Module III CNS Kerberos and its example.pptx

Authentication Applications
• developed to support application-level
authentication & digital signatures
• will discuss Kerberos – a private-key
authentication service
• discuss X.509 - a public-key directory
authentication service

Kerberos
• Authentication service developed as a part
of MIT’s Athena project
• provides centralized private-key third-party
authentication in a distributed network
– allows users access to services distributed
through network
– without needing to trust all workstations
– rather all trust a central authentication server
• two versions in use: 4 & 5

Athena
• An open distributed environment
• Any user can access services from any
workstation
• Several security threats exists in such an
environment:
– A user impersonate another user
– A user may change the network address of a w/s and
may make it look as another w/s
– A user may eavesdrop on a session and mount a
replay attak later

Kerberos Requirements
• its first report identified requirements as:
– secure
– reliable
– transparent
– scalable
• implemented using an authentication
protocol based on Needham-Schroeder

Kerberos v4 Overview
• a basic third-party authentication scheme
• have an Authentication Server (AS)
– users initially negotiate with AS to identify self
– AS provides a non-corruptible authentication
credential (ticket granting ticket TGT)
• have a Ticket Granting server (TGS)
– users subsequently request access to other
services from TGS on basis of users TGT

Kerberos v4 Dialogue
1. obtain ticket granting ticket from AS
• once per session
2. obtain service granting ticket from TGT
• for each distinct service required
3. client/server exchange to obtain service
• on every service request

Kerberos Realms
• a Kerberos environment consists of:
– a Kerberos server
– a number of clients, all registered with server
– application servers, sharing keys with server
• this is termed a realm
– typically a single administrative domain
• if have multiple realms, Kerberos servers
must share keys and trust each other

Kerberos Version 5
• developed in mid 1990’s to address the
deficiencies of v4
• provides improvements over v4
• encryption algorithm: DES is weak and vulnerable
to attacks. V5 allows a suit of encryption
algorithms.
• V5 breaks away from IP only networks
• V4 uses 8bit ticket lifetime.V5 uses start time and
end time.
•
•

X.509 Authentication Service
• part of CCITT X.500 directory service standards
– distributed servers maintaining user info database
• defines framework for authentication services
– directory may store public-key certificates
– with public key of user signed by certification authority
• also defines authentication protocols
• uses public-key crypto & digital signatures
– algorithms not standardised, but RSA recommended
• X.509 certificates are widely used

X.509 Certificates
• issued by a Certification Authority (CA), containing:
– version (1, 2, or 3)
– serial number (unique within CA) identifying certificate
– signature algorithm identifier
– issuer X.500 name (CA)
– period of validity (from - to dates)
– subject X.500 name (name of owner)
– subject public-key info (algorithm, parameters, key)
– issuer unique identifier (v2+)
– subject unique identifier (v2+)
– extension fields (v3)
– signature (of hash of all fields in certificate)
• notation CA<<A>> denotes certificate for A signed by CA

Obtaining a Certificate
• any user with access to CA can get any
certificate from it
• only the CA can modify a certificate
• because cannot be forged, certificates can
be placed in a public directory
• If there are a large number of users, one
CA may not be able to handle the load
• Also it is difficult to propagate the public
key of the CA securely.

Certificate Chaining
• if both users share a common CA then they are
assumed to know its public key
• What if both users have their certificates issued
by two different CAs? (and one does not know
the public key of the other CA)
• Suppose A’s certificate is issued by X1 and B’s
by X2
• And A does not know the public key of X2.
(A can not verify the public key of B).

Certificate chaining
• Suppose X1 and X2 have securely exchanged
their public keys.
• X1 can prepare a certificate for X2 and sends it
to A.
• A can request this certificate from X1, obtain the
public key of X2, and then verify B’s certificate.
• Notationally,
X1<<X2>>X2<<B>>
--Chain of two certficates.
--need not be limited to two certificates.

CA Hierarchy
• CAs can certify each other.
• CAs are linked by this relation.
• CAs can be organized in several structures
• X.509 suggests CA's must form a hierarchy
• use certificates linking members of hierarchy to
validate other CA's
– each CA has certificates for clients (forward) and
parent (backward)
• each client trusts parents certificates
• enable verification of any certificate from one CA
by users of all other CAs in hierarchy

CA Hierarchy
• A can verify B’s certificate using the following
certificate chain:
X<<W>>W<<V>>V<<Y>>Y<<Z>>Z<<B>>
-- There is chain of trust also.
• Likewise, B can verify A’s public key using the
following certificate chain:
Z<<Y>>Y<<V>>V<<W>>W<<X>>X<<A>>
--can obtain these certificates from the directory.

Certificate Revocation
• certificates have a period of validity
• may need to revoke before expiry, e.g.:
1. user's private key is compromised
2. user is no longer certified by this CA
3. CA's certificate is compromised
• CA’s maintain list of revoked certificates
– the Certificate Revocation List (CRL)
– CRL is advertised widely through directory.
• users should check certificates with CA’s CRL

Authentication Procedures
• X.509 includes three alternative
authentication procedures:
• One-Way Authentication
• Two-Way Authentication
• Three-Way Authentication
• all use public-key signatures
• It is assumed that the two parties know
each other’s public key.

One-Way Authentication
• 1 message ( A->B) used to establish
– the identity of A and that message is from A
– message was intended for B
– integrity & originality of message
• message must include timestamp, nonce,
B's identity and is signed by A
• may include additional info for B
– E.g., session key

Two-Way Authentication
• 2 messages (A->B, B->A) which also
establishes in addition:
– the identity of B and that reply is from B
– that reply is intended for A
– integrity & originality of reply
• reply includes original nonce from A, also
timestamp and nonce from B
• may include additional info for A

Three-Way Authentication
• 3 messages (A->B, B->A, A->B) which
enables above authentication without
synchronized clocks
• has reply from A back to B containing
signed copy of nonce from B
• means that timestamps need not be
checked or relied upon

X.509 Version 3
• has been recognised that additional
information is needed in a certificate
– email/URL, policy details, usage constraints
• rather than explicitly naming new fields
defined a general extension method
• extensions consist of:
– extension identifier
– criticality indicator
– extension value

Certificate Extensions
Extensions fall into three categories:
• key and policy information
– convey additional info about subject & issuer keys,
plus indicators of certificate policy
• certificate subject and issuer attributes
– support alternative names, in alternative formats for
certificate subject and/or issuer
• certificate path constraints
– allow constraints on use of certificates by other CA’s
(may restrict the type of certificate issued)

Summary
• have considered:
– Kerberos trusted key server system
– X.509 authentication and certificates

Public Key Infrastructure
(PKI)
Providing secure communications
and authentication over an open
network.

Topics
• Understanding the technology
– Cryptography, Digital Signatures, Third
Party Trust, and Public Key Certificates.
• Public Key Infrastructure
– Definitions, Components, Infrastructure,
Processes, and Issues.

Cryptography Methods
• 2 Types of Cryptography being used.
– Symmetric Key (shared secret) Cryptography
– Public Key Cryptography
• Each has a role in a Public Key
Infrastructure.

Symmetric Key Cryptography
• 1 Key known by both parties (shared)
• A message encrypted by the key can only
be decrypted using the same key.
• Issue: Hard to share the key securely.
Hello Ijfd82*7df Hello

Public Key Cryptography
• 2 keys generated. 1 private, 1 public.
• A message encrypted by 1 key can only be decrypted by
the other.
• Public keys are stored in a public repository and are
freely available.
• Private keys are stored on local system protected by a
password. Never transmitted over the network.
Hello 9klfms83f Hello
Bye Jf#f9j3f92 Bye
Private Public

Public key Cryptography
• 2 way encrypted communication
possible using 2 sets of public keys.
• Issue: Large resources required.
Hello 9klfms83f Hello
Bye Jf#f9j3f92 Bye
Party A’s
Public
Party B’s
Private
Party A’s
Private
Party B’s
Public
Party A Party B

Their roles in PKI
• Public keys are used
to securely transmit a
symmetric session
key.
• The symmetric key is
used to setup secure
encrypted
communications.
Party B’s
Private
Party B’s
Public
Party A Party B
Hello Ijfd82*7df Hello
Step 1: Party A
creates
symmetric key
and transmits it
to Party B using
their public key.
Step 2: Secure
communications
setup using the
symmetric key.

Digital Signature
• Private keys can be used to sign a document.
• The public key is used to decrypt the signature
which verifies that the message came from the
person who owns the private key.
• Issue: How does party B verify Party’s A Public
Key.
Party A’s
Public
Party A’s
Private
Party A Party B
Hello Bob
signed Jonny
Hello Bob
signed dfjlf9#fsi
Hello Bob
signed Jonny

Trusted Third Party
• A trusted third party is someone both
communicating parties trusts.
• This party authenticates Party A using older
style methods (ID Card) and verifies they own
the private key.
• This party then uses its own private key to
digitally sign party A’s public key.
• Since party B trusts the public key of the third
party, when it decrypts the signature on party
A’s Public key it can then trust A’s public key.
• Signed public keys can be used for
authentication.

Public Key Certificate (PKC)
• A public key certificate is a document that:
– Contains the public key of its owner.
– Contains a set of attributes that identifies its owner
– Is digitally signed by a trusted third party called a
Certificate Authority (CA).
– Has an life span (expiry date).
• Certificates are stored in public repositories.
• Used to authenticate, setup secure
communications and trust a digital signature.

Public Key Infrastructure (PKI)
• Defined by the IETF PKIX Working Group
as:
“The set of hardware, software, people, policies and
procedures needed to create, manage, store,
distribute, and revoke public key certificates based on
public key cryptography.”

PKI Component Definitions
• Certificate Authority (CA) : An authority trusted to create and
assign public key certificates. Required to validate user information
and verify they own the private key. Required to maintain CRLs.
• Registration Authority (RA) : An optional authority that can act on
behalf of a CA to validate user information and verify they own the
private key.
• Repository : A data base or directory used to store and distribute
Public Key Certificates and CRLs.
• Certificate Revocation Lists (CRL) : A list of certificates that have
been revoked due to their owners breaking one of the rules in the
certificate policy or by having its private key compromised.
• Certificate Policy (CP) : A set of rules which indicates how a
certificate is to be used by a community of users or set of
applications.
• Certificate Practice Statement (CPS) : A set of guidelines a CA
follows when issuing certificates.

Governed by Certificate Practice Statement.
Governed by Certificate Policy.
The Infrastructure
Repository
for PKCs
and CRLs
Certificate
Authority
Registration
Authority
User
Application
or Server
Certificate
and
revocation
list storage.
Certificate
and
revocation
list retrieval.
Certificate requests
Authentication and
Secure communication
Registration
process

Certificate use.
• During setup of connection between a server and user:
– Certificates are withdrawn from the repository for both
parties.
– Digital signatures are decrypted using the CA’s public key.
– The Certificate revocation list for the signing CA is
referenced to verify that the certificate has not been revoked.
– If all passes then authentication of the server and user has
been accomplished (i.e. each trusts that the private key is
owned by the person identified in the certificate).
• Secure communications are then setup by the user generating a
symmetric session key and transmitting it to the server using the
servers public key to encrypt it. Once the server has decrypted
the session key using its private key a secure socket is setup
using the session key.

The Repository(LDAP)
• A Repository:
– Requires an efficient directory capable of
authentication, replication and redundancy
– should be capable of storing more data than
just certificates and must be capable of
complicated searches
• LDAP provides all the requirements plus:
– can use Public Keys during its authentication
– is being integrated into many other
technologies
– Has a good set of standard APIs

Issues with PKI
• Certificate Revocation is still in its infancy.
• Trust
– Do we trust the commercial CAs out there. Why do we trust them to
authenticate information they are not the authority of.
– How do we trust repositories.
• Non PKI security holes
– How secure are clients, CAs, and repository systems from hackers
and virus attacks. Are they physically secure.
– How well guarded are private keys.
• Is the data in the certificate being check thoroughly.
• The idea of Non-Repudiation.
• Roaming Access (Smart Cards)

Types of electronic mail security
 Pretty Good Privacy
 S/Mime

Pretty Good Privacy
 Originator: Philip R. Zimmerman created
PGP in 1991.
 PGP provides a confidentiality and
authentication service that can be used for
electronic mail and file storage applications.
 Free, worldwide, works across a variety of
platforms.
 Based on known algorithms such as RSA

Why is PGP Popular?
Available free in source code on a variety of
platforms.
Based on publicly reviewed well known
algorithms.
Wide range of applicability
Not developed or controlled by
governmental or standards organizations

Operational Description
Consist of five services:
– Authentication
– Confidentiality
– Compression
– E-mail compatibility
– Segmentation

Authentication
 The steps for authentication are as follows
– The sender creates a message
– SHA-1 is used to generate 160-bit hash code
– Hash code is encrypted with RSA using senders
private key
– Receiver uses RSA to decrypt the hash code
– Receiver generates a new hash code and
compares with the decrypted one

Mod1: PGP Message with Authentication Only
MD=H(M)
PRa: A’s Private key for PK scheme
PUa: A’s Public key for PK scheme
EP : Public Key Encryption
DP : Public Key Decryption
EC: Symmetric Encryption
DC: Symmetric Decryption
H : Hash Function
|| : Concatenation
Z : Compression using ZIP Algorithm
R64 : Conversion to Radix 64 ASCII format
MD: Message Digest
EP[PRa, H(M) ]
EP[PRa, H(M) ]
Sent Message
M
MD’
MD
M || EP[PRa, H(M) ]

Confidentiality
 The steps to obtain confidentiality are as follows
– The sender generates a message and a random 128-bit
number called the session key
– The message is encrypted with CAST-128
– The session key is encrypted with recipients’ public key
using RSA
– The recipient uses RSA with its private key to decrypt the
session key
– The session key decrypts the message

Mod2: PGP Message with Confidentiality Only
Sent Message
PE(PUb, Ks)
PE(PUb, Ks)
Ks
Encrypted M
Ks: Session Key
H : Hash Function
|| : Concatenation
MD: Message Digest
Receiver user B
Sender user A
ECKs[ Z(M)] || PE(PUb, Ks)
Z(M)
ECKs[ Z(M)]

Confidentiality and Authentication
 To have both confidentiality and
authentication
– The sender first signs the message using it’s own
private key
– Then encrypts the message with the session with
the session key
– Then encrypts the session key with the recipient’s
private key

Mod3: PGP Message with Confidentiality & Authentication
Ks: Session Key
H : Hash Function
|| : Concatenation
MD: Message Digest
PE(PUb, Ks)
Signed
Message
PE(PUb, Ks)
Ks
H(M)=MD
PE(PRa, MD)
PE(PRa, MD)
MD’
MD
Sender user A
Receiver user B
M

Compression
 PGP compresses files using a ZIP algorithm
– The signature is generated before compression
 To store the uncompressed message with the signature
 Would interfere with compression because of multiple
compression algorithms exist.
– Message encryption is after compression
 To strengthen cryptographic security, as it reduces
redundancy

Compatibility
 E-mail sends only ASCII characters
– Because of this PGP converts message to ASCII
 Converts three octets into four ASCII characters
 Expands message by 33%
 After compression, there is a net reduction by a third

Segmentation and Reassembly
 Some mail providers impose a maximum
length of 50,000 octets
– PGP will automatically subdivide any message
too large into small enough segments to send via
e-mail
 This is done after all other processing

Sumary of PGP Services
Function Algorithm Used
Digital Signature DSS/SHA or
RSA/SHA
Message
Encryption
CAST or IDEA or
three -key triple DES
with Diffie -Hellman
or RSA
Compression ZIP
E-mail
Compatibility
Radix -64 conversion
Segmentation -

* = Fields used to index table
PGP Key Rings

PGP Message Generation from A to B
(Confidentiality and authentication)
Table
Decryption
Key
Password
Signature=PE(PUb, MD)
EP(PUb, Ks)
Ks: Session Key)
H : Hash Function
|| : Concatenation
R64 : Conversion to Radix 64 ASCII
format
MD: Message Digest

PGP Message from A, Reception on B site
(Confidentiality and authentication)
Table
Decryption
Key
Password
EP(PUb, Ks)
PE(PUb, MD)
Signature=
Message digest: MD’
Message digest: MD
Ks: Session Key
H : Hash Function
|| : Concatenation
R64 : Conversion to Radix 64 ASCII
format
MD: Message Digest

PGP Session Key and IV Generation
(Random Number Generator)

PGP Trust Model Example
Two partially trusted partner
Required to accept key!
Partially trusted
Fully trusted
Trusted By
two level trust
chain

Revoking Public Keys
The owner issue a key revocation
certificate.
Normal signature certificate with a revote
indicator.
Corresponding private key is used to sign
the certificate.

Cryptographic Keys
 PGP uses four types of keys
– Session keys
– Public keys
– Private Keys
– Passphrase keys

Cryptographic Keys
 Three requirements for the keys
– Needs a mean of generating unpredictable
session keys
– Would like a way to allow each user to have
multiple public/private key pairs
– Maintain a file of the public/private key pairs

Session Key Generation
 Random 128-bit numbers are generated
using CAST-128
 Input to the number generator takes in is a
128-bit key and two 64-bit blocks of plaintext.
– Input is determined by keystrokes and the times
the keystrokes are made
– Input is also effected by previous key outputs

Key identifiers
 With multiple private/public key pairs, there
needs to be a way for the receiver to know
which to use
– How this is done is through the combination of a
64 bit key ID, which is unique to a user ID.
 With this key ID, the receiver can retrieve the correct
public key of the sender to decrypt the message.
 A list of these key ID’s are placed in what is called a key
ring.

Key Rings
 There are both public and private key rings
– A user needs a passphrase key in order to retrieve a
private key, or to encrypt with a private key
– When creating a private key
 The user selects the passphrase to be used
 The system generates a new public/private key pair using
RSA, and using SHA-1 a 160-bit hash code is generated
from the passphrase
 The system encrypts the private key using CAST-128 with
the 128 bits of the hash code of the key and then the hash
code is discarded

Key Ring – Signing the message
 PGP retrieves sender’s private key using
user-id as an index.
 PGP prompts the user for the passphrase to
recover unencrypted private key
 Constructs signature component of the
message

Key Ring – Encrypting the Message
 PGP generates session key and uses it to
encrypt the message
 PGP retrieves the recipient’s public key from
it’s public-key ring using their user ID as an
index
 The session key of the message is
constructed

Key Ring – Decrypting the Message
 PGP retrieves the receiver’s private key from
the private-key ring using the key ID in the
session key component of the message as
an index
 PGP prompts the user for the passphrase to
recover the unencrypted private key
 PGP recovers the session key and decrypts
the message.

Key Ring – Authenticating the Message
 PGP retrieves the sender’s public key from
the public-key ring using the key ID from the
signature portion of the message as an
index.
 PGP recovers the transmitted message
digest
 PGP computers the message digest for the
received message

The Use of Trust
 Each public-key ring has a signature and a
signature trust entry for each public key
– This entry indicates the degree the PGP user
trusts the signer to certify public keys.
 Each public-key ring has a owner trust field
– This entry indicates the degree to which the public
key is trusted to sign other public key certificates.

S/MIME
 Stands for Secure/Multipurpose Internet Mail
Extension
 Security enhancement to the MIME internet
e-mail format

MIME – Header Files
 There are five message header fields
– MIME-Version
– Content-Type
– Content-Transferring Encoding
– Content-ID
– Content-Description

MIME – Content Types
 Text
– Plain
– Enriched
 Multipart
– Mixed
– Parallel
– Alternative
– Digest

MIME – Content Types
 Message
– Rfc822
– Partial
– External-body
 Image
– Jpeg
– Gif
 Video
– mpeg

Mime – Content Type
 Audio
– Basic
 Application
– PostScript
– Octet-stream

MIME – Content Transferring Encoding
 Two types
– Quoted printable
 Used when data consists largely of octets.
 Limits message lines to 76 characters.
– Base64 transfer encoding
 Common for encoding arbitrary binary data.

S/MIME Functionality
 S/MIME provides the following functions
– Enveloped Data
 Consists of encrypted content of any type of encrypted
content encryption keys
– Signed Data
 Contains a digital signature
– Clear-signed data
 Encoded digital signature
– Signed and enveloped data
 Encrypted and Signed data

S/MIME – Cryptographic Algorithms
 Create message digest to form digital
signature
– Must use SHA-1, Should support MD5
 Encrypt message digest to form signature
– Must support DSS, Should support RSA
 Encrypt session key for transmission
– Should support Diffie-Hellman, Must support RSA

S/MIME – Cryptographic Algorithms
 Encrypt message for transmission with one-
time session key
– Must support triple DES, Should support AES,
Should support RC2/40
 Create a message authentication code
– Must support HMAC with SHA-1, Should support
HMAC with SHA-1

S/MIME – User Agent Role
 Key generation
– Generating key with RSA
 Registration
– Register a user’s public key must be registered
with a certification authority
 Certificate storage and retrieval
– Access to a local list of certificates in order to
verify incoming signatures and encrypt outgoing

S/MIME – Enhanced Security Services
 Signed receipts
– The receiver returns a signed receipt back to the
sender to verify the message arrived
 Security labels
– Permission, priority or role of message being sent
 Secure mailing lists
– Sending to multiple recipients at once securely by
using a public key for the whole mailing list

IP Security
1. Overview
2. Architecture
3. Authentication Header
4. Encapsulating Security Payload
5. Combining security Associations
6. Internet Key Exchange.

IP Security
 have a range of application specific
security mechanisms
◦ eg. S/MIME, PGP, Kerberos, SSL/HTTPS
 however there are security concerns that
cut across protocol layers
 would like security implemented by the
network for all applications

IPSec
 general IP Security mechanisms
 provides
◦ authentication
◦ confidentiality
◦ key management
 applicable to use over LANs, across public
& private WANs, & for the Internet

Benefits of IPSec
 in a firewall/router provides strong
security to all traffic crossing the
perimeter
 in a firewall/router is resistant to bypass
 is below transport layer, hence
transparent to applications
 can be transparent to end users
 can provide security for individual users
 secures routing architecture

IP Security Architecture
 specification is quite complex
 defined in numerous RFC’s
◦ incl. RFC 2401/2402/2406/2408
◦ many others, grouped by category
 mandatory in IPv6, optional in IPv4
 have two security header extensions:
◦ Authentication Header (AH)
◦ Encapsulating Security Payload (ESP)

IPSec Services
 Access control
 Connectionless integrity
 Data origin authentication
 Rejection of replayed packets
◦ a form of partial sequence integrity
 Confidentiality (encryption)
 Limited traffic flow confidentiality

Security Associations
 a one-way relationship between sender &
receiver that affords security for traffic
flow
 defined by 3 parameters:
◦ Security Parameters Index (SPI)
◦ IP Destination Address
◦ Security Protocol Identifier
 has a number of other parameters
◦ seq no,AH & EH info, lifetime etc
 have a database of Security Associations

Authentication Header (AH)
 provides support for data integrity &
authentication of IP packets
◦ end system/router can authenticate user/app
◦ prevents address spoofing attacks by tracking
sequence numbers
 based on use of a MAC
◦ HMAC-MD5-96 or HMAC-SHA-1-96
 parties must share a secret key

Encapsulating Security Payload (ESP)
 provides message content confidentiality &
limited traffic flow confidentiality
 can optionally provide the same authentication
services as AH
 supports range of ciphers, modes, padding
◦ incl. DES,Triple-DES, RC5, IDEA, CAST etc
◦ CBC & other modes
◦ padding needed to fill blocksize, fields, for traffic flow

Encapsulating Security Payload

Transport vs Tunnel Mode ESP
 transport mode is used to encrypt &
optionally authenticate IP data
◦ data protected but header left in clear
◦ can do traffic analysis but is efficient
◦ good for ESP host to host traffic
 tunnel mode encrypts entire IP packet
◦ add new header for next hop
◦ good forVPNs, gateway to gateway security

Combining Security Associations
 SA’s can implement either AH or ESP
 to implement both need to combine SA’s
◦ form a security association bundle
◦ may terminate at different or same
endpoints
◦ combined by
 transport adjacency
 iterated tunneling
 issue of authentication & encryption
order

Combining Security Associations

Web Security
 HTTP is not a secure protocol
◦ simple and stateless client/server application running
over TCP/IP
 Added security measures needed
◦ we will see SSL (Secure Socket Layer) and TLS
(Transport Layer Security)
◦ HTTPS
 Secure HTTP protocol
◦ SSL support is provided for several other TCP/IP
applications as well
 POP3, SMTP, FTP, News, ...

Web Security
 Threats
◦ Integrity
 data modification, insertion
 cryptographic checksums (HMAC)
◦ Confidentiality
 eavesdropping on the net
 can be prevented by encryption
 theft from server machine
 on-site security measures needed
◦ Authentication
 impersonation, data forgery
 we will see some cryptographic techniques
◦ Denial of service, hacked web servers
Scope
of
SSL
/
TLS

Where to provide security?
 Long-lasting discussion, no ultimate
answer
have seen this lecture
have seen and
will see

SSL (Secure Socket Layer)
 originally developed by Netscape
 version 3 designed with public input
 subsequently Internet standardization
effort started at IETF
◦ TLS (Transport Layer Security) working group
established
◦ TLS can be viewed as SSL v3.1 and compatible
with SSL v3

SSL Protocol Stack
 adds security
features
◦ reliable and secure
end to end data
transfer
 SSL is not a single
protocol
◦ two-layers of
protocols
• makes use of TCP (reliable end to end
data transfer)

Two SSL concepts
 SSL session
◦ an association between client and server
◦ define a set of cryptographic parameters created by
the Handshake Protocol
◦ may be shared by multiple SSL connections
 SSL connection
◦ a transient, peer-to-peer, secure communication link
◦ associated with (derived from) a SSL session
 Both are characterized by several parameters
◦ that define a session state or connection state

Session state parameters
 Session identifier
◦ chosen by server
 Peer certificate
◦ certificate of the peer entity (server’s if the entity is client, client’s if the
entity is server)
◦ may be null (which is the likely case for server)
 Compression method
◦ algorithm used for compression
 Cipher Spec
◦ bulk data encryption algorithm (DES, etc.) - may be null (rarely)
◦ hash algorithm used in cryptographic checksum (MD5 or SHA-1)
 Master Secret
◦ 48-bytes secret shared between client and server
 Is resumable
◦ a flag that specifies if the session can be used later

Connection State Parameters
 Random numbers
◦ server and client exchange
◦ used as nonces during key exchange
 MAC secret
◦ secret key used for MAC operations
 conventional encryption key
 initialization vector
◦ if CBC mode is used
 sequence numbers
◦ each party maintains separate sequence numbers

SSL Record Protocol
 serves to SSL connections
◦ uses connection parameters
 provides confidentiality and integrity
 also fragments (into 214
bytes chunks) and optionally
compresses data (in practice no compression)
 confidentiality
◦ IDEA, RC2-40, DES-40, DES, 3DES, Fortezza, RC4-40,
RC4-128
◦ message is optionally compressed before encryption
 message integrity
◦ using a MAC with shared secret key
◦ similar to HMAC but pads are concatenated rather than
XORed

SSL Record Protocol
header fields
 content type (higher layer protocol)
 Version
 fragment length

Change Cipher Spec Protocol
 very simple protocol that uses the record
protocol
 the new state established by the handshake
protocol is a pending state
◦ that is not yet valid
 change cipher spec protocol (actually a single
command exchanged between client and
server) makes this pending state the current
one
◦ connection parameter changes
 will see its use in handshake protocol

Alert Protocol
 conveys SSL-related alerts to peer entity
 secured using the record protocol
◦ and with current connection state parameters
 each message is two bytes
◦ one byte for level (severity)
 warning (connection may resume) or fatal (connection is terminated)
◦ one byte for the alert code
 unexpected message, bad record MAC, decompression failure
 handshake failure (no common ground), illegal parameters
(inconsistent or unrecognizable parameters)
 close notify
 no certificate, bad certificate, unsupported certificate, certificate
revoked, certificate expired, certificate unknown

Handshake Protocol
 The most complex part of SSL
 Allows server and client
◦ to authenticate each other
◦ to negotiate encryption and MAC algorithms
◦ to negotiate cryptographic keys to be used
 handshake is done before any data is
transmitted
◦ so cannot assume a secure record protocol
 handshake is performed (in an abbreviated way)
even if an old session is used

Handshake Protocol
 a series of messages in phases
◦ Establish Security Capabilities
◦ Server Authentication and Key Exchange
◦ Client Authentication and Key Exchange
◦ Finish
 Handshake message format
 Message types

Handshake Phase 1 – Establish Security
Capabilities
 Client Hello (a list of client’s preferences)
◦ version: highest version supported by client
◦ client’s random
 also includes a timestamp
 against replay attacks
◦ session ID
 nonzero means client wants to use an existing session state for a
new connection state; zero means new connection on a new
session
◦ compression methods supported by client
◦ Cipher Suite
 a list that contains the combination of crypto algorithms supported
by the client in order of preference
 each entry is a key exchange algorithm and a cipher spec

Handshake Phase 1 – Establish Security
Capabilities
 Server Hello (response to client’s requests)
◦ version: version proposed by client if also supported
by server, otherwise highest supported by server
◦ server’s random
 same structure as client’s but independent
◦ session ID
 if client offered one and it is also supported by server, then
the same ID
 otherwise a new ID assigned by server
◦ compression methods chosen from the client’s list
◦ Cipher Suite selected from the client’s list

Key exchange methods
 how the conventional encryption and
MAC keys are exchanged?
◦ actually first pre-master secret is exchanged
◦ master secret is derived from it
◦ other keys are derived from the master secret

Key exchange methods – cont’d
 Rephrase question: how the pre-master secret is
exchanged?
◦ RSA
 server provides an RSA certificate, client encrypts the pre-
master secret and sends it
◦ Fixed Diffie-Hellman (DH)
 Server DH parameters are fixed and sent in a certificate
◦ Ephemeral DH
 server certificate contains an RSA or DSS key
 server creates DH parameters (used one-time) and signs by this
key
◦ Anonymous DH
 no certificates, no authentication, just send out DH parameters
 vulnerable to man-in-the-middle-attacks

Some Cipher Specs Fields
 Cipher algorithm
◦ RC4, RC2, DES, 3DES, DES40 (40-bit DES), IDEA
 Hash algo. for MAC
◦ MD5 or SHA-1
 Cipher type
◦ stream or block
 Is Exportable
◦ binary
 IV size
◦ size of the init. vector for CBC mode

Handshake Phase 2: Server Auth. and Key
Exchange
 Certificate is needed if anon-DH is not used
(which is the case most of the time)
◦ needed for server authentication
◦ if fixed DH, then certificate contains enough
information for key exchange (so server key exchange
message is not needed)

Exchange
 Server Key Exchange
◦ not needed for
 fixed DH and RSA key exchange (if RSA key is not signature only)
◦ message content depends on the key exchange method agreed
 Anon-DH
 message contains two DH public parameters and server’s public key
 Ephemeral DH
 same as anon-DH plus a signature on them
 RSA key exchange (if server’s RSA key is signature-only)
 server sends a temporary RSA encryption key to client in a signed message
◦ Signatures contain random values to resist against replay attacks

Exchange
 Certificate Request Message
◦ although not common in practice, server may request
client to send its certificate
 to authenticate the client
◦ two fields: certificate type and acceptable CAs
 a list of them
◦ Certificate types
 fixed DH (certificate may be signed with RSA or DSS)
 ephemeral DH (certificate may contain RSA or DSS key)
 signature only (not used for key exchange but for auth.)
 RSA or DSS
 Server Hello Done message
◦ server is finished and will wait for client’s response

Handshake Phase 3: Client Auth. and Key
Exchange
 Upon receipt of server hello done
◦ client checks the server certificate and server hello
parameters
◦ after that client starts sending its own messages
 Client’s Certificate
◦ is sent if requested and available

Handshake Phase 3: Client Auth. and Key Exchange
 Client Key exchange message
◦ content depends on the key exchange method agreed
◦ RSA
 48-byte pre-master secret is encrypted using server’s RSA key
(obtained at phase 2)
◦ fixed-DH
 client DH params are in client certificate, so key exchange
message is null
◦ Anon or ephemeral DH
 Client DH params and public key are sent
 no signature even for ephemeral DH
◦ no client authentication and authenticated key exchange
so far

Handshake Phase 3: Client Auth. and Key
Exchange
 CertificateVerify message
◦ in client key exchange message, the client is not
authenticated
 anyone could send the key exchange message
◦ a method for authentication is the certificate verify
message
 client shows ownership of private key corresponding the public
key in client certificate by signing a hash that contains the master
secret and handshake messages
 except for fixed DH that does not contain a signature key
◦ what about authentication for fixed DH case?
 no authentication but the attacker cannot produce the pre-
master and master secrets since it does not know the DH
private key

Handshake Phase 4: Finish
 Wrap-up
 Change cipher spec messages
◦ to make the pending cipher spec the current one

Handshake Phase 4: Finish
 Finish message
◦ a MAC on exchanged handshake messages using the
master secret
◦ to verify that handshake is successful and both parties
have the same master secret
◦ client’s finished is verified by server and vice versa
◦ the connection state of the record protocol that
encrypts and MACs finished message is the new one
 so this is also verification of all the keys created

TLS (Transport Layer Security)
 TLS is a proposed Internet Standard (RFC
2246)
◦ similar to SSL v3, some difference are given here
 Version number
◦ record format is the same, but the major version 3,
minor version 1 (v3.1)
 MAC
◦ TLS uses HMAC with pads XORed (unlike SSL where
pads are appended)
 additional alert codes

TLS (Transport Layer Security)
 Same cipher suites of SSL except Fortezza
◦ actually it is not common in SSL v3 either
 No ephemeral client certificates inTLS
◦ since signature-only certificates are used for that
purpose
 some changes in certificate verify and finished
message calculations
 a different Pseudorandom function (PRF)
◦ master secret and key block calculations use PRF in
TLS

INTRUDERS:
 One of the most publicized attacks to security is
the intruder, generally referred to as hacker or cracker.
 An individual who seizes supervisory control of the
system and uses this control to evade auditing and
access controls or to suppress audit collection.
 Someone who intrudes on the privacy or property of
another without permission.
 Cryptographic system audit consists of multiple layers
— examining the cryptographic protocol for risks and
flaws, making sure the implementation conforms with
the protocol and reviewing the code itself for bugs and
mistakes.

Types of intruders
 MASQUERADER:
 In case of an insider attack, a masquerade attacker
gains access to the account of a legitimate user either
by stealing the victim's account ID and password, or
by using a keylogger.
 For example, if a legitimate user leaves the terminal
or session open and logged in, a co-worker may act as
a masquerade attacker.
 If an authorization process is not fully protected, it can
become extremely vulnerable to a masquerade attack.
 Masquerade attack involves impersonating legitimate
sources and creating fake identities.

Types of intruders
 MISFEASOR:
 Generally an insider.
 Performs unauthorized access to data,
programs or resources.
 Misuses his/her preveilages.
 CLANDESTINE USER
 Can be either an insider or outsider.
 Seizes supervisory control of the system and
uses it to evade auditing and access controls or
to suppress audit collection.

Intrusion techniques:
 The objective of the intruders is to gain access to a
system or to increase the range of privileges
accessible on a system.
 Generally, this requires the intruders to acquire
information that should be protected. In most cases,
the information is in the form of a user password.
• Asymmetric Routing.
• Buffer Overflow Attacks.
• Common Gateway Interface Scripts.
• Protocol-Specific Attacks.
• Traffic Flooding
• Trojans.
• Worms.

Asymmetric routing:
 In Asymmetric Routing network packets leave via one
path and return via a different path.
 In this method, the attacker attempts to utilize more
than one route to the targeted network device.
Buffer Overflow Attacks:
 This approach attempts to overwrite specific sections
of computer memory within a network, replacing
normal data in those memory locations with a set of
commands that will later be executed as part of the
attack.
 In most cases, the goal is to initiate a denial of service
(DoS) situation, or to set up a channel through which
the attacker can gain remote access to the network.

 Scripts:
 The Common Gateway Interface (CGI) is routinely used
in networks to support interaction between servers
and clients on the Web.
 But it also provides easy openings—such as
"backtracking"—through which attackers can access
supposedly secure network system files.
 When systems fail to include input verification or check
for backtrack characters, a covert CGI script can easily
add the directory label ".." or the pipe "|" character to
any file path name and thereby access files that should
not be available via the Web.

 Protocol-Specific Attacks:
 When performing network activities, devices obey
specific rules and procedures.
 These protocols—such as ARP, IP, TCP, UDP, ICMP, and
various application protocols—may inadvertently leave
openings for network intrusions via protocol
impersonation ("spoofing") or malformed protocol
messages.
 For example, Address Resolution Protocol (ARP) does
not perform authentication on messages, allowing
attackers to execute "man-in-the-middle" attacks.
 Protocol-specific attacks can easily compromise or even
crash targeted devices on a network.

 Traffic flooding :
 An ingenious method of network intrusion simply
targets network intrusion detection systems by
creating traffic loads too heavy for the system to
adequately screen. In the resulting congested and
chaotic network environment, attackers can
sometimes execute an undetected attack and even
trigger an undetected "fail-open" condition.
 Trojans:
 These programs present themselves as benign and do
not replicate like a virus or a worm. Instead, they
instigate DoS attacks, erase stored data, or open
channels to permit system control by outside
attackers. Trojans can be introduced into a network
from unsuspected online archives and file repositories,

 Worms
 A common form of standalone computer virus, worms
are any computer code intended to replicate itself
without altering authorized program files.
 Worms often spread through email attachments or
the Internet Relay Chat (IRC) protocol.
 Undetected worms eventually consume so many
network resources, such as processor cycles or
bandwidth, that authorized activity is simply squeezed
out.
 Some worms actively seek out confidential
information—such as files containing the word
"finance" or "SSN"—and communicate such data to

Intrusion Detection System (IDS)
 An Intrusion Detection System (IDS) is a system that
monitors network traffic for suspicious activity and
issues alerts when such activity is discovered.
 It is a software application that scans a network or a
system for harmful activity or policy breaching.
 Any malicious venture or violation is normally
reported either to an administrator or collected
centrally using a security information and event
management (SIEM) system.
 A SIEM system integrates outputs from multiple
sources and uses alarm filtering techniques to
differentiate malicious activity from false alarms.

Classification of Intrusion Detection System:
 Network Intrusion Detection System (NIDS):
Network intrusion detection systems (NIDS) are set up at a planned point
within the network to examine traffic from all devices on the network. It
performs an observation of passing traffic on the entire subnet and matches
the traffic that is passed on the subnets to the collection of known attacks.
Once an attack is identified or abnormal behavior is observed, the alert can
be sent to the administrator. An example of an NIDS is installing it on the
subnet where firewalls are located in order to see if someone is trying crack
the firewall.
 Host Intrusion Detection System (HIDS):
Host intrusion detection systems (HIDS) run on independent hosts or devices
on the network. A HIDS monitors the incoming and outgoing packets from
the device only and will alert the administrator if suspicious or malicious
activity is detected. It takes a snapshot of existing system files and compares
it with the previous snapshot. If the analytical system files were edited or
deleted, an alert is sent to the administrator to investigate. An example of
HIDS usage can be seen on mission critical machines, which are not
expected to change their layout.

Classification of Intrusion Detection System:
 Protocol-based Intrusion Detection System (PIDS):
Protocol-based intrusion detection system (PIDS) comprises of a system or
agent that would consistently resides at the front end of a server,
controlling and interpreting the protocol between a user/device and the
server. It is trying to secure the web server by regularly monitoring the
HTTPS protocol stream and accept the related HTTP protocol.
 Application Protocol-based Intrusion Detection System (APIDS):
Application Protocol-based Intrusion Detection System (APIDS) is a system
or agent that generally resides within a group of servers. It identifies the
intrusions by monitoring and interpreting the communication on
application specific protocols. For example, this would monitor the SQL
protocol explicit to the middleware as it transacts with the database in the
web server.
 Hybrid Intrusion Detection System :
Hybrid intrusion detection system is made by the combination of two or
more approaches of the intrusion detection system. In the hybrid intrusion
detection system, host agent or system data is combined with network
information to develop a complete view of the network system. Hybrid
intrusion detection system is more effective in comparison to the other

Detection Methods of IDS:
1. Signature-based Method:
Signature-based IDS detects the attacks on the basis of the
specific patterns such as number of bytes or number of 1’s or
number of 0’s in the network traffic. It also detects on the basis of
the already known malicious instruction sequence that is used by
the malware. The detected patterns in the IDS are known as
signatures.Signature-based IDS can easily detect the attacks
whose pattern (signature) already exists in system but it is quite
difficult to detect the new malware attacks as their pattern
(signature) is not known.
2. Anomaly-based Method:
Anomaly-based IDS was introduced to detect the unknown
malware attacks as new malware are developed rapidly. In
anomaly-based IDS there is use of machine learning to create a
trustful activity model and anything coming is compared with
that model and it is declared suspicious if it is not found in model.
Machine learning based method has a better generalized
property in comparison to signature-based IDS as these models

Intrusion prevention:
 Password Management :
 The front line of defense against intruders is the
password system.
 Virtually all multiuser systems require that a user
provide not only a name or identifier (ID) but also a
password.
 The password serves to authenticate the ID of the
individual logging on to the system. In turn, the ID
provides security in the following ways:
 The ID determines whether the user is authorized to
gain access to a system.
 The ID determines the privileges accorded to the user.

 Password Selection Strategies:
 The goal is to eliminate guessable passwords while
allowing the user to select a password that is
memorable. Four basic techniques are in use:
◦ User education.
◦ Computer-generated passwords.
◦ Reactive password checking.
◦ Proactive password checking.

 User education
◦ Users can be told the importance of using hard-to-guess passwords
and can be provided with guidelines for selecting strong passwords.
 Computer-generated passwords
◦ passwords are quite random in nature
 Reactive password checking
◦ the system periodically runs its own password cracker to find
guessable passwords. The system cancels any passwords that are
guessed.
 Proactive password checking approaches:
◦ Rule enforcement:
 All passwords must be at least eight characters long.
 The passwords must include at least one each of uppercase,
lowercase, numeric digits, and punctuation marks.
 Another possible procedure is simply to compile a large dictionary of
possible "bad" passwords.

 How to Choose a secure password?
 Do NOT use words or phrases that have personal
significance.
 Mix letters, numbers and symbols, and use case sensitivity
 Try to memorize the password, and avoid writing it down
 Do not use the same password for everything
 Use a password manager (PM). It is a utility that creates an
encrypted file where your passwords are stored.
 Try to use "nonsense words."
 Do not tell anybody your password.

Index
 Introduction
 Terminologies
 Categories
 Backdoors
 Logic Bomb
 Trojan Horse
 Nimda attack
 Important questions

Introduction
Malicious software also known commonly as
malware refers to a more sophisticated type of threat
to the computer system which are presented by
programs that exploit vulnerabilities in computing
systems.

More about Malware
 Malware is a software designed to cause
damage to or use up the resources of a
target computer
 It is concealed within or masquerades
as legitimate software
 In some cases it spreads itself via email
or infected floppy disks

Terminologies
 Virus: malware that when executed tries
to replicate itself into another
executable code , this code is said to be
infected .when the infected code is
executed the virus also executes.
 Worm: a program that can run
independently and propagate a
working version of itself onto other
hosts.

 Mobile code: software that can be shipped to
a collection of platforms and execute with
identical semantics.
 Auto-rooter: malicious hacker tools used to
break into new machines remotely.
 Kit : set of tools to generate virus
automatically.
 Spammer programs : used to send large
volumes of unwanted emails.
 Flooder : used to attack networked
computer with a large volume of traffic to
carry out a denial of service attack.

 Root kit : set of tools used after attacker has
broken into a system and gained root level
control.
 Zombie ,bot :program activated on infected
machine that is further activated to launch
attacks on other machines.
 Spyware : software that collects information
from a computer and transmits it into another
system.
 Adware : advertising that is integrated into software
that can result in pop ups or redirection of a
browser onto a commercial site.

Categories Of Malware
• Referred to as parasitic
• Essential fragments of
programs which cannot
exists independently
• Ex viruses ,logic bombs
,back doors
Needs a
host
• Self contained programs that
can be run on the o.s
• Ex worms , bot programs
Independent

..Also classified into
Activated
by a trigger
Ex logic
bombs ,bac
k doors ,
bot
programs
Do not
replicate When
executed may
produce one
or more
copies of itself
and can be
activated on
same systems
Ex viruses
and worms
Do
replicate

Backdoors
 It is a secret entry point into a program that
allows to gain access without going through usual
security procedures . It is a threat when
unscrupulous programmers use them for
unauthorized access.
 Programmers use backdoors legitimately to
debug and test programs such a backdoor is
called maintenance hook .This is usually done
when programmers develop application that has
an authorization procedure in it.
 To secure the backdoor the security measures
should focus on the program development and
software update activities.

Logic Bombs
 This is one of the oldest type of program
threat .
 Logic bomb is basically a program
embedded into a software by an
intruder ,which lies dormant until a
predefined condition is met after which
the program then triggers an
unauthorized act.

Trojan horse
 It is a program or a command procedure
containing a hidden code that when
invoked performs some unwanted or
harmful function.
 Trojan horse can be used to change the
file permission for the file to be shared
within many users.
 It can be also used for destroying the file
when it seems to be doing a particular task.

Nimda Attack
 Referred to as a simple worm , it uses four
distribution methods which are as follows:
 Email: a user on a host opens an infected
email attachment , nimda looks for email
addresses on the host and sends copies of
itself to these addresses.
 Windows shares : it scans hosts for
unsecured file shares and infects the files on
that host , so as when a user runs an infected
file , which will activate nimda on that host.

 Web Servers : Nimda scans web serves
and if it finds a vulnerable server ,it
attempts to transfer a copy of itself and
infect it.
 Web Clients : if a vulnerable web client
visits a web server that has been
infected by Nimda the workstation of
the web client will become infected.

Malicious Logic
 Malicious logic is a set of instructions that
cause a site’s security policy to be
violated.
◦ Trojan horses
◦ viruses
◦ worms
16

Viruses and ”Malicious” Programs
 Computer “Viruses” and related programs have the ability to
replicate themselves on an ever increasing number of
computers. They originally spread by people sharing floppy
disks. Now they spread primarily over the Internet (a
“Worm”).
 Other “Malicious” Programs may be installed by hand on a
single machine. They may also be built into widely distributed
commercial software packages. These are very hard to
detect before the payload activates (Trojan Horses,Trap
Doors, and Logic Bombs).
16

Taxonomy of Malicious Programs
Need Host
Program
Independent
Trapdoors Logic
Bombs
Trojan
Horses
Viruses Bacteria Worms
Malicious
Programs
16

Definitions
 Virus - code that copies itself into other
programs
 A “Bacteria” replicates until it fills all disk space,
or CPU cycles
 Payload - harmful things the malicious program
does, after it has had time to spread
 Worm - a program that replicates itself across
the network (usually riding on email messages
or attached documents (e.g., macro viruses)
 Macro - virus composed of sequence of
instructions that are interpreted rather than
executed directly 16

Definitions
 Boot Sector is used to bootstrap a system or
mount a disk- executed when the system
“sees” the disk for the first time
 Boot sector infector - virus that inserts itself
into the boot sector of a disk
16

Definitions
 TSR - terminate and stay resident virus - stays
active in memory after the application has
terminated
 Stealth viruses - conceal the infection of files
 Polymorphic - viruses that change form each
time it inserts itself into a program
16

Definitions
 Trojan Horse - instructions in an otherwise good
program that cause bad things to happen (sending your
data or password to an attacker over the net).
 Logic Bomb - malicious code that activates on an event
(e.g., date).
 Trap Door (or Back Door) - undocumented entry point
written into code for debugging that can allow
unwanted users.
 Easter Egg - extraneous code that does something
“cool.” A way for programmers to show that they
control the product.
17

Virus Phases
 Dormant phase - the virus is idle
 Propagation phase - the virus places an
identical copy of itself into other
programs
 Triggering phase – the virus is
activated to perform the function for
which it was intended
 Execution phase – the function is
performed
17

Virus Protection
Have a well-known virus protection program, configured to
scan disks and downloads automatically for known viruses.
Do not execute programs (or "macro's") from unknown
sources (e.g., PS files, Hypercard files, MS Office documents,
Avoid the most common operating systems and email
programs, if possible.
17

Virus Techniques
 Stealth viruses
◦ Infect OS so that infected files appear normal to user
 Macro viruses
◦ A macro is an executable program embedded in a word
processing document (MS Word) or spreadsheet (Excel)
◦ When infected document is opened, virus copies itself into global
macro file and makes itself auto-executing (e.g., gets invoked
whenever any document is opened)
 Polymorphic viruses
◦ Viruses that mutate and/or encrypt parts of their code with a
randomly generated key
17

Types ofViruses
 ParasiticVirus - attaches itself to executable files as part of their
code. Runs whenever the host program runs.
 Memory-residentVirus - Lodges in main memory as part of the
residual operating system.
 Boot SectorVirus - infects the boot sector of a disk, and spreads
when the operating system boots up (original DOS viruses).
 StealthVirus - explicitly designed to hide fromVirus Scanning
programs.
 PolymorphicVirus - mutates with every new host to prevent
signature detection.
17

Firewall
 Definition: A Network Firewall is a system or
group of systems used to control access between
two networks -- a trusted network and an untrusted
network -- using pre-configured rules or filters.
176

 Firewall is device that provides secure
connectivity between networks (internal/
external).
 It is used to implement and enforce a security
policy for communication between networks.
 A firewall may be a hardware, software or a
combination of both that is used to prevent
unauthorized program or internet users from
accessing a private network or a single computer.
177

 All messages entering or leaving the intranet pass
through the firewall, which examines each message &
blocks those that do not meet the specified security
criteria.
178

Why do we need a firewall?
 To protect confidential
information from those who do
not explicitly need to access it.
 To protect our network & its
resources from malicious users
& accidents that originate
outside of our network.
179

Types of firewall
1. Hardware firewall
2. Software firewall
180

1. Hardware Firewall
 It is a physical device.
 It can be installed between the
modem and computer.
 It can be incorporated into a
broadband router being used to
share the internet connection.
 Protects an entire network.
181

 Usually more expensive, harder to configure.
 E.g.- Cisco pix, Netscreen,Watchfuard etc.
182

2. Software Firewall
 It is a software application.
 It is installed onto the computer system
that you wish to protect .
 Protects a single computer.
 This is usually the computer with
modem attached to it.
183

 Usually less expensive, easier to configure.
 E.g.- Norton internet security, MacAfee
internet security etc.
184

Types of firewall technique
 Packet filter
 Application gateway
 Circuit-level gateway
 Bastion host
185

Packet filter
 It looks at each packet entering or leaving the
network and accepts or rejects it based on
user-defined rules.
186

 Packet filtering is fairly effective & transparent
to users, but it is difficult to configure.
 In addition, it is susceptible to IP spoofing.
187

Application gateway
 In such type of firewall remote host or
network can interact only with proxy server,
proxy server is responsible for hiding the
details of the internal network i.e. intranet.
 Users uses TCP/IP application, such as FTP &
Telnet servers.
188

• This is very effective, but can impose a
performance degradation.
189

Circuit – level Firewall
 This can be a stand – alone system or it can be a
specialized functions performed by an application –
level gateway for certain applications.
 It does not permit an end – to – end TCP
connection; rather, the gateway sets two TCP
connections.
 A typical use of the circuit – level gateway is a
situation in which the system administrator trusts
the internal users. 190

• The gateway can be configured to support
application level or proxy service on
inbound connections and circuit level
functions for outbound connections.
191

Bastion Host
 Bastion host is a special purpose computer on
a network specifically designed and configured
to withstand attacks.
192

 It generally hosts a single application, provides
platform for application gateway and circuit-
level gateway.
 It supports limited/specific applications to
reduce the threat to the computer.
 Include application-Telnet, SMTP, FTP
193

What a personal firewall can do
 Stop hackers from accessing your computer.
 Protect your personal information.
 Blocks “pop up” ads and certain cookies.
 Determines which programs can access the
internet.
 Block invalid packets. 194

What a personal firewall can not
do
 Cannot prevent e-mail viruses
-only an antivirus product with update definitions
can prevent e-mail viruses.
 After setting it initially, you cannot forget about it
-The firewall will require periodic updates to the
rule sets and the software itself.
195

Module III CNS Kerberos and its example.pptx

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