network security - iitkgp

1

Network Security

2

Security Attacks and Services

3

Security Attacks

• Any action that compromises the security of information.

• Four types of attack:1. Interruption2. Interception3. Modification4. Fabrication

• Basic model:

DSource Destination

S

4

• Interruption:– Attack on availability

• Interception:– Attack on confidentiality

S D

S D

I

5

• Modification:– Attack on integrity

• Fabrication:– Attack on authenticity

S D

S D

I

I

6

Passive and Active Attacks

• Passive attacks– Obtain information that is being transmitted

(eavesdropping).– Two types:

• Release of message contents:- It may be desirable to prevent the opponent from learning the contents of the transmission.

• Traffic analysis:- The opponent can determine the location and identity of communicating hosts, and observe the frequency and length of messages being exchanged.

– Very difficult to detect.

7

• Active attacks– Involve some modification of the data stream or

the creation of a false stream.– Four categories:

• Masquerade:- One entity pretends to be a different entity.• Replay:- Passive capture of a data unit and its

subsequent retransmission to produce an unauthorized effect.

• Modification:- Some portion of a legitimate message is altered.

• Denial of service:- Prevents the normal use of communication facilities.

8

Security Services

• Confidentiality • Authentication • Integrity • Non-repudiation • Access control • Availability

– Denial of Service Attacks– Virus that deletes files

9

Network Access Security Model

10

Cryptography: Basic Concepts

11

Introduction

• Most important concept behind network security is encryption.

• Two forms of encryption are in common use:– Private (or Symmetric)

• Single key shared by sender and receiver.• Examples: DES, AES, IDEA

– Public-key (or Asymmetric)• Separate keys for sender and receiver.• Examples: RSA, Diffie-Hellman

12

Some Terminologies

• Plaintext: – the data that is to be encrypted

• Ciphertext: – the encrypted form of the data

• Encryption/Decryption algorithm– the algorithm used to carry out the transformation.

• Key– Usually a secret entity.– Used as parameter to the encryption/decryption

algorithm.

13

Private Key Cryptography

14

Simplified Model of Conventional Encryption

EncryptionAlgorithm

Decryption Algorithm

PlaintextP

PlaintextP

Ciphertext C

Shared Key K Shared Key K

15

Classical Techniques

• Broadly falls under two categories:1. Substitution ciphers

• Each letter of group of letters of the plaintext are replaced by some other letter or group of letters, to obtain the ciphertext.

2. Transposition ciphers• Letters of the plaintext are permuted in some

form.

16

Substitution Ciphers

1. Caesar Cipher– Earliest known substitution cipher.– Replace each letter of the alphabet with the letter

three places after that alphabet.– Alphabets are assumed to be wrapped around

(Z is followed by A, etc.).

P: H A P P Y N E W Y E A RC: K D S S B Q H Z B H D U

17

– We can generalize the idea by replacing each letter by the kth following letter.

– If we assign a number to each letter (A=1, B=2, etc), then

C = E (P) = (P + k – 1) % 26 + 1P = D (C) = (C – k + 25) % 26 + 1

– Drawback:• Brute force attack is easy• Try out all the 25 possible keys

18

2. Mono-alphabetic Cipher– Allow any arbitrary substitution.– There can be 26! or 4x1026 possible keys.– A typical key may be:

(ZAQWSXCDERFVBGTYHNMJUIKLOP)– Drawback:

• We can make guesses by observing the relative frequency of letters in the text.

• Compare it with standard frequency distribution charts in English (say).

• Also look at the frequency of digrams and trigrams, for which tables are also available.

• Easy to break in general.

19

3. Poly-alphabetic Cipher– Use different mono-alphabetic substitutions as

we proceed through the plaintext message.– Vigenere cipher is the best known cipher of

this class.• Consists of 26 Caesar ciphers, with shifts of 0 to 25.• Each cipher is denoted by a key letter, which is the

ciphertext letter that substitutes for the plaintext letter ‘a’.

• To encrypt a message, a key is needed that is as long as the message (usually, a repeating keyword).

• Decryption is just the reverse.

20

– Drawback:• Key and the plaintext share the same

frequency distribution of letters.• The best thing would have been to use a

keyword which is as large as the plaintext, and has no statistical relationship to it.

21

Transposition Cipher

• Many techniques were proposed under this category.

• A simple scheme:– Write out the plaintext in a rectangle, row by row,

and read the message column by column, by permuting the order of the columns.

– Order of the column becomes the key.

22

• An exampleP: we have enjoyed the workshop in jadavpur

Key: 4 3 1 2 5 6 7w e h a v e en j o y e d th e w o r k sh o p i n j ad a v p u r -

C: howpv ayoip ejeoa wnhhd vernu edkjr etsa-

23

– Drawback:• The ciphertext has the same letter frequency

as the original plaintext.• Guessing the number of columns and some

probable words in the plaintext holds the key.

24

Some Important Issues

• Security of the scheme– Depends entirely on the secrecy of the key. – Does not depend on the secrecy of the algorithm.

(Has to be public for criticism!)• So, the assumptions that we make:

– Algorithms for encryption/decryption are known to the public.

– Keys used are kept secret.

25

What is meant by What is meant by ““Security lies in the KeysSecurity lies in the Keys””

2.15 milliseconds232 = 4.3 x 10932

5.9 x 1030 years2168 = 3.7 x 1050168

5.4 x 1018 years2128 = 3.4 x 1038128

10 hours256 = 7.2 x 101656

Time required at 106

decryptions / µsNumber of

Alternative KeysKey Size

(bits)

26

Practical Encryption Algorithms

• Data Encryption Standard (DES)– Block size is 64 bits.– Key is 56 bits.

• IDEA– Block size is 64 bits.– Key size is 128 bits.

• Advanced Encryption Standard (AES)– Also known as Rijndael cryptosystem.– Block size can be 128, 192, or 256 bits.– Key size can be 128, 192, or 256 bits.

27

Block Encryption Algorithms

• Data Encryption Standard (DES)– The most widely used encryption scheme.– Known as the Data Encryption Algorithm (DEA).– It is a block cipher.

• The plaintext is 64-bits in length.• The key is 56-bits in length.• Longer plaintexts are processed in 64-bit blocks.

28

Initial Permutation

Round 1

Round 2

Round 16

32-bit Swap

Reverse InversePermutation

Permuted Choice 2

Permuted Choice 2

Permuted Choice 2

Permuted Choice 1

Left circular shift

Left circular shift

Left circular shift

P (64-bit)

C (64-bit)

K (56-bit)

General Schematicof DES Algorithm

K16

K1

K2

29

Single Iteration of DES Algorithm

30

DES

• The overall processing at each iteration:Li = Ri-1

Ri = Li-1 ⊕ F(Ri-1, Ki)

• Concerns about:– The algorithm and the key length (56-bits).– Longer key lengths essential for critical

applications.

Fiestel Structure

31

Problems with DES

• 56-bit key size considered to be too small for providing acceptable level of security for most applications.

• Broken by various cryptanalysis groups.

32

Hence, Triple DES!• Use three keys and three executions of

the DES algorithm (encrypt-decrypt-encrypt).

C = EK3 [DK2 [EK1 [P]]]

– C = ciphertext– P = Plaintext– EK[X] = encryption of X using key K– DK[Y] = decryption of Y using key K

• Effective key length of 168 bits.

33

Triple DES: Illustration

E D E

D E D

P

P

C

C

K1

K1

K2

K2

K3

K3

X Y

Y X

34

Some Points to Observe

• Key distribution problem of secret key systems:– Establish key before communication.– Need n(n-1)/2 keys with n different parties.

A

B E

DC

35

Key Distribution

• Two parties A and B trying to communicate.– A key could be selected by A and physically

delivered to B.– A third party could select the key and physically

deliver it to both A and B.– If A and B have previously used a key, one party

could transmit the new key to the other, encrypted using the old key.

– If A and B each have an encrypted connection to a third party C, C could deliver a key on the encrypted links to A and B.

36

Key Distribution (contd.)

• Session key:– Data encrypted with a one-time session key.– At the conclusion of the session the key is

destroyed.• Permanent key:

– Used between entities for the purpose of distributing session keys.

37

Public Key Cryptography

38

Basic Concept

• Uses two keys for every simplex logical communication link.

a) Public keyb) Private key

• Every communication node will have a pair of keys.– For n number of nodes, total number of keys

required is 2n.

39

Encryption using Public Key System

EncryptionAlgorithm

Decryption Algorithm

PlaintextP

PlaintextP

Ciphertext C

A B

B’s public key KUB B’s private key KRB

40

Authentication using Public Key System

EncryptionAlgorithm

Decryption Algorithm

PlaintextP

PlaintextP

Ciphertext C

A B

A’s private key KRA A’s public key KUA

41

Applications

• Three categories:a) Encryption/decryption:

• The sender encrypts a message with the recipient’s public key.

b) Digital signature / authentication: • The sender signs a message with its private key.

c) Key exchange: • Two sides cooperate to exhange a session key.

42

Requirements

• Computationally easy for a party B to generate a key pair– Public key KUB

– Private key KRB

• Easy for sender to generate ciphertext: C = E (M, KUB)

• Easy for the receiver to decrypt ciphertext using private key:

M = D (C, KRB) = D (E (M, KUB), KRB)

43

• Computationally infeasible to determine KRB knowing KUB.

• Computationally infeasible to recover message M, knowing KUB and ciphertext C.

• Either of the two keys can be used for encryption, with the other used for decryption:

M = D (E (M, KUB), KRB) = D (E (M, KRB), KUB)

44

The RSA Public Key Algorithm

• RSA Algorithm– Developed by Ron Rivest, Adi Shamir and Len

Adleman at MIT, in 1977.– A block cipher.– The most widely implemented.

45

The RSA Algorithm – Key Generation

1. Select p,q p and q both prime2. Calculate n = p x q3. Calculate 4. Select integer e5. Calculate d6. Public Key KU = {e,n}7. Private key KR = {d,n}

)1)(1()( −−=Φ qpn)(1;1)),(gcd( neen Φ<<=Φ

)(mod1 ned Φ= −

φ(n) is the number of positive numbers less than nand relatively prime to n (called Euler totient).

46

The RSA Algorithm - Encryption

• Plaintext: M < n

• Ciphertext: C = Me (mod n)

47

The RSA Algorithm - Decryption

• Ciphertext: C

• Plaintext: M = Cd (mod n)

48

Example

• Select two prime numbers, p=7 and q=17.• Calculate n = pq = 7×17 = 119.• Calculate φ(n) = (p-1)(q-1) = 96.• Select e such that e is relatively prime to φ(n)=96,

and less than φ(n).– In this case, e=5.

• Determine d such that de = 1 (mod 96) and d<96.– d=77, because 77×5 = 385 = 4×96+1.

• Public key KU = {5,119}• Private key KR = {77,119}

49

Example (contd.)

50

The Security of RSA

• RSA is secure since– We use large number of bits in e and d.– The problem of factoring n into two prime factors is

computationally very difficult.• Knowing p and q will allow us to know Φ(n).• This will help an intruder to know the values of e and d.• Until recently, this was felt to be infeasible for numbers in the

range of 100 decimal digits or so (approximately 300 bits).• A worldwide team cooperating over the internet and using

1600 computers recently cracked the code in eight months.• Currently, a 1024-bit key size (about 300 decimal digits) is

considered strong enough for virtually all applications.– Key sizes in the range of 1024 to 2048 bits seems safe.

51

Private and Public Key Systems: a Comparison

• Symmetric encryption/decryption is much faster than asymmetric encryption/ decryption:

RSA: kilobits/secondDES: megabits/second

⇓DES is about 100 times faster than RSA

52

Diffie-Hellman Key Exchange

• Proposed in 1976.• Allows group of users to agree on secret

key over insecure channel.• Cannot be used to encrypt and decrypt

messages.• Depends for its effectiveness on the

difficulty of computing discrete logarithms.

53

D-H Algorithm

• A and B want to agree on shared secret key.– They agree on two large numbers n and g, such

that 1<g<n.– A choose random x, computes X=gx mod n, and

sends X to B.– B chooses random y, computes Y=gy mod n, and

sends Y to A.– A computes k1= Yx mod n .– B computes k2= Xy mod n .

• Note: k1 = k2 = gyx mod n .

54

B

X=gx mod n

Y=gy mod n

Choose x Choose y

Compute Yx mod n Compute Xy mod n

A

55

D-H Algorithm: contd.

• Requires no prior communication between A and B.

• Security depends on difficulty of computing x, given X = gx mod n .

• Choices for g and n are critical.– Both n and (n-1)/2 should be prime.– n should be large.

• Susceptible to intruder-in-the-middle (man-in-the-middle) attack.– Active intruder.

56

A B

Intruder

Man-in-the-Middle Attack

57

A Comparison• Symmetric encryption/decryption is much

faster than asymmetric encryption/ decryption:

RSA: kilobits/secondDES: megabits/second

⇓DES is about 100 times faster than RSA

• Key size:– RSA: selected by user– DES: 56 bits

58

Message Authenticationand

Hash Functions

59

Approaches to Message Authentication

• Authentication using conventional encryption– Only the sender and receiver should share a key.

• Message authentication without message encryption– An authentication tag is generated and appended to

each message.• Message authentication code (MAC)

– Calculate the MAC as a function of the message and the key

MAC = F(M, K)

60

Basic Idea of MAC

• Generate a small hash code (usually of fixed size) from a message of arbitrary size.– Transmit the hash code along with the message.– Common algorithms: MD5, SHA-0, SHA-1.

M

H MAC

M

MAC

61

62

Hash Functions: Classification

• Unkeyed hash function– Also known as modification detection code (MDC).– Used to preserve integrity of message.

• Keyed hash function– Also known as message authentication code (MAC)– Used to authenticate the source of a message in

addition to preserving integrity of the message.

63

Hash Functions: Desirable Properties

• Collision: – A hash function H maps an infinite set to a finite

one.– So there must exist messages x and x′ such that

H(x) = H(x′). Such a pair (x,x′) of messages is called a collision for H.

• First preimage resistance: – Except for few hash values y, it should be difficult

to find a message x such that H(x) = y.

64

• Second preimage resistance: – Given a message x, it should be difficult to find

another message x′ with the property that H(x) = H(x′).

• Collision resistance: – It should be difficult to find two messages x and x′

with H(x) = H(x′).

65

Secure HASH Functions

• Purpose of the HASH function is to produce a fingerprint.

• Desirable properties of a HASH function H:– H can be applied to a block of data at any size.– H produces a fixed length output.– H(x) is easy to compute for any given x.– For any given block x, it is computationally infeasible to find

x such that H(x) = h.– For any given block x, it is computationally infeasible to find

some y, with H(y) = H(x), x ≠ y.– It is computationally infeasible to find any pair (x, y) such

that H(x) = H(y), x ≠ y.

66

Hash Functions: Examples

• Custom-designed hash functions work based on the general principle described earlier.

• The MD family: MD2, MD4 and MD5 (128-bit hash).

• The SHA family: SHA-1 (160-bit), SHA-256 (256-bit), SHA-384 (384-bit) and SHA-512 (512-bit).

• RIPEMD-128 (128-bit), RIPEMD-160 (160-bit).

67

One-way HASH function

68

One-way HASH function

• Secret value is added before the hash and removed before transmission.

69

Message Digest Generation Using SHA-1

70

SHA-1 Processing of single 512-Bit Block

71

Other Secure HASH functions

160 (5 paired rounds of 16)

64 (4 rounds of 16)

80 (4 rounds of 20)

Number of steps

264-1 bitsMaximum message size

512 bits512 bits512 bitsBasic unit of processing

160 bits128 bits160 bitsDigest length

RIPEMD-160MD5SHA-1

∞ ∞

72

HMAC

• Use a MAC derived from a cryptographic hash code, such as SHA-1.

• Motivations:– Cryptographic hash functions executes faster in

software than encryption algorithms such as DES.– Library code for cryptographic hash functions is

widely available.– No export restrictions from the US.

73

• HMAC (a keyed hash function)• Notations:

M = the message to be hashedH = an unkeyed hash functionK = key for HMACP,Q = short padding blocks (not secret).

HMAC(M) = H (K || P || H (K || Q || M))

• HMAC involves two calls of H.• HMAC is efficient, since the outer call involves

computation of hash of a short message.

74

HMAC Structure

75

Hash Function: Attacks

• Birthday attack: – Let H be a hash function that produce n-bit hash values. If

about 2n/2 random messages are hashed by H, then it is highly probable that we have found two messages x and x′satisfying H(x) = H(x′).

– The bit-size n of hash values should be at least as large as 128. The values greater than or equal to 160 are recommended.

• Other attacks:– Attacks on the compression function– Chaining attacks– Attacks on the underlying block cipher

76

Digital Signatures

77

Digital Signatures: Introduction

• Digital equivalent of hand-written signatures.• Bind pieces of digital data with particular entities.• Based on public-key technology.• Signing:

– The signer uses his private key ‘d’ to sign.• Difficulty of forging:

– An entity without knowledge of this private key ‘d’cannot generate a valid signature on a new piece of data.

78

• Verifying: – Anybody having access to the signer’s public key

‘e’ can verify the signature.• Non-repudiation:

– An entity should not be allowed to deny valid signatures made by him.

79

Digital Signatures: Classification

• Signature with appendix:– A representative H(M) of the message M is computed. The

signing transformation is applied on H(M). Verification requires the message M.

– Signature generation:• m = H(M)

s = fs(m,d)Output the signed message (M,s).

– Signature verification:• Compute m = H(M)

Compute m′ = fv(s,e)If (m=m′) output “signature verified” else output “signature not verified”

80

• Signature with message recovery: – The signing transformation is applied to the message itself.

The verification transformation retrieves the message.– Signature generation:

• Compute the signature s = fs(M,d)– Signature verification:

• Recover the message M′ = fv(s,e)If M′ looks like a valid message,

output “signature verified”else

output “signature not verified”

81

Digital Signatures: Classification

• Deterministic signatures: – For a given message the same signature is generated on

every occasion the signing algorithm is executed.• Probabilistic signatures:

– On different runs of the signing algorithm different signatures are generated, even if the message remains the same.

• Probabilistic signatures offer better protection against some kinds of forgery.

• Deterministic signatures are of two types:– Multiple-use signatures:

• Slow. Parameters are used multiple times.– One-time signatures:

• Fast. Parameters are used only once.

82

Digital Signatures: Examples

• RSA with appendix

– Parameter selection:• Select two large primes p and q.

Compute n = pq and f(n) = (p - 1)(q - 1).Select a random integer e with gcd(e,f(n)) = 1.Compute an integer d satisfying ed = 1 (mod f(n)).

Publish (e,n) as the verification key.Keep d secret as the signing key.

83

• Signature generation:– Generate the short representative m = H(M) of M.

Sign m as s = md (mod n).Output (M,s) as the signed message.

• Signature verification:– Generate the short representative m = H(M) of M.

Invert signature as m’ = se (mod n).If (m = m’), output “signature verified”,else output “signature not verified”.

• Forging: – If any key other than d is used to sign m, the equality m = m’ will

not be satisfied. So forging RSA signatures is as difficult as knowing d.

84

Digital Signatures: Examples

• RSA with message recovery– Parameter selection: Same as before.

• Signature generation:– Compute s = Md (mod n).

Output the signature s.• Signature verification:

– Compute M’ = se (mod n).If M’ is a valid message, output “signature verified”,else output “signature not verified”.

• Forging:– If any key other than d is used to generate s, the recovered

message M’ will almost certainly look like a meaningless sequence of symbols.

85

Digital Signatures: Blind Signatures

• The signer is not allowed to know the message to sign. Still hisactive participation is necessary for signing.

• Blind RSA signature:

Parameter generation: As before.

Signature generation:

– A generates a random integer k coprime to n.A blinds m as m* = mke (mod n).B signs s* = (m*)d (mod n).A retrieves B’s signature s = s*k-1 (mod n).

• Signature verification: As before.

86

Digital Signatures: Undeniable Signatures

• An active participation of the signer is necessary during signature verification.

• A signer is not allowed to deny a legitimate signature made by him.

• An undeniable signature comes with a denial or disavowal protocol that generates one of the following three outputs:– Signature verified– Signature forged– The signer is trying to deny his signature by not properly

participating in the protocol.

87

Digital Signatures: Attacks

• Total break: – An attacker knows the signing key or has a

function that is equivalent to the signature generation transformation.

• Selective forgery: – An attacker can generate signatures (without the

participation of the legitimate signer) on a set of messages chosen by the attacker.

• Existential forgery: – The attacker can generate signatures on certain

messages over which the attacker has no control.

88

Digital Signatures: Attacks

• Key-only attack: – The attacker knows only the verification (public)

key of the signer. This is the most difficult attack to mount.

• Known-message attack: – The attacker knows some messages and the

signatures of the signer on these messages.• Chosen-message attack:

– This is similar to the known-message attack except that the messages for which the signatures are known are chosen by the attacker.

89

• Adaptive chosen-message attack: – The messages to be signed are adaptively chosen

by the attacker.

90

Digital Certificates

91

Digital Certificates: Introduction

• Bind public-keys to entities.• Required to establish the authenticity of public keys.• Guard against malicious public keys.• Promote confidence in using others’ public keys.• Require a Certification Authority (CA) whom every

entity over a network can believe. Typically, a government organization or a reputed company can be a CA.

• In case a certificate is compromised, one requires to revoke it.

• A revoked certificate cannot be used to establish the authenticity of a public key.

92

Digital Certificates: Contents

• A digital certificate contains particulars about the entity whose public key is to be embedded in the certificate. It contains:– Name, address and other personal details of the entity.– The public key of the entity. The key pair may be

generated by either the entity or the CA. If the CA generates the key pair, then the private key is handed over to the entity by trusted couriers.

• The certificate is digitally signed by the private key of the CA.

• If signatures are not forgeable, nobody other than the CA can generate a valid certificate for an entity.

93

Digital Certificates: Revocation

• A certificate may become invalid due to several reasons:– Expiry of the certificate– Possible or suspected compromise of the entity’s private

key• An invalid certificate is revoked by the CA.• The CA maintains a list of revoked certificates — the

Certificate Revocation List (CRL).• An entity A willing to use B’s public key looks up the

certificate for B’s public key. If the CA’s signature is verified on this certificate and if the certificate is not found in the CRL, then A gains the desired confidence to use B’s public key.

94

Network Security Principles in Use

95

Authentication Application:: KERBEROS

• Users wish to access services on servers.• Three threats exist:

– User pretend to be another user.– User alter the network address of a computer.– User eavesdrop on exchanges and use a replay

attack.

96

• Provides a centralized authentication server (AS) to authenticate users to servers and servers to users.

• Relies on conventional encryption.– Makes no use of public-key encryption.

• Two versions: version 4 and 5.• Version 4 makes use of DES.

97

98

Electronic Mail Security:: Pretty Good Privacy (PGP)

• PGP provides a confidentiality and authentication service that can be used for electronic mail and file storage applications.

• Why popular?– It is available free on a variety of platforms.– Based on well known algorithms.– Wide range of applicability.

99

• Summary of PGP services:

Radix-64 ConversionEmail Compatibility

ZIPCompression

CAST or IDEA or 3-key Triple DES with Diffie-Hellman or RSA

Message Encryption

DSS/SHA or RSA/SHA

Digital Signature

Algorithm UsedFunction

100

Authentication Only Service

MH EP || Z

KRA

Z-1M

DP

KUA

H

=?

Sour

ce A

Des

tinat

ion

B

101

Confidentiality Only Service

MZ EC ||

EP

KUB

KS

DP

KRB

DC Z-1M

KS

Sour

ce A

Des

tinat

ion

B

102

PGP Cryptographic Functions

103

Secure Socket Layer (SSL)

• SSL was first used by Netscape.– To ensure security of data sent through HTTP,

LDAP or POP3.• Uses TCP to provide reliable end-to-end

secure service.• In general, SSL can be used for secure data

transfer for any network service running over TCP/IP.

104

HTTP LDAP POP3

SSL

TCP/IP

Application Layer

Network Layer

105

• The main objectives of SSL are:– Authenticate the client and server to each other.– Ensure data integrity.– Ensure data privacy.

• Required for both the protocol data and also the application data.

106

SSL Architecture

• SSL consists of two layers of protocols:– SSL Record Protocol

• Ensures data security and integrity.– Protocols required to establish SSL connection.

• Three protocols used in this layer: SSL Handshake ProtocolSSL ChangeCipherSpec ProtocolSSL Alert Protocol

107

SSL Handshake

Protocol

SSL ChangeCipherSpec

Protocol

SSL Alert

Protocol

Application Protocol

(HTTP, etc.)

SSL Record Protocol

TCP

IP

108

SSL Record Protocol

• Mainly responsible for data encryption and integrity.

• Basic function:– Take an application message to be sent.– Fragment the application message data.

• 16 Kbytes or smaller.– Encapsulate it with appropriate headers and

create an object called a record.– Encrypt the record and forward it to TCP.

109

Application Data

Fragments

Compressed data

Add MACMAC

Encrypt data

HTCP packetH: SSL record

header

110

The Higher Layer Protocols

• SSL Alert Protocol– Used to send session messages associated with

data exchange and functioning of the protocol.– Each message consists of two bytes:

• First byte is either 1 (warning) or 2 (fatal). If “fatal”, the SSL session is terminated.

• Second byte contains one of the defined error codes.

111

• SSL ChangeCipherSpec Protocol– Consists of a single message that carries the

value of 1.– Purpose of this message is to cause the pending

session state to be established as a fixed state.• Define the set of protocols to be used.• Must be sent from client to server, and vice versa.

112

• SSL Handshake Protocol– Used to initiate a session between the server and

the client.– Within the application data, algorithms and keys

used for data encryption can be negotiated.– Provides mutual authentication.– Process of negotiation divided into four phases.

113

– Client sends to the server• SSL version• Random (used to protect key exchange)• Session ID• CipherSuite

– Server sends back• SSL version• Random (a different number is generated)• Session ID• CipherSuite

114

Transport Layer Security (TLS)

• Extension of SSL.• Aim is to provide security and data integrity

features at the transport layer between two web applications.

• Supported my most web servers and browsers today.

115

Secure Shell (SSH)

• Originally developed in 1995.– As a secure replacement for telnet, rlogin, rcp,

etc.– Allows port forwarding (tunneling over SSH).– Built-in support for proxies/firewalls.

• Widely used nowadays.

116

• In SSHv1 protocol, the server uses two keys:– Long-term server identification key.

• Binds the connection to the server.• 1024 bit RSA.

– Short-term encryption key, changed every hour.• Makes later recovery impossible.• Short-term keys are regenerated as a background task.• 768 bit RSA.

117

• Multiple authentication mechanisms– Straight passwords (protected by SSH encryption).– RSA based authentication.

• Client decrypts a challenge from the server; returns the hash to the server.

– Plug-in mechanisms (biometrics, smartcard, etc.).

118

IP Security (IPSec)

• Security built into the IP layer.– Provides host-to-host (or firewall-to-firewall)

encryption and authentication.– Required for IPv6, but optional for IPv4.

• Consists of two parts:– IPSec proper (for encryption and authentication).– IPSec key management.

119

IPSec

• Provides two modes of protection– Tunnel Mode– Transport Mode

• Authentication and Integrity• Confidentiality• Replay Protection

120

• Protection in Tunnel Mode– Encapsulates the entire IP packet within IPSec

protection.– Tunnels can be created between several different

node types:• Firewall to firewall• Host to firewall• Host to host

121

• Protection in Transport Mode– Encapsulates only the transport layer information

within IPSec protection.– Can only be created between host nodes.

• Authentication and Integrity– Verifies the origin of data.– Assures that data sent is the data received.– Assures that the network headers have not

changed since the data was sent.

122

• Confidentiality– Encrypts data to protect against eavesdropping.– Can hide data source when encryption is used

over a tunnel.• Replay Prevention

– Causes transmitted packets to be dropped.

123

• Problems with IPSec– Excessively complex and difficult to use.– Does now allow use of NAT.– Routers need to be made IPSec aware.

124

Secure HTTP (S-HTTP)

• An extension to the HTTP protocol to support sending data securely over the web.

• Difference from SSL:– SSL is designed to establish a secure connection

between two hosts.– s-HTTP is designed to send individual messages

securely.

125

• Some Features:– Provides a variety of security mechanisms to HTTP

clients and servers.– Does not require client-side public certificates (or

public keys), as it supports symmetric key-only operation modes.

– Provides full flexibility of cryptographic algorithms.• s-HTTP and HTTPS are not the same.• HTTPS is an alternative to s-HTTP.

– HTTP runs on top of SSL or TSL.

126

Penetration Testing and Ethical Hacking

127

What is a Penetration Test?

• A process of actively evaluating the information security measures in an organization.

• Most common procedure:– The security measures are actively analyzed for

design weaknesses, technical flaws and vulnerabilities.

– Results are delivered in a comprehensive report.

128

Ethical Hacking ….

• Definition of ethical hacking– A situation where a computer and network expert

attacks a security system on behalf of its owners, seeking vulnerabilities that a malicious hacker could exploit.

– To test a security system, ethical hacking uses the same methods as their less principled counterparts (hackers), but report problems instead of taking advantage of them.

• Also called penetration testing.

129

Why is it Required?

• There are several reasons why organizations choose to perform a penetration test.– To identify the threats facing the information

assets of the organization.– Reduce the IT security costs by identifying and

resolving vulnerabilities and weaknesses.– Provide the organization with information

assurance.– Gain and maintain certification to an industry

regulation (BS7789, HIPAA, etc.).

130

Types of Tests

1. External Penetration Testing– This is the traditional approach.– The testing is focused on servers, infrastructure,

and the underlying software (OS, database, etc.).– Two broad approaches:

• Black box testing: performed with no prior knowledge of the infrastructure to be tested.

• White box testing: performed with full disclosure of the topology and the environment.

131

– This basically involves the following:• Comprehensive analysis of publicly available

information about the target.• Identification and analysis of the target hosts.• Analysis of the behavior of security devices like

screening routers and firewalls.• Identification and analysis of the vulnerabilities within

the target hosts.

132

2. Internal Penetration Testing– Follows a methodology similar to external

testing.– Provides a more complete view of the site

security.– Testing is typically carried out from a number of

network access points, representing each logical and physical segment.• Can include DMZ, VPNs, subnets, etc.

133

3. Application Security Assessment– To identify and assess threats to the organization

through proprietary applications or systems.– The application must not expose the underlying

servers and software to attack.– A malicious user should not be able to access,

modify, or destroy data or services within the system.

– Even in a well-deployed and secured infrastructure, a weak application can expose the organization’s assets to risk.

134

4. Remote Access Security Assessment– This addresses the security risks associated

with an increasingly mobile workforce.• Working from home• Broadband always-on Internet access• 802.11 wireless networking

– Increased exposure by extending the traditional perimeter of the organization.

135

Some of the Typical Areas

• Network Security– Network surveying– Port scanning– System identification– Services identification– Router testing– Firewall testing– Intrusion detection system testing– Trusted systems testing– Password cracking– Denial of service testing

136

• Wireless Security– Wireless networks testing– WEP security testing– Infrared systems testing– Cordless communications testing

Other areas include “Information Security”, “Social Engineering”, “Physical Security”, etc.

137

About the Tools to be Used

• Tools are essential for automating the penetration testing process.

• Where to find the tools?– A number of commercial tools are available.

• They work by using sets of thousands of pre-defined signatures that can identify vulnerabilities in a system.

• Signatures need to be updated on a regular basis.– Open-source tools that are freely available.

• Also called “hacker tools”, and are often made available on “hacker” web sites.

138

• May range from very professionally developed and maintained tools, to poorly documented scripts meant to perform specific tasks.

• Special care must be taken to ensure that the tools themselves do not contain virus or any other malicious contents.

– Specialist penetration testing providers can develop their own tools.

• Because available tools are mostly incomplete, and multiple tools need to be used.

• Many vulnerabilities are not covered by them.

139

Hacker Web SItes

• Some pointers …. there are many many morehttp://www.hackthissite.org/http://www.happyhacker.org/http://www.2600.com/http://hackaday.com/

140

141

142

143

144

References

1. Cryptography and Network Security: Principles and Practice, 2nd

Edition, William Stallings, Prentice Hall, New Jersey, 1999.2. Network Security Essentials: Applications and Standards, William

Stallings, Pearson Education Asia, 2000.3. Applied Cryptography, B. Schneier, Wiley, New York, 1996.4. Internet Cryptography, R. Smith, Addison Wesley, MA, 1997.5. Handbook of Applied Cryptography, A.J. Menezes, et al.

http://www.cacr.math.uwaterloo.ca/hac/6. Journals

– IEEE Transactions on Information Theory– Computer Security– Communications of the ACM– IEEE Communications Magazine– Computer Communications Review

7. The Internet

145