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CryptographicProtocol
Building Block Protocols
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Contents
Communications Using Symmetric Cryptography
Definition
Type of Protocol
Digital Signatures with Encryption
Communications Using Public-Key Cryptography
One-Way Functions & One-Way Hash Functions
Random and Pseudo-Random-Sequence Generation
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Definition
Step
Order
ProtocolProtocolProtocolProtocol
Task
Two or More Parties
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Key Words
Series of steps:
Each step must be well-defined
There must be specified action for every possiblesituation
Everyone involved must know the steps and follow
Two or more parties:
Parties can be friends and trust each other or
adversaries and mistrust each other Accomplish a task:
This can involve sharing (parts of) a secret,confirming an identity, signing a contract, etc.
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Protocol Cryptography
Protocol Cryptography is a protocol that usecryptography algorithm.
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Characteristic
Establishedin advance
Complete
u ua ysubscribed
Unambiguous
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Definition
Everyone involved in the protocol must know theprotocol and all of the steps to follow in advance.
Everyone involved in the protocol must agree to.
The protocol must be unambiguous.
The protocol must be complete
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Definition
Use Cryptography
Involves Some Cryptographic algorithm
Confidentiality
Integrity
Authentication
ProtocolProtocolProtocolProtocol
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Examples
Face To face
Rely on peoples presence to ensure fairness and
security
Over Computer Network
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Buying goods over the internet,
playing online poker
E-voting in an election
Internet Banking
Rekening Bersama
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Type of Protocols
Arbitrated Protocols
Adjudicated Protocols
Self-Enforcing Protocols
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Arbitrated Protocols
An arbitrator is a disinterested third party trustedto complete the protocol
Has no allegiance to any party involved All people participating trust that he is acting honestly
and correctly
Arbitrators can help complete protocols betweenparties that dont trust each other
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Arbitrated Protocols
In the real world, lawyers, public notaries, andbanksact as arbitrators
For example, Bob can buy a car from Alice usinganarbitrated protocol
1. Bob writes a check and gives it to the bank(Trent)
2. an puts enoug money on o to covercheck and certifies the check3. Alice gives the title to Bob and Bob gives the
certified check to Alice
4. Alice deposits the check This works, because Alice trusts the banks
certification
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Arbitrated Protocols
There are some problems with arbitratedprotocols in the virtual world:
Its more difficult for people to trust a faceless entitysomewhere in the network
An arbitrator can become a bottleneck, as he has to
deal with every transaction This may lead to even more delay (due to the arbitrator
theres always some delay)
Lots of damage can be caused if arbitrator is
subverted
Someone has to pay for running an arbitration
service
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Adjudicated Protocols
Arbitrators have high costs, so arbitratedprotocols can be split into two sub-protocols:
A non-arbitrated part An arbitrated part that is executed only if there is
adispute
This special kind of arbitrator is called anadjudicator
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Adjudicated Protocols
An adjudicator is a third party who can judgewhether a transaction was performed fairly, in
case of a dispute. Example: a notary public, who attests the
authenticit of a si ned document
An adjudicated protocol allows an adjudicator toexamine transaction data to decide whether twodisputing parties acted fairly.
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Adjudicated Protocols
Example :
Nonarbitrated subprotocol (executed every
time):(1) Alice and Bob negotiate the terms of the contract.
(2) Alice signs the contract.
(3) Bob signs the contract.
Adjudicated subprotocol (executed only in caseof a dispute):(4) Alice and Bob appear before a judge.
(5) Alice presents her evidence.
(6) Bob presents his evidence.(7) The judge rules on the evidence.
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Adjudicated Protocols
Issues with adjudicated protocols in the virtualworld
Protocols rely in a first instance on the parties beinghonest
provides enough evidence to be able to detect this In a good adjudicated protocol, this evidence also
identifies the cheating party
Instead of preventing cheating, adjudicated protocolsdetect cheating
The (inevitability of) detection acts as a deterrent
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Self-Enforcing Protocols
A self-enforcing protocol guarantees fairness ofa transaction, without the presence of a third
party. During the transaction, either partys attempt to
cheat becomes immediatel obvious to the other
party.
Example :
There are several ways two people can divide a pieceof cake in half.
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Self-Enforcing Protocols
Advantages:
no such cost as those associated with arbitrated or
adjudicated protocols
There is not a self-enforcing protocol for everysituation.
www.themegallery.com Company Logo
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Attack on Protocols
Attack
Cheaters
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Attacks
Eavesdropping
Modification
Replay
Preplay
e a vesary enganges n arun o e pro oco pr orto a run by legitimate principals
Reflection
The adversary send protocols messages back to theprincipal who sent them
Denial of Services
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Attack
Typing Attack
The adversary replaces a (normally encrypted)
protocol message field of one type with (normallyencrypted) messages fields of another type
Cryptanalysis
Certificate Manipulation The adversary chooses or modifies certificate
information to attack one or more protocol run.
Protocol Interaction The adversary chooses a new protocol to interact with
known protocol
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Cheaters
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Communications Using Symmetric Cryptography
(1) Alice and Bob agree on a cryptosystem
(2) Alice and Bob agree on a key
(3) Alice takes her plaintext message and encrypts it using
.
ciphertext message.
(4) Alice sends the ciphertext message to Bob.
(5) Bob decrypts the ciphertext message with the samealgorithm and key and reads it.
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Problem
Keys must be distributed in secret
If a key is compromised (stolen, guessed,
extorted, bribed, etc.), the security has been
gone.
The total number of keys increases rapidly as
the number of users increases
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One-Way Functions
One-way functions are relatively easy to
compute, but significantly harder to reverse.
Given xit is easy to compute f(x), but given f(x) it
is hard to compute x
Problem :A trapdoor one-way function
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One-Way Hash Functions
a.k.a compression function, contraction function,message digest, fingerprint, cryptographic
checksum, message integrity check (MIC), andmanipulation detection code (MDC).
a variable-length input string (called a pre-image)
an converts t to a xe - engt genera y sma eroutput string (called a hash value)
Collision-Free
Message Authentication Codes A.k.a. data authentication code (DAC)
is a one-way hash function with the addition of a secret key
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Communications Using Public-Key Cryptography
In 1976, Whitfield Diffie and Martin Hellman
General protocol(1) Alice and Bob agree on a public-key cryptosystem.
2 Bob sends Alice his ublic ke .
(3) Alice encrypts her message using Bobs public keyand sends it to Bob.
(4) Bob decrypts Alices message using his private key.
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Problems
Public-key algorithms are slow, Large Number
requirement Public-key cryptosystems are vulnerable to
-
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Hybrid Cryptosystem
(1) Bob sends Alice his public key.
(2) Alice generates aK, encrypts it using Bobs
public key, and sends it to Bob. EB(K) randomsession key,
key to recover the session key. DB(EB(K)) = K
(4) Both of them encrypt their communicationsusing the same session key.
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Digital Signatures
Requirement
The signature is authentic The signature is unforgeable
The signature is not reusable
The signed document is unalterable The signature cannot be repudiated
In reality, none of these statements about signatures is
completely true.
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Signing Documents with Symmetric Cryptosystemsand an Arbitrator
(1) Alice encrypts her message to Bob with KA and
sends it to Trent.(2) Trent decrypts the message with KA.
3 Trent takes the decr ted messa e and a
statement that he has received this message fromAlice, and encrypts the whole bundle with KB.
(4) Trent sends the encrypted bundle to Bob.
(5) Bob decrypts the bundle with KB. He can now readboth the message and Trents certification thatAlice sent it.
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Advantages
All requirements on Signatures are fulfilled
The signature is authentic
The signature is unforgeable The signature is not reusable
The si ned document is unalterable
The signature cannot be repudiated Because the scheme use the Trent
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Problems
The protocol take too much time for Trent
Hard to creating and maintaining good Trent
Trent is bottlenecks in any communication
system
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Digital Signature Trees
Ralph Merkle proposed a digital signaturesscheme using a tree structure.
Use Root, node and sub node hierarchical
Signing Documents with Public-Key
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Signing Documents with Public KeyCryptography
Basic Protocols
(1) Alice encrypts the document with herprivate key, thereby signing the document.
(3) Bob decrypts the document with Alices publickey, thereby verifying the signature.
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Advantages
Fulfilled the requirement of Signatures
Trent is not needed to either sign and verify
signatures.
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Other Digital Signature
Signing Documents and Timestamps
Signing Documents with Public Key
Cryptography and One-Way Hash Functions1. Alice produces a one-way hash of a document.
2. Alice encrypts the hash with her private key, therebysigning the document.
3. Alice sends the document and the signed hash toBob.
4. Bob produces a one-way hash of the document that
Alice sent. He then, using the digital signaturealgorithm, decrypts the signed hash with Alicespublic key. If the signed hash matches the hash he
generated, the signature is valid.
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Multiple Signatures
(1) Alice signs the hash of the document.
(2) Bob signs the hash of the document.
(3) Bob sends his signature to Alice.
(4) Alice sends the document, her signature, and .
(5) Carol verifies both Alices signature and Bobssignature.
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Digital Signatures with Encryption
The signature provides proof of authorship andthe envelope provides privacy.
(1) Alice signs the message with her private key. SA(M)(2) Alice encrypts the signed message with Bobs
public key and sends it to Bob. EB(SA(M))
(3) Bob decrypts the message with his private key.DB(EB(SA(M))) = SA(M)
(4) Bob verifies with Alices public key and recovers themessage. VA(SA(M)) = M
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Random and Pseudo-Random-Sequence Generation
Pseudo-Random Sequences
It looks random. This means that it passes all the
statistical tests of randomness Cryptographically Secure Pseudo-Random
Sequences
It is unpredictable Use Key, The Key is generally the seed used to set
the initial state of generator
Real Random Sequences It cannot be reliably reproduced
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B i P t l
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Basic Protocols
Key Exchange
Authentication
Authentication and Key Exchange
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