understanding cryptography - Regional Security through Data ...

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21 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Serbia 2003


INTRODUCTION

to

CRYPTOGRAPHY

Fred Piper

Codes & Ciphers Ltd

12 Duncan Road, Richmond

Surrey, TW9 2JD

ENGLAND

Royal Holloway, University of London

Egham Hill, Egham

Surrey TW20 0EX

ENGLAND

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Sender


Am I happy that the whole world sees this ?


What am I prepared to do to stop them ?


What am I allowed to do to stop them ?

Recipient


Do I have confidence in :


the originator


the message contents and message stream


no future repudiation.

Network Manager


Do I allow this user on to the network ?


How do I control their privileges ?


Some Security Issues

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Cipher System

cryptogram

c


Enciphering

Algorithm

Deciphering

Algorithm

Key
k(E)

Key
k(D)

message

m

message

m

Interceptor

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The Attacker’s Perspective

Deciphering

Algorithm

Unknown Key

k(D)

Known

c

Wants
m

Note
:
k(E)
is not needed unless

it helps determine
k(D)

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Two Types of Cipher System



Conventional or Symmetric

k(D)

easily obtained from
k(E)



Public or Asymmetric

Computationally infeasible to determine
k(D)

from
k(E)

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Mortice Lock.

If you can lock
it, then you can
unlock it.

Bevelled Sprung Lock.

Anyone can lock it,
only keyholder can
unlock it.

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Types of Attack



Ciphertext only


Known plaintext


Chosen ciphertext

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Assumptions About Attacker (1)

Military/Government:


Try to keep details of system (including algorithm)
secret

Worst Case Conditions:

Commercial:



Assume he knows:


System (including algorithm)


All ciphertext


Some corresponding plaintext/ ciphertext

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Warning

THE FACT THAT AN ALGORITHM HAS
BEEN PUBLISHED SAYS NOTHING
ABOUT ITS STRENGTH.

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Breaking Algorithm


Finding a method of determining
message from cryptogram without
being given deciphering key.

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Exhaustive Key Search Attacks


The security depends on the secrecy of
the deciphering key.



One potential attack, if the algorithm is
known, is to try all possible deciphering
keys and to eliminate all incorrect ones.



To withstand this type of attack a large key


space is required.

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Exhaustive Key Searches

Estimating time required for key
search requires assumptions about the
attacker’ resources

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Saints or Sinners ?

Receiver

Interceptor

Sender

Who are the ‘good’ guys ?

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If Someone Wants Your Plaintext


Give it to them


Give them the decryption key


They may break algorithm


They may ‘find’ plaintext in system


They may ‘find’ key in system

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Practical Considerations when Implementing
Encryption


Key management is the difficult part


Keys need to be generated, distributed,
stored, changed securely


History shows that most cryptanalytic
attacks exploit poor implementation
and/or key management


Example: Enigma in World War 2


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Cryptography is used to provide:

1.

Confidentiality


2.

Data Integrity


3.

Entity/Origin Verification


4.

Non
-
Repudiation


5.

Access Control

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Choosing an Algorithm

The choice of algorithm depends upon

the application.


Applications of encryption include :


Data confidentiality


Data integrity


Digital Signatures.

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Misuse of Encryption







Grade

Good student




xxxxx

Bad student




xxxxx


Grades can be changed

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Classification of Techniques



Bit / Block operation


Positional dependence/independence


Message dependence/independence


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Vernam Cipher

Random sequence
k
1
,k
2
,…,k
n

Message
m
1
,m
2
,…,m
n

+

Ciphertext

k
1

m
1
,k
2

m
2
,…,k
n

m
n

The message and key are bit strings

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Stream Cipher

Plaintext data

Keystream

sequence

Ciphertext

Key

Sequence

Generator

XOR

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Stream Cipher


Enciphers bit by bit


Positional dependence


Security depends on properties of
the keystream

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Stream Ciphers

Applications


Widely used for military and
paramilitary applications for both data
and digitised speech


The main reason for their wide use is
that military communications are
often over poor channels and error
propagation is unacceptable

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Symmetric Block Cipher System

Key dependent

permutation

on
s
-
bit blocks

s
-
bit

plaintext block

s
-
bit

ciphertext block

Key

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Block Ciphers : Key Sizes



Depends on security requirement



Key searches on size 2
90

are
currently considered infeasible


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DES: Key Search on Internet (1997)


DES has 2
56
keys


DES key found


Search took 140 days


Search used over 10,000 computers


Peak rate: 7.10
9

keys/sec


‘Might’ have taken 32 days

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DES Breaker (1998)

Electronic Frontier Foundation

Design cost




$ 80,000

Manufacturing cost


$130,000

Test key found in



56 hours

Complete search in


220 hours

90 Billion keys per second

Design details published

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DES : Double Length Key


k = (k1,k2)


k1,k2 DES keys


E
k
(m) = E
k1
(D
k2
(E
k1
(m)))


key is 112 bits


key search with 2
112

trials is
infeasible.

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Advanced Encryption Standard
(AES)


Block ciphers


Block size 128 bits


Key lengths 128, 192, 256 bits


Must be faster than triple DES

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AES (Continued)

June 1998
:


15 candidates

August 1998
:


11

April 1999:



5

Decision




October 2000





Rijndael

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The following slides will not be
discussed but are included for
completeness

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Applications



Access Control


Authentication

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The Challenge / Response Principle

Key

Key

Random

number

Challenge

PIN
-
Controlled

A

A

Response

A
-

Encipher or OWF

Y/N

= ?

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Digital Signatures


According to ISO, the term Digital
Signature is used: ‘to indicate a
particular authentication technique
used to establish the origin of a
message in order to settle disputes of
what message (if any) was sent’.

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Digital Signatures

A signature on a message is some data


that validates a message and verifies its origin


a receiver can keep as evidence


a third party can use to resolve disputes.


It depends on


the message


a secret parameter only


available to the sender

It should be



easy to compute



(by one person only)



easy to verify



difficult to forge

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Principle of Digital Signatures

There is a (secret) number which:


Only one person can use


Is used to identify that person


‘Anyone’ can verify that it has been
used

NB:

Anyone who knows the value of a
number can use that number.

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Certification Authority

Aim :

To guarantee the authenticity of public keys.


Method :

The Certification Authority guarantees the
authenticity by signing a certificate containing
user’s identity and public key with its secret key.


Requirement :

All users must have an authentic copy of the
Certification Authority’s public key.

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Certification Process

Verifies

credentials

Creates

Certificate

Receives

(and checks)

Certificate

Presents Public

Key and

credentials

Generates

Key Set


Distribution

Centre

Owner

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How Does it Work?





The Certificate can accompany all Fred’s
messages

The recipient must directly or indirectly:


Trust the CA


Validate the certificate

The CA certifies

that Fred Piper’s

public key

is………..

Electronically

signed by

the CA

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Fundamental Requirement

Internal infrastructure to support
secure technological implementation