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Chapter: Computer Networks

Transport layer

1. Data compression 1.1 Lossless compression techniques 2. Introduction to jpeg 2.1 Jpeg compression 3 Introduction to mpeg 3.1 Video compression (mpeg) 3.2 Frame types 4. Introduction to mp3 5. Cryptography 5.1 Transposition cipher 5.2 Polyalphabetic cipher 5.3 Types of ecryption 5.6 Types of encryption keys 6. Symmetric key 7. Public-key 8. Authentication 9. Key distribution 9.1 Key distribution mechanisms 10. Key agreement 11. PGP 12. SSH 13. Transport security 14. IP security 15. Wireless security





·         Multimedia data, comprising audio, video, and still images, now makes up the majority of traffic on the Internet by many estimates.


     This is a relatively recent development—it may be hard to believe now, but there was no YouTube before 2005.


·         Part of what has made the widespread transmission of multimedia across networks possible is advances in compression technology.


·         Because multimedia data is consumed mostly by humans using their senses—vision and hearing—and processed by the human brain, there are unique challenges to compressing it.


·         You want to try to keep the information that is most important to a human, while getting rid of anything that doesn’t improve the human’s perception of the visual or auditory experience.


·         Hence, both computer science and the study of human perception come into play.


·         In this section we’ll look at some of the major efforts in representing and compressing multimedia data.


·         To get a sense of how important compression has been to the spread of networked multimedia, consider the following example.


·         A high-definition TV screen has something like 1080 × 1920 pixels, each of which has 24 bits of color information, so each frame is 1080 × 1920 × 24 = 50Mb and so if you want to send 24 frames per second, that would be over 1Gbps.


·         That’s a lot more than most Internet users can get access to, by a good margin.


·         By contrast, modern compression techniques can get a reasonably high quality HDTV signal down to the range of 10 Mbps, a two order of magnitude reduction, and well within the reach of many broadband users.


·         Similar compression gains apply to lower quality video such as YouTube clips—web video could never have reached its current popularity without compression to make all those entertaining videos fit within the bandwidth of today’s networks.


·         Lossless Compression Techniques


     In many ways, compression is inseparable from data encoding.


That is, in thinking about how to encode a piece of data in a set of bits, we might just as well think about how to encode the data in the smallest set of bits possible.


            For example, if you have a block of data that is made up of the 26 symbols A through Z, and if all of these symbols have an equal chance of occurring in the data block you are encoding, then encoding each symbol in 5 bits is the best you can do (since 25 = 32 is the lowest power of 2 above 26).

            If, however, the symbol R occurs 50% of the time, then it would be a good idea to use fewer bits to encode the R than any of the other symbols.


·         In general, if you know the relative probability that each symbol will occur in the data, then you can assign a different number of bits to each possible symbol in a way that minimizes the number of bits it takes to encode a given block of data.


·         This is the essential idea of Huffman codes, one of the important early developments in data compression.


            1.1 Lossless Compression Techniques


·         Run length Encoding


o     Run length encoding (RLE) is a compression technique with a brute-force simplicity.


o     The idea is to replace consecutive occurrences of a given symbol with only one copy of the symbol, plus a count of how many times that symbol occurs—hence the name “run length.”


o     For example, the string AAABBCDDDD would be encoded as 3A2B1C4D.


1.2 Differential Pulse Code Modulation


o     Another simple lossless compression algorithm is Differential Pulse Code Modulation (DPCM).


o     The idea here is to first output a reference symbol and then, for each symbol in the data, to output the difference between that symbol and the reference symbol.


o     For example, using symbol A as the reference symbol, the string AAABBCDDDD would be encoded as A0001123333 since A is the same as the reference symbol, B has a difference of 1 from the reference symbol, and so on.


o     Dictionary based Methods


o     The final lossless compression method we consider is the dictionary-based approach, of which the Lempel-Ziv (LZ) compression algorithm is the best known.

o     The Unix compress and gzip commands use variants of the LZ algorithm.


o     The idea of a dictionary-based compression algorithm is to build a dictionary (table) of variable-length strings (think of them as common phrases) that you expect to find in the data, and then to replace each of these strings when it appears in the data with the corresponding index to the dictionary.

o     Dictionary based Methods


o     For example, instead of working with individual characters in text data, you could treat each word as a string and output the index in the dictionary for that word.


o     To further elaborate on this example, the word “compression” has the index 4978 in one particular dictionary; it is the 4978th word in /usr/share/dict/words.


o     To compress a body of text, each time the string “compression” appears, it would be replaced by 4978.



1.2 Image Representation and Compression


Given the increase in the use of digital imagery in recent years—this use was spawned by the invention of graphical displays, not high-speed networks—the need for standard representation formats and compression algorithms for digital imagery data has grown more and more critical.


In response to this need, the ISO defined a digital image format known as JPEG, named after the Joint Photographic Experts Group that designed it. (The “Joint” in JPEG stands for a joint ISO/ITU effort.)



1.3 Image Representation and Compression

o   JPEG is the most widely used format for still images in use today.


o   At the heart of the definition of the format is a compression algorithm, which we describe below.


o   Many techniques used in JPEG also appear in MPEG, the set of standards for video compression and transmission created by the Moving Picture Experts Group.


o   Digital images are made up of pixels (hence the megapixels quoted in digital camera advertisements).


o   Each pixel represents one location in the two-dimensional grid that makes up the image, and for color images, each pixel has some numerical value representing a color.


o   There are lots of ways to represent colors, referred to as color spaces: the one most people are familiar with is RGB (red, green, blue).





2.1 JPEG Compression


DCT Phase


DCT is a transformation closely related to the fast Fourier transform (FFT). It takes an


8 × 8 matrix of pixel values as input and outputs an 8 × 8 matrix of frequency coefficients.


You can think of the input matrix as a 64-point signal that is defined in two spatial dimensions (x and y); DCT breaks this signal into 64 spatial frequencies. DCT, along with its inverse, which is performed during decompression, is defined by the following formulas:

where pixel(x, y) is the grayscale value of the pixel at position (x, y) in the 8×8 block being compressed; N = 8 in this case


Quantization Phase

The second phase of JPEG is where the compression becomes lossy.


DCT does not itself lose information; it just transforms the image into a form that makes it easier to know what information to remove.


Quantization is easy to understand—it’s simply a matter of dropping the insignificant bits of the frequency coefficients


Quantization Phase


n The basic quantization equation is QuantizedValue(i, j) = IntegerRound(DCT(i, j)/Quantum(i, j)) Where


n Decompression is then simply defined as DCT(i, j) = QuantizedValue(i, j) × Quantum(i, j)

     Encoding Phase


§    The final phase of JPEG encodes the quantized frequency coefficients in a compact form.


§    This results in additional compression, but this compression is lossless.


§    Starting with the DC coefficient in position (0,0), the coefficients are processed in the zigzag sequence.


§    Along this zigzag, a form of run length encoding is used—RLE is applied to only the 0 coefficients, which is significant because many of the later coefficients are 0.


§    The individual coefficient values are then encoded using a Huffman code.




3.1Video Compression (MPEG)

 We now turn our attention to the MPEG format, named after the Moving Picture Experts Group that defined it.

To a first approximation, a moving picture (i.e., video) is simply a succession of still images—also called frames or pictures—displayed at some video rate.

Each of these frames can be compressed using the same DCT-based technique used in JPEG


3.2 Frame Types


MPEG takes a sequence of video frames as input and compresses them into three types of frames, called I frames (intrapicture), P frames (predicted picture), and B frames (bidirectional predicted picture).


Each frame of input is compressed into one of these three frame types. I frames can be thought of as reference frames; they are self-contained, depending on neither earlier frames nor later frames.



The most common compression technique used to create CD-quality audio is based on the perceptual encoding technique. This type of audio needs at least 1.411 Mbps, which cannot be sent over the Internet without compression. MP3 (MPEG audio layer 3) uses this technique.


The basic perceptual model used in MP3 is that louder frequencies mask out adjacent quieter ones. People can not hear a quiet sound at one frequency if there is a loud sound at another


This can be explained better by the following figures presented by Rapha Depke

The audio signal passes through 32 filters with different frequency


Joint stereo coding takes advantage of the fact that both channels of a stereo channel pair contain similar information


These stereophonic irrelevancies and redundancies are exploited to reduce the total bitrate


Joint stereo is used in cases where only low bitrates are available but stereo signals are desired.



A typical solution has two nested iteration loops


            Distortion/Noise control loop (outer loop)

            Rate control loop (inner loop)


Rate control loop

·        For a given bit rate allocation, adjust the quantization steps to achieve the bit rate.


            This loop checks if the number of bits resulting from the coding operation exceeds the number of bits available to code a given block of data.


            If it is true, then the quantization step is increased to reduce the total bits. This can be achieved by adjusting the global gain




What Is Cryptography

Cryptography is the science of hiding information in plain sight, in order to conceal it from unauthorized parties.


Substitution cipher first used by Caesar for battlefield communications


Encryption Terms and Operations


Plaintext – an original message

Ciphertext – an encrypted message

Encryption – the process of transforming plaintext into ciphertext (also encipher)

Decryption – the process of transforming ciphertext into plaintext (also decipher)



Encryption key – the text value required to encrypt and decrypt data


Encryption methodologies


Substitution Cipher

Plaintext characters are substituted to form ciphertext

“A” becomes “R”, “B” becomes “G”, etc.

Character rotation

Caesar rotated three to the right (A > D,

B > E, C > F, etc.)

A table or formula is used

ROT13 is a Caesar cipher

Image from Wikipedia (link Ch 5a)

Subject to frequency analysis attack


5.1 Transposition Cipher

Plaintext messages are transposed into ciphertext


Plaintext: ATTACK AT ONCE VIA NORTH BRIDGE Write into columns going down


Read from columns to the right Ciphertext:


AKCNBTAEORTTVRIAOITDCNAHG Subject to frequency analysis attack Monoalphabetic Cipher


One alphabetic character is substituted or another


Subject to frequency analysis attack


5.2 Polyalphabetic Cipher

Two or more substitution alphabets


Not subject to frequency attack


Running-key Cipher Plaintext letters converted to numeric (A=0, B=1, etc.)


Plaintext values “added” to key values giving ciphertext


Modulo arithmetic is used to keep results in range 0-26


Add 26 if results < 0; subtract 26 if results > 26 One-time Pad


Works like running key cipher, except that key is length of plaintext, and is used only once


Highly resistant to cryptanalysis


5.3 Types of ecryption

Block cipher


Encrypts blocks of data, often 128 bits Stream cipher


Operates on a continuous stream of data Block Ciphers


Encrypt and decrypt a block of data at a time Typically 128 bits


Typical uses for block ciphers


Files, e-mail messages, text communications, web Well known encryption algorithms


DES, 3DES, AES, CAST, Twofish, Blowfish, Serpent


Block Cipher Modes of Operation

Electronic Code Book (ECB)

Cipher-block chaining (CBC)

Cipher feedback (CFB)

Output feedback (OFB)

Counter (CTR)


5.4 Initialization Vector (IV)


Starting block of information needed to encrypt the first block of data IV must be random and should not be re-used


WEP wireless encryption is weak because it re-uses the IV, in addition to making other errors


Block Cipher: Cipher-block Chaining (CBC)


Ciphertext output from each encrypted plaintext block is used in the encryption for the next block


First block encrypted with IV (initialization vector)


Block Cipher: Cipher Feedback (CFB)


Plaintext for block N is XOR’d with the ciphertext from block N-1. In the first block, the plaintext XOR’d with the encrypted IV

5.5 Stream Ciphers

Used to encrypt a continuous stream of data, such as an audio or video transmission


A stream cipher is a substitution cipher that typically uses an exclusive-or (XOR) operation that can be performed very quickly by a computer.


Most common stream cipher is RC4 Other stream ciphers


5.6 Types of Encryption Keys Symmetric key

A common secret that all parties must know

Difficult to distribute key securely


Used by DES, 3DES, AES, Twofish, Blowfish, IDEA, RC5


Asymmetric key

Public / private key


Openly distribute public key to all parties

Keep private key secret

Anyone can use your public key to send you a message

Used by RSA. El Gamal, Elliptic Curve

Asymmetric Encryption Uses

Encrypt message with recipient's public key


Only recipient can read it, using his or her private key Provides confidentiality


Sign message


Hash message, encrypt hash with your private key Anyone can verify the signature using your public key


Provides integrity and non-repudiation (sender cannot deny authorship) Sign and encrypt


Both of the above




·        most symmetric block ciphers are based on a Feistel Cipher Structure

·        needed since must be able to decrypt ciphertext to recover messages efficiently

·        block ciphers look like an extremely large substitution

·        would need table of 264 entries for a 64-bit block

·        instead create from smaller building blocks

·        using idea of a product cipher

·        Horst Feistel devised the feistel cipher

            based on concept of invertible product cipher

·        partitions input block into two halves

            process through multiple rounds which

            perform a substitution on left data half

            based on round function of right half & subkey

            then have permutation swapping halves

·        implements Shannon’s substitution-permutation network concept

·        block size

            increasing size improves security, but slows cipher

·        key size


            increasing size improves security, makes exhaustive key searching harder, but may slow cipher


·        number of rounds

            increasing number improves security, but slows cipher

·        subkey generation

            greater complexity can make analysis harder, but slows cipher


·        round function

            greater complexity can make analysis harder, but slows cipher

·        fast software en/decryption & ease of analysis

            are more recent concerns for practical use and testing


            7. PUBLIC-KEY


Rapidly increasing needs for flexible and secure transmission of information require to use new cryptographic methods.


The main disadvantage of the classical cryptography is the need to send a (long) key through a super secure channel before sending the message itself.


In secret-key (symetric key) cryptography both sender and receiver share the same secret



In public-key ryptography there are two different keys: a public encryption key



a secret decryption key (at the receiver side).

Basic idea: If it is infeasible from the knowledge of an encryption algorithm ek to construct the corresponding description algorithm dk, then ek can be made public.

Toy example: (Telephone directory encryption)

Start: Each user U makes public a unique telephone directory tdU to encrypt messages for U and U is the only user to have an inverse telephone directory itdU.


Encryption: Each letter X of a plaintext w is replaced, using the telephone directory tdU of the intended receiver U, by the telephone number of a person whose name starts with letter X.


Decryption: easy for Uk, with an inverse telephone directory, infeasible for others. Analogy:


Secret-key cryptography 1. Put the message into a box, lock it with a padlock and send the box. 2. Send the key by a secure channel.


Public-key cryptography Open padlocks, for each user different one, are freely available. Only legitimate user has key from his padlocks. Transmission: Put the message into the box of the intended receiver, close the padlock


Main problem of the secret-key cryptography: a need to make a secure distribution (establishment) of secret keys ahead of transmissions.


Diffie+Hellman solved this problem in 1976 by designing a protocol for secure key establishment (distribution) over public channels.


Protocol: If two parties, Alice and Bob, want to create a common secret key, then they first agree, somehow, on a large prime p and a primitive root q (mod p) and then


they perform, through a public channel, the following activities.


Alice chooses, randomly, a large 1 Ł x < p -1 and computes X = q x mod p.


Bob also chooses, again randomly, a large 1 Ł y < p -1 and computes Y = q y mod p.


Alice and Bob exchange X and Y, through a public channel, but keep x, y secret.

Alice computes Y x mod p and Bob computes X y mod p and then each of them has the

Key K = q xy mod p.


The following attack by a man-in-the-middle is possible against the Diffie-Hellman key establishment protocol.


. Eve chooses an exponent z.

Eve sends q z to both Alice and Bob. (After that Alice believes she has received q x and Bob believes he has received q y.)

Eve intercepts q x and q y.

When Alice sends a message to Bob, encrypted with KA, Eve intercepts it, decrypts it,

then encrypts it with KB and sends it to Bob without any need forsecret key distribution (Shamir's no-key algorithm)

Basic assumption: Each user X has its own


secret encryption function eX secret decryption function dX


and all these functions commute (to form a commutative cryptosystem). Communication protocol


with which Alice can send a message w to Bob.

        Alice sends eA (w) to Bob


        Bob sends eB (eA (w)) to Alice

        Alice sends dA (eB (eA (w))) = eB (w) to Bob

        Bob performs the decryption to get dB (eB (w)) = w.



            8. AUTHENTICATION


·         fundamental security building block


     basis of access control & user accountability


·         is the process of verifying an identity claimed by or for a system entity


·         has two steps:


     identification - specify identifier


     verification - bind entity (person) and identifier


distinct from message authentication

four means of authenticating user's identity


based one something the individual


·         knows - e.g. password, PIN


·         possesses - e.g. key, token, smartcard


·         is (static biometrics) - e.g. fingerprint, retina


·         does (dynamic biometrics) - e.g. voice, sign


can use alone or combined


all can provide user authentication all have issues


Authentication Protocols

used to convince parties of each others identity and to exchange session keys


may be one-way or mutual


key issues are


·         confidentiality – to protect session keys


·         timeliness – to prevent replay attacks


where a valid signed message is copied and later resent


·         simple replay


·         repetition that can be logged


·         repetition that cannot be detected


·         backward replay without modification


countermeasures include


·         use of sequence numbers (generally impractical)


·         timestamps (needs synchronized clocks)


·         challenge/response (using unique nonce)


One-Way Authentication

·         required when sender & receiver are not in communications at same time (eg. email)


·         have header in clear so can be delivered by email system


·         may want contents of body protected & sender authenticated


·         as discussed previously can use a two-level hierarchy of keys


·         usually with a trusted Key Distribution Center (KDC)


     each party shares own master key with KDC


     KDC generates session keys used for connections between parties


     master keys used to distribute these to them


            9. KEY DISTRIBUTION


In designing the key distribution protocol, the authors took into consideration the following requirements:


Security domain change: The Certifiction Authority (CA) of the receiving security domain must be able to authenticate the agent which comes from another security domain.


Trust establishment: The key distribution process should start with a very high trust relationship with the Certifiction Authority (CA) .


Secure key distribution: The key distribution process should be conducted in a secure manner (e.g., trusted path).


Efficiency: The key distribution process should not consume a lot of resources, such as machines CPUs and network bandwidth.


Scalability: The key distribution process must be scalable enough, so that the mobile agent can have the ability to roam widely.


Transparency: The mobile agents should not include a code which is proper to key distribution.


This will ease programming as agents programmers will concentrate on the programming logic rather than the key obtaining issues.


Portability: The protocol should not be platform specific and should be ported to any mobile agent platform.


Ease of administration: The key distribution protocol should not be a burden on the administrator. The protocol is an automated infrastructure that should require minimum administrator’s intervention.


9.1 Key Distribution Mechanisms.

System Components:

A key distribution system for mobile agents includes the following components:


Agent: An agent is a software component which executes on behalf of a particular user who is the user of the agent.


An agent can be mobile and move from one host server to another under its own control to achieve tasks on these hosts servers.

·        Agent Server: Each host, as part of the mobile agent platform, runs an execution environment, the agent server.


·        Messaging System. A messaging system is part of an agent execution environment. It provides facilities for agents to communicate both locally and remotely


·        The CA. It is a trusted third party which provides digital certificates for mobile agents, users and agent servers.


All digital certificates are signed by the CA for further verification of their authenticity and validity.


·        Keystore: Each agent server has a local database which is used to store and retrieve its own private/public key pair and the digital certificate.


·        It also stores the digital certificate of the trusted CA and other agent severs, mobile agents, and CAs with which the agent server has prior communication.


·        Similarly, each CA has a local keystore .


·        Security Domain: A security domain consists of a group of agent servers which are under one common CA. In the security domain, the agent servers have the digital certificate of their local CA stored in their local keystores.


·        When a mobile agent moves, it can move within the same security domain or changes a security domain.


            10. KEY AGREEMENT


–   Flexibility in credentials

–   Modern, publically analysed/available cryptographic primitives


–   Freshness guarantees

–   PFS?

–   Mutual authentication

–   Identity hiding for supplicant/end-user

–   No key re-use


–   Fast re-key

–   Fast handoff

–   Efficiency not an overarching concern:

·        Protocol runs only 1/2^N-1 packets, on average

–   DOS resistance


Credentials flexibility

Local security policy dictates types of credentials used by end-users

Legacy authentication compatibility extremely important in market

·       Examples:

–   username/password

–   Tokens (SecurID, etc)

–   X.509 certificates




·       Algorithms must provide confidentiality and integrity of the authentication and key agreement.


·       Public-key encryption/signature

–   RSA

–   ECC

–   DSA


·       PFS support D-H

·       Most cryptographic primitives require strong random material that is “fresh”.

–   Not a protocol issue, per se, but a design requirement nonetheless

·       Both sides of authentication/key agreement must be certain of identity of other party.

·       Symmetric RSA/DSA schemes (public-keys on both sides)

·       Asymmetric schemes

–   Legacy on end-user side

–   RSA/DSA on authenticator side


            11. PGP


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


PGP was developed be Phil Zimmermann in 1991 and since then it has grown in popularity. There have been several updates to PGP.


A free versions of PGP is available over the Internet, but only for non-commercial use. The latest (Jan. 2000) current version is 6.5.


Commercial versions of PGP are available from the PGP Division of Network Associates


For three years, Philip Zimmermann, was threatened with federal prosecution in the United States for his actions. Charges were finally dropped in January 1996.


At the close of 1999, Network Associates, Inc. announced that it has been granted a full license by the U.S. Government to export PGP world-wide, ending a decades-old ban.

PGP enables you to make your own public and secret key pairs.


PGP public keys are distributed and certified via an informal network called "the web of trust".


            Most experts consider PGP very secure if used correctly. PGP is based on RSA, DSS, Diffie-Hellman in the public encryption side, and CAST.128, IDEA, 3DES for conventional encryption. Hash coding is done with SHA-1.


            PGP has a wide range of applicability from corprorations that wish to enforce a standardized scheme for encryptin files and messages to individuals who wish to communicate securely with each others over the interent.


            The actual operation of PGP consists of five services: authentication, confidentiality, compression, e-mail compatibility and segmentation (Table 12.1.)



            The digital signature service is illustrated in Fig 12.1a.


– EC is used for conventional encryption, DC for decryption, and EP and ED correspondingly for public key encryption and decryption.


            The algorithms used are SHA-1 and RSA. Alternatively digital signatures can be generated using DSS/SHA-1.


            Normally digital signatures are attached to the files they sign, but there are exceptions


– a detached signature can be used to detect a virus infection of an executable program.


–   sometimes more than one party must sign the document.

a separate signature log of all messages is maintained


•        Confidentiality service is illustrated in Fig 12.1b.


·        Confidentiality can be use for storing files locally or transmitting them over insecure channel.


·        The algorithms used are CAST-128 or alternatively IDEA or 3DES. The ciphers run in CFB mode.


·        Each conventional key is used only once.

–   A new key is generated as a random 128-bit number for each message.


– The key is encrypted with the receivers public key (RSA) and attached to the message.


– An alternative to using RSA for key encryption, ELGamal, a variant of Diffie-Hellman providing also encryption/decryption, can be used.


·        The use of conventional encryption is fast compared to encryption the whole message with RSA.


·        The use of public key algorithm solves the use session key distribution problem. In email application any kind of handshaking would not be practical.


12. SSH


protocol for secure network communications


–   designed to be simple & inexpensive

SSH1 provided secure remote logon facility


–   replace TELNET & other insecure schemes

–   also has more general client/server capability

            Can be used for FTP, for example

SSH2 was documented in RFCs 4250 through 4254


SSH clients & servers are widely available (even in OSs)


Identification string exchange


–   To know which SSH version, which SSH implementation

Algorithm Negotitation


– For the crypto algorithms (key exchange, encryption, MAC) and compression algo.


–   A list in the order of preference of the client


– For each category, the algorithm chosen is the first algorithm on the client's list that is also supported by the server.


key exchange


–   Only two exchanges

–   Diffie-Hellman based

–   Also signed by the server (host private key)


– As a result (i) two sides now share a master key K. (ii) the server has been authenticated to the client.


·         Then, encryption, MAC keys and IV are derived from the master key


·         End of key exchange


–   To signal the end of key exchange process

–   Encrypted and MACed using the new keys

·         Service Request: to initiate either user authentication or connection protocol


·         Authentication of client to server


·         First client and server agree on an authentication method


–   Then a sequence of exchanges to perform that method

–   Several authentication methods may be performed one after another

·         authentication methods


–   public-key

     Client signs a message and server verifies


–   password


     Client sends pasword which is encrypted and MACed using the keys agreed

–   runs on SSH Transport Layer Protocol

–   assumes secure authentication connection

–   which is called tunnel

–   used for multiple logical channels

–   SSH communications use separate channels

–   either side can open with unique id number

–   flow controlled via sliding window mechanism

–   have three stages:

–   opening a channel, data transfer, closing a channel


            13. TRANSPORT SECURITY


transport layer security service


originally developed by Netscape


version 3 designed with public input


subsequently became Internet standard known as TLS (Transport Layer Security)


uses TCP to provide a reliable end-to-end service


SSL has two layers of protocols


SSL connection


·         a transient, peer-to-peer, communications link


·         associated with 1 SSL session


SSL session


·         an association between client & server


·         created by the Handshake Protocol


·         define a set of cryptographic parameters


·         may be shared by multiple SSL connections



using symmetric encryption with a shared secret key defined by Handshake Protocol


     AES, IDEA, RC2-40, DES-40, DES, 3DES, Fortezza, RC4-40, RC4-128


     message is compressed before encryption


·         message integrity


     using a MAC with shared secret key


     similar to HMAC but with different padding


·         one of 3 SSL specific protocols which use the SSL Record protocol


·         a single message


·         causes pending state to become current


·         hence updating the cipher suite in use




RFC 2401  - Overall security architecture and services offered by IPSec.


Authentication Protocols


·         RFC 2402 – IP Authentication Header processing (in/out bound packets )


·         RFC 2403 – Use of MD-5 with Encapsulating Security Payload and Authentication Header


·         RFC 2404 - Use of Sha1with Encapsulating Security Payload and Authentication Header

ESP Protocol


·         RFC 2405 – Use of DES-CBS which is a symmetric secret key block algorithm (block size 64 bits).


·         RFC 2406 – IP Encapsulating Security Payload processing (in/out bound packets)


RFC 2407 – Determines how to use ISAKMP for IPSec


RFC 2408 (Internet Security Association and Key Management Protocol - ISAKMP)


·         Common frame work for exchanging key securely.

·         Defines format of Security Association (SA) attributes, and for negotiating, modifying, and deleting SA.



·         Security Association contains information like keys, source and destination address, algorithms used.

·         Key exchange mechanism independent.


RFC 2409 – Internet key exchange


·         Mechanisms for generating and exchanging keys securely.


·         Designed to provide both confidentiality and integrity protection


·         Everything after the IP header is encrypted


·         The ESP header is inserted after the IP header


·         Designed for integrity only


·         Certain fields of the IP header and everything after the IP header is protected


·         Provides protection to the immutable parts of the IP header





·         Wireless connections need to be secured since the intruders should not be allowed to access, read and modify the network traffic.

·         Mobile systems should be connected at the same time.


·         Algorithm is required which provides a high level of security as provided by the physical wired networks.


·         Protect wireless communication from eavesdropping, prevent unauthorized access.


·         Access Control


     Ensure that your wireless infrastructure is not used.


·         Data Integrity


     Ensure that your data packets are not modified in transit.


·         Confidentiality


     Ensure that contents of your wireless traffic is not leaked.


·         WEP relies on a secret key which is shared between the sender (mobile station) and the receiver (access point).


·         Secret Key : packets are encrypted using the secret key before they are transmitted.


·         Integrity Check : it is used to ensure that packets are not modified in transit


·         To send a message to M:


     Compute the checksum c(M). Checksum does not depend on the secret key ‘k’.


     Pick a IV ‘v’ and generate a key stream RC4(v,k).


     XOR <M,c(M)> with the key stream to get the cipher text.


     Transmit ‘v’ and the cipher text over a radio link.


     WEP uses RC4 encryption algorithm known as “stream cipher” to protect the confidentiality of its data.


     Stream cipher operates by expanding a short key into an infinite pseudo-random key stream.

     Sender XOR’s the key stream with plaintext to produce cipher text.


     Receiver has the copy of the same key, and uses it to generate an identical key stream.


     XORing the key stream with the cipher text yields the original message.


     Passive Attacks


     To decrypt the traffic based on statistical analysis (Statistical Attack)


     Active Attacks


     To inject new traffic from authorized mobile stations, based on known plaintext.


     Active Attacks


     To decrypt the traffic based on tricking the access point


     Dictionary Attacks


     Allow real time automated decryption of all traffic.





Effective means of protection a local system or network of systems from network-based security threats while affording access to the outside world via WAN`s or the Internet


Information systems undergo a steady evolution (from small LAN`s to Internet connectivity)


Strong security features for all workstations and servers not established

The firewall is inserted between the premises network and the Internet


Establish a controlled link

·        Protect the premises network from Internet-based attacks

·        Provide a single choke point


            Design goals:


·        All traffic from inside to outside must pass through the firewall (physically blocking all access to the local network except via the firewall)


·        Only authorized traffic (defined by the local security police) will be allowed to pass


Four general techniques:

            Service control


·        Determines the types of Internet services that can be accessed, inbound or outbound


            Direction control

·        Determines the direction in which particular service requests are allowed to flow


            User control

·        Controls access to a service according to which user is attempting to access it


            Behavior control

·        Controls how particular services are used (e.g. filter e-mail)


Types of Firewalls

            Three common types of Firewalls:

–   Packet-filtering routers

–   Application-level gateways


– Circuit-level gateways (Bastion host


Packet-filtering Router

            Packet-filtering Router


– Applies a set of rules to each incoming IP packet and then forwards or discards the packet


–   Filter packets going in both directions


– The packet filter is typically set up as a list of rules based on matches to fields in the IP or TCP header


–   Two default policies (discard or forward)



–   Simplicity

–   Transparency to users

–   High speed



–   Difficulty of setting up packet filter rules

–   Lack of Authentication


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