Network Security Philadelphia UniversityAhmad Al- Ghoul 2010-20111 Module 3 Encryption Protocols and...

Network Security Philadelphia University

Ahmad Al- Ghoul 2010-2011 1

Module 3Module 3 Encryption Protocols and Encryption Protocols and

PracticesPracticesMModified by :Ahmad Al GhoulPPhiladelphia UniversityFFaculty Of Administrative & Financial SciencesBBusiness Networking & System Management DepartmentRRoom Number 32406EE-mail Address: [email protected]



Objectives

Definition of Prtocol. Types Of Protocols. Key Distribution. Digital Signature. Key Escrow. Voting By Computer. Oblivious Transfer. Certified Mail.



Definition of Protocols

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.



Protocols

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 two- computer communication, both computers must follow the same protocol in order for either to participate.



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.



Kinds of Protocols 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



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



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.



Arbitrated Protocols disadvantages

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.



Types of Protocols Adjudicated ProtocolsIts 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



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.



Kinds of Protocols



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



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



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



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



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.



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

Algorithm Ciphertext

Encryption key



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



Cryptography Examples of encryption algorithms…

– Asymmetrical :

(or public key)

• RSA• ElGamal• DSA

– Symmetrical:

(or secret key)

• DES and triple DES (3DES)

• IDEA• RC-4, RC-5, RC-6 • AES



Cryptography

Digital signatures

Objective: to ensure the integrity of the data and the authentication of the message

Characteristics: authentic, unalterable, cannot be re-used or copied

– The signature is a cryptography.

– The signature is different every time.

– Operates in the same manner as encrypted public keys



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




– Generates keys that are difficult to crack

= 000000000000000000000...




– Certifies that they are usable

– Protects them at the time of their use

– Revokes them when they are compromised



Cryptography Secret key management

KDC

computer pirate

shared key

sharedkey

shared key

sharedkey

sharedkey



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



Symmetric Key Digital Signature (1)

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

30Ahmad Al- Ghoul 2010-2011Network Security Philadelphia University

Symmetric Key Digital Signature(2)

Symmetric Key Digital Signature

– using arbiter, A

• S(sender) share a secret key with A

• R(receiver) share a secret key with A

– using symmetric encryption

– protocol : SKDS-1

SK

RK

arbiter

S R

),( .1 SKME ))),,(,,(( .2 RS KKMESME



Digital Signature without Encryption(1)

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.

32Ahmad Al- Ghoul 2010-2011Network Security Philadelphia University


Digital Signature without Encryption– using arbiter, A– using cryptographic sealing function:

• e.g. keying hash function like HMAC-SHA

– protocol : SKDS-2

arbiter

S R

)(, .1 MfM S),(),(,, .2 SMfMfSM RS




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



Preventing Reuse or Alternation



Digital signaturePaul

Plain text

Signature

Plaintext Plaintext

Simon private key Simon public key

Sign Verify

Verifies?Yes, NO

Simon



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



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



Hash Function RequirementsA 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 yx with h(y)=h(x)

It is computationally infeasible to find any pair (x, y) such that h(x)=h(y)



Message Authentication Using a One-way Hash Function (1)

H

H

E

D

Mes

sag

e Mes

sag

e Mes

sag

e

Compare

K

K

Using conventional encryption

A B



Message Authentication Using a One-way Hash Function (2)

H

H

E

D

Mes

sag

e Mes

sag

e Mes

sag

e

Compare

Kprivate

Kpublic

Using public-key encryption (Digital Signature)

A B



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.

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