Transcript lecture 3 - Philadelphia University Jordan

Module 3
Encryption Protocols and Practices
MModified by :Ahmad Al Ghoul
PPhiladelphia University
FFaculty Of Administrative & Financial Sciences
BBusiness Networking & System Management Department
RRoom Number 32406
EE-mail Address: [email protected]
Network Security
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Ahmad Al- Ghoul 2010-2011
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Objectives
 Definition of Prtocol.
 Types Of Protocols.
 Key Distribution.
 Digital Signature.
 Key Escrow.
 Voting By Computer.
 Oblivious Transfer.
 Certified Mail.
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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.
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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 twocomputer communication, both computers
must follow the same protocol in order for
either to participate.
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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.
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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
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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
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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.
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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.
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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
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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.
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Kinds of Protocols
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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
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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
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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
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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
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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.
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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
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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
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Cryptography
 Examples of encryption algorithms…
– Symmetrical:
(or secret key)
• DES and triple
DES (3DES)
• IDEA
• RC-4, RC-5, RC-6
• AES
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– Asymmetrical
(or public key)
:
• RSA
• ElGamal
• DSA
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Cryptography
Digital signatures
Objective:
the
to ensure the integrity of the data and
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
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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
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Cryptography
 Key management
– Generates keys that are difficult to crack
= 000000000000000000000...
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Cryptography
 Key management
– Certifies that they are usable
– Protects them at the time of their use
– Revokes them when they are compromised
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Cryptography  Secret key
management
shared
key
KDC
shared
key
shared
key
shared
key
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shared
key
computer pirate
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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
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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
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Symmetric Key Digital Signature(2)
 Symmetric Key Digital Signature
– using arbiter, A
• S(sender) share a secret key
• R(receiver) share a secret key
– using symmetric encryption
– protocol : SKDS-1
with A
KS
with A
KR
2. E (( M , S , E ( M , K S )), K R )
1. E ( M , K S )
arbiter
R
S
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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.
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Digital Signature without Encryption(2)
 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
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Digital Signature without Encryption(3)
 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
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 Preventing Reuse or Alternation
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Digital signature
Pau
Plain text
Plaintext
Simon
Sign
Verify
Signature
Simon private key
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Plaintext
Ahmad Al- Ghoul 2010-2011
Verifies?
Yes, NO
Simon public key
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 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
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Cryptography
 Hashing functions
 Produces a 128-bit checksum
 Processes incoming data in blocks of 512
MD5
bits
(16 32-bit blocks)
 4 repetitions of 16 operations
 The result is 4 32-bit blocks
 Produces a 160-bit checksum
 Processes incoming data in blocks of 512
SHA
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bits
(16 32-bit blocks–> 80 32-bit
blocks)
 4 repetitions of 20 operations
 The result is 5 32-bit blocks
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Hash Function Requirements
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)
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Message Authentication Using a Oneway Hash Function (1)
Message
B
Message
Message
A
H
K
H
Compare
K
D
E
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Using conventional
encryption
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Message Authentication Using a Oneway Hash Function (2)
H
Message
B
Message
Message
A
H
Kpublic Compar
e
Kprivate
D
E
Using public-key encryption (Digital
Network Security
Signature)
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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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