Basics_of_Cryptography.pdf

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Basics of Cryptography


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Definition of Cryptography

Conversion of plain text data into a secret code

We will announcethe merger at noon

on Friday

We will announcethe merger at noon

on Friday

Plain Text Data

px pbdd tgghnvxmax fxlzxl tm ghhg

hg ylbwtr

px pbdd tgghnvxmax fxlzxl tm ghhg

hg ylbwtr

Secret Code


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Brief History

Dates as far back as 1900 B.C in Egypt

Julius Caesar (100-44 B.C.) used simple

substitution For example, substitute each letter in the alphabet with a

different letter

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

M N O P Q R S T U V W X Y Z A B C D E F G H I J K L


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Brief History (cont.)

Sir Francis Bacon (1623) Bi-literal cipher


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Thomas Jefferson (1790) Wheel Cipher

Read this

line on the

wheel


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William Frederick Friedman Father of USCryptanalysis

US Government cryptanalyst 1918 Index of Coincidence and its Applications inCryptography

WWII Code Machines: Germanys Enigma Japans Purple


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1970s 1976 DES standardized

1976 Public key cryptography introduced

1977 RSA cipher introduced

1980s Stronger ciphers (128 bit) introduced (e.g., IDEA)

1990s Integration of cryptography into commercial

applications Post 2K:

AES standardized to replace DES

Emergence on the use of elliptic curve cryptography


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Encrypting Data

Encrypting data primarily requires two

components

Secret key Algorithm

We will announce

the merger atnoon on Friday

We will announcethe merger atnoon on Friday

Plain Text

px pbdd tgghnvx

max fxlzxl tmghhg hg ylbwtr

px pbdd tgghnvxmax fxlzxl tm

ghhg hg ylbwtr

Cipher Text

Crypto

Algorithm

Crypto

Algorithm

Secret Key


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We willannounce the

merger on Friday

We willannounce the

merger on Friday

Plain Text

Decrypting Data

Reverse process of encryption


ghhg hg ylbwtr


ghhg hg ylbwtr

Cipher Text

Crypto

Algorithm

Crypto

Algorithm

Secret Key


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Types of Cryptographic Algorithms

Symmetric

Asymmetric

One-Way Hashes


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Symmetric Cryptography

Use the same secret key to encrypt and decrypt

data

SharedSecret Key

I LoveYou! apdojqlejz I LoveYou!AlgorithmAlgorithm AlgorithmAlgorithm

Alice Bob

S i C h


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Symmetric Cryptography:

Pros and Cons

Pros:

Fast Short keys

Well known

Key generation simple

Cons: Secrecy of keys

Mgmt of keys

Nos. of keys


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Asymmetric Cryptography

Use one key for encryption, and one key for

decryption

One key is considered the private (secret) key, while the otheris considered the public key

Keys are mathematically related to one another


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An example (encrypting):

I LoveYou!

I LoveYou!

Alice Bob

AlgorithmAlgorithm

Bobs Public Key

AlgorithmAlgorithm

Bobs Private Key

apdojqlejz

Cipher Text


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An example (signing):

I LoveYou!

Alice Bob

AlgorithmAlgorithm

Alices Private Key

AlgorithmAlgorithm

Alices Public Key

I LoveYou!

SignedPlain Text

I LoveYou!

Authentic

Asymmetric Cryptography:


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Asymmetric Cryptography:

Pros and Cons

Pros:

No shared secrets Key mgmt easier

Provides secrecy and

authenticity

Cons:

Slow

Large keys

Key generation ismore difficult


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One-Way Hashes

Calculates a fixed size value using input of any size

Result is typically 128 or 160 bits

Newer algorithms will produce larger outputs

Key Properties:

No two results are the same

One-way function (i.e., cannot determine input from output)


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One-Way Hashes

An example:

I LoveYou! Hash Algorithm

Hash Algorithm Hash1

160 Bits

NYCPhoneBook

Hash AlgorithmHash Algorithm Hash2

160 Bits

Hash1= Hash2

Any change to input

produces new output

DATA INTEGRITY


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Combining Cryptographic Methods

Encryption:

Encrypt data with a temporary symmetric key

Encrypt symmetric key to recipient using recipients public key

Decryption:

Decrypt temporary symmetric key using own private key

Decrypt message using symmetric key


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Encryption (An Example):

I LoveYou!

Alice Bob

Temp Symmetric Key

SymmetricAlgorithmSymmetricAlgorithm

apdojqlejz

Cipher Text

0010101

11010100010110AsymmetricAlgorithm

Asymmetric

Algorithm

Bobs Public Key

EncryptedSymmetric Key Message


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Decryption (An Example):

Bob

I LoveYou!

SymmetricAlgorithmSymmetricAlgorithm

Temp Symmetric Key

AsymmetricAlgorithm

Asymmetric

Algorithm

Bobs Private Key

Message

apdojqlejz

Cipher Text

0010101

11010100010110

EncryptedSymmetric Key


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Signing:

Hash the data to be signed (data integrity)

Sign the hash (authentication) Verification:

Verify signature on hash (authentication)

Verify hash against a calculated hash of data (data integrity)


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Signing (An Example):

I LoveYou!

Alice Bob

HashAlgorithm

HashAlgorithm

0010101

11010100010110

Hash

Alices Private Key01100101110101

0100101

Asymmetric

Algorithm

Asymmetric

Algorithm

Signature(Encrypted Hash) Message


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Signature Verification (An Example):

Bob

001010111010100010110

HashAlgorithm

HashAlgorithm

Message

I LoveYou!

Message

0110010

11101010100101

Signature(Encrypted Hash)

Alices Public Key

AsymmetricAlgorithm

AsymmetricAlgorithm

0010101

11010100010110

Are thehashesequal??

S


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Summary

Different types of cryptographic algorithms

address different requirements

Each type of algorithm has its set of pros and cons Combining types of algorithms provides complete

and useful solutions

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