 MModified by :Ahmad Al Ghoul
 PPhiladelphia University
 FFaculty Of Administrative & Financial Sciences
 BBusiness Networking & System Management Department
 RRoom Number 32406
 EE-mail Address: ahmad4_2_69@hotmail.com
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
1

Objectives

Definition of Prtocol.

Types Of Protocols.

Key Distribution.

Digital Signature.

Key Escrow.

Voting By Computer.

Oblivious Transfer.

Certified Mail.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
2




Definition of Protocols
– A protocol is an orderly sequence of steps two or more parties take to accomplish some task
A good protocol has the following
Characteristics
–
Established in advance : the protocol is completely designed before it is used.
–
Mutually subscribed : all parties to the protocol agree to follow its steps, in order.
–
Unambiguous : no party can fail to follow a step properly because the party has misunderstand the step.
–
Complete: for every situation that can occur there is a prescribed action to be taken.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
3


Protocols are also used in computer- to – computer communication. A computer needs to know when to “speak”, when to “listen”, with whom it is communicating, whether it has received all of a particular communication, and so forth. In a twocomputer communication, both computers must follow the same protocol in order for either to participate.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
4

Kinds of Protocols

Certain task, such as negotiating contracts, voting, distributing information, are simple human activities.
However, many of these tasks depend on a witness to ensure fairness.

We must develop protocols by which two suspicious parties can interact with each other and be convinced of fairness.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
5


Process vs. Mechanism
– protocols separate the process of accomplishing a task from the mechanism by which it is done
– a protocol specifies only the rules of behavior
• we verify the correctness of the process at a high level
– after becoming convinced of the correctness of the design, we implement the protocol using some mechanism
• using some particular language or encryption system
• we need only verify that the mechanism correctly reflects the design
• we can later change the implementation without affecting the design
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
6

Kinds of Protocols

Fairness
– in order to use computers more effectively, we must develop protocols by which two suspicious parties can interact with each other and be convinced of fairness

Types of Protocols
– Arbitrated Protocols
– Adjudicated Protocols
– Self-Enforcing Protocols
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
7

Types of Protocols


Arbitrated Protocols
In a computer protocol arbiter is a trustworthy third party who ensures fairness. The arbiter might be a person , a program, or a machine. For example, in a network an arbiter might be a program running on one machine of the network. The program receives and forwards messages between users. The user trust that when the arbiter forwards a message saying it comes from
A, the message really did come from user A.the notion of an arbiter is the basis for type of secure protocol called an arbitrated protocol.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
8






1- The two sides may not be able to find a neutral third party that both sides trust. Suspicious users are rightfully suspicious of unknown arbiter in a network.
2- Maintaining the availability of an arbiter represents a cost to the users or the network ; that cost may be high.
3- Arbitration causes a time delay in communication because a third party must receive, act on, and then forward every transaction.
4- If the arbitration service is heavily used, it may become a bottleneck in the network as many users try to access a single arbiter.
5- Secrecy becomes vulnerable, because the arbiter has access to much sensitive information
.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
9

Types of Protocols

Adjudicated Protocols
Its disinterested third party to judge fairness based on evidences.
Not only can a third party determine whether two parties acted fairly, that is, within the rules of the protocol, but third party can also determine who cheated.
Adjudicated protocols involve the services of a third party only in case of a dispute. Therefore, they are usually less costly, in terms of machine time or access to a trusted third party software judge, than arbitrated protocols. However, adjudicated protocols detect a failure to cooperate only after the failure has occurred
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
10

Types of Protocols

Self-Enforcing Protocols
Is one that guarantees fairness. If either party tries to cheat, that fact becomes evident to the other party. No outsider is needed to ensure fairness.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
11

Kinds of Protocols
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
12

Protocols to Solve Problems

When two humans interact directly, they do so differently than if there is a computer between them. Although there may be differences in the approach, we want to devise protocols for the automated environment by which people can carry out everyday tasks.

The following problems will be considered

Voting by Computer

Oblivious Transfer

Contract Signing

Certified Mail
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
13

Digital Signature

A digital signature is a protocol that produces the same effect as a real signature
– two primary conditions
• Unforgeable : if a person P signs message M with signature S(P,M), it is impossible for anyone else to produce the pair [M,S(P,M)]
• Authentic: if a person R receives the pair [M,S(P,M)] each one from P, R can check that the signature is really from P. only P could have created this signature, and the signature is firmly attached to M
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
14

Digital Signature
– two desirable conditions
• not alterable :after being transmitted, M cannot be changed by S,R, or an interceptor.
• not reusable : a pervious message presented will instantly detected by R
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
15

Cryptography

Why?
– The word cryptography comes from the Greek:
• Kruptos which means hidden
• Graphien which means to write
– Cryptography is the art of encoding data by means of the encryption process.
– To satisfy the following needs:
• Authentication
• Confidentiality
• Integrity
• Non-repudiation
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
16


Cryptography
A few definitions (cont’d)!
– Encryption Algorithm:
• A series of operations used to encrypt and decrypt data
– Encryption Key:
• A sequence of symbols used by the encryption algorithm when data encryption and decryption takes place. Without it, it is impossible to return to plaintext.
– Cryptology:
• Cryptology is a mathematical science comprising two streams: cryptography and cryptanalysis.
– Cryptanalysis:
• The opposite of cryptography, cryptanalysis seeks weaknesses in order to decrypt ciphertext.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
17

Cryptography
– Regular text <=> algorithm <=> crypto-text
• Algorithm only
– Secret and vulnerable algorithm
• Algorithm + key = lock
– Public algorithm remains secure.
– The key is the key to the secret.
Plaintext
Encryption key
Network Security
Philadelphia University
Algorithm
Ahmad Al- Ghoul 2010-2011
Ciphertext
18

Cryptography

Limitations of the public key
– Keys that are shorter than 512 bits (154 digits) are not secure
– 1,024 is ideal (308 digits)
– Keys can be distributed or reset on a public key server
– Key authentication is necessary
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
19

Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
20

Cryptography

Examples of encryption algorithms…
–
Symmetrical:
(or secret key)
–
Asymmetrical :
(or public key)
• DES and triple
DES (3DES)
• IDEA
• RC-4, RC-5, RC-6
• AES
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
• RSA
• ElGamal
• DSA
21

Cryptography

Digital signatures

Objective: the to ensure the integrity of the data and authentication of the message

Characteristics: authentic, unalterable, cannot be reused or copied
– The signature is a cryptography.
– The signature is different every time.
– Operates in the same manner as encrypted public keys
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
22


Cryptography
Key management
 … involves five major steps:
– Generates keys that are difficult to crack
– Distributes them in a secure manner
– Certifies that they are usable
– Protects them at the time of their use
– Revokes them when they are compromised
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
23

Cryptography

Key management
– Generates keys that are difficult to crack
= 000000000000000000000...
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
24

Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
25

Cryptography

Key management
– Certifies that they are usable
– Protects them at the time of their use
– Revokes them when they are compromised
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
26

Cryptography 
Secret key management shared key
KDC shared key shared key
Network Security
Philadelphia University shared key shared key
Ahmad Al- Ghoul 2010-2011 computer pirate
27

Cryptography

Digital certificates
– Digital certificates are electronic folders containing the public key and information on the user as well as the CA’s signature.
– The following information is found on a X.509 certificate:
• The certificate’s version number
• The certificate’s serial number
• The algorithm used to sign the certificate
• The certificate’s issuer
• The certificate’s retention period
• The public key issuer
• Information on the public key
• Extensions introduced by versions 2 and 3
• The issuer’s digital signature
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
28


With a private key encryption system the secrecy of the key guarantees the authenticity of the massage, as well as its secrecy.

Symmetric key encryption doesn't prevent forgery, the receiving party can create an identical message, because it has the same key, so there is no protection against the repudiation ( denial of sending a message ).

To solve this problem, there must be trusted third party A
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
29


Symmetric Key Digital Signature
– using arbiter, A
• S(sender) share a secret key with A
K
– using symmetric encryption
K
R
– protocol : SKDS-1
S
Network Security
Philadelphia University
1 .
E ( M , K
S
) arbiter
2 .
E (( M , S , E ( M , K
S
)), K
R
)
R
Ahmad Al- Ghoul 2010-2011
30









If S and R are not concerned with secrecy, they can agree on a cryptographic sealing to use as a signature, the seal is a stamp, mark to prove the authenticity. A sealing function is a mathematical function affected by every bit of its input.
Suppose S and R have each registered a personal sealing function with arbiter
Let fs and fR be these two functions.
S sends M and fs(m) to A.
A also computes fs(M) from the copy of M received from S.
If two values match the massage is authenticated.
A sends M, s, fs(m), and fR((M,S)) to R.
R cannot interrupt fs(M) but can use it as evidence that S sent M.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
31


Digital Signature without Encryption
– using arbiter, A
– using cryptographic sealing function:
• e.g. keying hash function like HMAC-SHA
– protocol : SKDS-2
1 .
M , f
S
( M ) 2 .
M , S , f
S
( M ), f
R
( M , S ) arbiter
R S
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
32


Preventing Reuse or Alternation
– SKDS-1 and SKDS-2
• satisfy two primary conditions for digital signature: unforgeable and authentic
– do not satisfy two desirable conditions for digital signature: not alterable and not reusable
– solution
• not reusable : use time stamp
• not alterable : make each piece depend on time stamp
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
33


Preventing Reuse or Alternation
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
34

Paul
Simon
Plaintext
Sign
Plain text
Signature
Plaintext
Verify
Verifies?
Yes, NO
Simon private key
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
Simon public key
35


Public key solution is much less cumbersome than the single key solution

One disadvantage is that the message is authentic but not private (as everyone can decrypt with the public key)

This can be overcome by using two encryption
– in the first you encrypt the message with the public key of the recipient - the message can be read only by the recipient it is private - confidentiality is provided
– in the second (the envelope) you encrypt the encrypted message with your secret key - the message was sent only by you as only you know your private key - the message is authentic
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
36

Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
37

Cryptography

Hashing functions
MD5

Produces a 128-bit checksum

Processes incoming data in blocks of 512 bits (16 32-bit blocks)

4 repetitions of 16 operations

The result is 4 32-bit blocks
SHA
 Produces a 160-bit checksum

Processes incoming data in blocks of 512 bits (16 32-bit blocks–> 80 32-bit blocks)

4 repetitions of 20 operations

The result is 5 32-bit blocks
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
38

Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
39



A hash function H must have the following properties:
 H can be applied to a block of data of any size
H produces a fixed-length output
H(x) is relatively easy to compute for any given x, making both hardware and software implementations practical
 For any given code h, it is computationally infeasible to find x such that h(x)=h
 For any given block x, it is computationally infeasible to find y
 x with h(y)=h(x)
 It is computationally infeasible to find any pair (x, y) such that h(x)=h(y)
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
40

A B
H
H
K
E
Network Security
Philadelphia University
Using conventional
K
Compare
D
41

A B
H
K public
Compar e
H
K private
E
Using public-key encryption (Digital
Philadelphia University Ahmad Al- Ghoul 2010-2011
D
42

Report

Discus one of these topics

1- man in middle attack.

2(RSA)Rivest, Shamir, and Adleman, A method for obtaining digital signatures and public-key cryptosystems
.

3-Brute force attack.
Network Security
Philadelphia University Ahmad Al- Ghoul 2010-2011
43