nonce”, – MKm 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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Key distributionKey distribution Above:

– KA and KB are keys used by A and B, respectively, to communicate with C

– IDA identifies entity A

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Key distributionKey distribution Secure operation of these schemes, against:

– Masquerade– replay attacks

Other issues:– Hierarchy of keys– Lifetime of a session key– Generation of Nonce or Random numbers

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