453 Network Security

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Nov 2, 2013 (3 years and 8 months ago)

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453 Network Security

Section 7: IPSec/SSL

Dr. E.C. Kulasekere


Sri Lanka Institute of Information
Technology
-

2006

IP Security


S/MIME (Multipurpose Internet Main
Extension), PGP, Kerberos are some
examples of application specific security
mechanisms


However there are security concerns that
cut across protocol layers


Would like security implemented by the
network for all applications, even for
security
-
ignorant applications.

Need for IP Security


Here is an example of a secure private
TCP/IP network with


Disallowing links to untrusted sites


Encrypting packets that leave the premises


Authenticating packets that enter the
premises


All of the above varying requirements can
be met if you provide IP level security.


With IP security applications of all kinds
will be covered.


IPsec


The functional areas general IP Security
mechanisms provides


Authentication: This provides source
authentication.


Confidentiality: encryption


key management: transfer of keys securely.


applicable to use over LANs, across public
& private WANs, & for the Internet

IP Security Overview


This was introduced to address: IP
Spoofing, eavesdropping, packet sniffing
etc.


Came after IPv4, actually for IPv6. But can
also be used with IPv4.


The principle feature of IPsec is that it
supports varied applications and
authenticate all traffic at the IP level. This
is primarily good for distributed
applications.

Applications of IPsec


Secure branch office connectivity over the
internet.


A company can build a secure VPN over the
internet.


Hence the internet is used heavily and no
private network is required.


Saving in cost and network management
overhead is the end result.

Applications of IPsec


Secure remote access over the internet.


The office system has to be equipped with
IPsec.


The user dials into the ISP and then gains
access securely to the company network.


This reduces the toll charges for traveling
employees and telecommuters.


Enhancing e
-
commerce security


Web and eCommerce applications have built
in security protocols, the use of IPsec will
enhance this.

IPSec Uses

Benefits of IPSec


in a firewall/router provides strong security
to all traffic crossing the perimeter. The
traffic within the company or workgroup
will not incur additional overhead.


is resistant to bypass in a firewall.


is below transport layer, hence transparent
to applications


can be transparent to end users as well.


can provide security for individual users if
desired

Routing Applications of IPsec


In addition to protecting the premises and
users it will additionally play a role in
routing architectures in internetworking. It
can assure that


A router advertisement comes from an
authorized router.


A neighbor advertisement comes from an
authorized router.


A redirect message comes from the router to
which the initial packet was sent.


A routing update is not forged.

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. In
both cases the security features are
implemented as extension headers that
follow the main IP header.


Areas Addressed in the IPsec
Documentation


Architecture. General concepts and
requirements.


Encapsulating security payload (ESP):
packet format for encryption.


Authentication header (AH): packet format
for authentication.


Encryption algorithm: different algorithms.


Key management:

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


IPsec Services Provided by AH and ESP
Protocols

Security Associations (SA)


This is the key concept common to both
authentication and confidentiality.


An association is a one
-
way relationship
between the sender and the receiver that
affords security services to the traffic carried
on it.


If this is required to have a two way secure
communication, then TWO SAs are required.


Security services are afforded to an SA for
the use of AH or ESP, but not both. Since
both require different packet formats which
cannot be achieved simultaneously


Security Associations


defined by 3 parameters:


Security Parameters Index (SPI): only of local
significance. The SPI is carried in AH and ESP
headers to enable the receiving system to select the
SA under which a received packet will be processed.


IP Destination Address: The address of the
destination endpoint of the SA eg. End user system
firewall or router


Security Protocol Identifier: This indicates whether the
association is an AH or ESP security association.


has a number of other parameters: seq no, AH & EH
info, lifetime etc


have a database of Security Associations: Security
policy database (SPD)

Modes of Use for AH and ESP


Transport Mode: This mode provides
protection primarily for upper layer
protocols


ESP in transport mode encrypts and
optionally authenticates the IP payload but not
the IP header (used for end
-
to
-
end
communications)


AH in transport mode authenticates the IP
payload and selected portions of the IP
header.


Modes of Use for AH and ESP


Tunnel Mode: This mode provides
protection to the entire packet.


After the AH and ESP fields are added to the
IP packet, the entire packet plus the security
fields are treated as the payload of new
`outer’ IP packet with a new outer IP header.


No routers on the way are able to examine
the inner IP header.


Since the total is encapsulated, the
destination and source addresses may be
completely different adding security to the
packet during transition.

Tunnel and Transport Mode
Functionality

Authentication Header (AH)


provides support for data integrity &
authentication of IP packets in transit


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

Authentication Header (pp.491)

Anti
-
Replay Service


This is the type of attack where the
attacker obtains a copy of an
authenticated packet and later transmit it
to the intended destination.


This is circumvented using the sequence
number.


The sequence number is initialized when
the new SA is established and then
incremented for each subsequent packet.

Anti
-
Replay Service …


Since IP is connectionless, the delivery of the
packet is not guaranteed. Hence the receiver
implements a window size of W before it
determines that the packet expires.

Transport & Tunnel Modes

END to END & END to INTERMEDIATE

pp. 495

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 most common


pad to meet block size, 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

Transport vs. Tunnel Mode ESP …


tunnel mode encrypts entire IP packet


add new header for next hop


good for VPNs, 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 bundle


have 4 cases (see next)

Combining SAs


Case 1


All security is provided between end
-
systems


The secret key should be shared by end systems


The following combinations possible


AH in transport mode, ESP in transport mode, AH followed
by ESP in transport mode, any of the above inside an AH
or ESP in tunnel mode

Combining SAs


Case 2


Security is provided between gateways routers
firewalls etc and no host implements IPsec.


A single tunnel SA using AH, ESP or ESP with
authentication can be used. Nested tunnels are
not required.

Combining SAs


Case 3


Builds on case 2 by adding end
-
to
-
end security.

Combining SAs


Case 4


Provides support for a remote host that uses the
internet to reach an organization’s firewall and
then to gain access to some server or
workstation behind the firewall. Only tunnel
mode is required.

Web Security


Web now widely used by business,
government, individuals


but Internet & Web are vulnerable


have a variety of threats


integrity


confidentiality


denial of service


authentication


need added security mechanisms

A Comparison of Threats on the
Web

Categorization of Web Security


Active/Passive attacks


Passive attack


Eavesdropping of network
traffic between browser and server and
gaining information about the web server that
is suppose to be restricted.


Active attacks


Impersonating, altering
messages in transit between the client and
the web server and in extreme cases altering
web site content.


Security threat based on location.


At web server, at web browser, transit
between browser and server, etc.

Web Security based on the Relative
location on the TCP/IP Stack

Web Traffic Security Approaches.


Network level (using IPSec)


Adv: transparent to end user+application, general
purpose solution, has IP filtering capabilities (will not
add overhead to all traffic)


Transport level


Just above TCP stack. Eg. SSL. Transparent to
application if provided in the protocol. Or can be
embedded in specific applications such as Netscape.


Application level


Embedded in application, advantage of being able to
customize the security t the application. Eg. SET