Basic Cryptography

1
Basic Cryptography

• Contents
• Cryptography
• Symmetric Cryptography
• Public Key Cryptography
• Digital Signature
• Chapter 2 pp15 23 of Web security

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What is Cryptography?

• The word originally comes from the Greek. It
  means secret writing.
• It is commonly used in militaries.
• For example, when you say OS to your friend
  using secret word. It means that see you in old
  place on Saturday night at 800 p.m. Only you
  and your friend can understand, third party
  cannot understand this meaning.

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Example of plain text and cipher text
HITYVf TYISHB jasdfsdf jsdf7643sga87dv8asvas
8dv9as87va8s7v8d7va8d7 56GTEUC

• Hi DCO20203
• The answers to quiz 1 are now available in
  general office at P1234.
• Charlie

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Cryptographic System

• No matter what method you adopt, it consists of
  four parts
• Plaintext the original message such as I will
  be there.
• Ciphertext The encrypted message after
  encrypting from plaintext such as I will be
  there becomes ibnj89asdf (.
• Cryptographic algorithm The method to convert
  from plaintext to ciphertext such as DES (data
  encryption standard). It is common that many
  people can use it.
• Key This is the key to encrypt and decrypt the
  message (encrypt it means from plaintext to
  ciphertext, decrypt from ciphertext to
  plaintext.)

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Four basic parts
DES
Algorithim
How are you?
IUt670,.
Plaintext
Ciphertext
1234
Key
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Caesar Cipher

• The scheme was proposed by Julius Caesar. The
  character of a plain text message is replaced by
  other character. The following example is to
  shift to right by two characters.

After
Example
Before
DCO students
FEQ uvwfgpvu
7
ASCII Table related to your assignment I, below
is an example of shifting by five
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How to Crack?(1) (means how to break the
ciphertext)

• Cryptanalysis try to analyse the ciphertext to
  guess the meaning.
• For example, if the plaintext How are you? is
  converted to uyi89rty89qwe89, we understand
  that 89 is used to replace the space. Of
  course, most of them are complicate and cannot be
  analyzed in this way.
• Brute-force attackGuess the key with all
  combinations. (if a digit lock consists of three
  numbers, you could try from 000 to 999 and you
  can then break it.)

1 digit
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How to Crack?(2)

• If the key is two-digit number from 00 to 99, we
  can try 100 times, then we know the key. The
  average attempt is 100/2 50 times, as the key
  might be 34 or 78, we dont need to try all of
  them. (In this case, the lock is algorithm.)
• For example, a numerical lock consists of three
  digits and you take 3 seconds to attempt one
  combination. On average, how long do you think
  you can break this lock? 3 x 1000/2 1500s 25
  minutes

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Types of Cryptography

• Symmetric use the same key to encrypt and
  decrypt the message, not very safe (For example,
  you use the same key to lock and unlock your
  door.)
• Asymmetric sender uses recipients public key to
  encrypt the document and the recipient uses his
  own private key to decrypt the document. (safe,
  but less efficient, will explain to you later)

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Symmetric Cryptography DES (data encryption
standard)
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Symmetric Cryptography - example

• Graphic From
• from this http//www.w3.org/Talks/971008-tpp71j/s
  ymmetric.gif

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Asymmetric Cryptography

• Use two keys recipient's public key and private
  key to perform encryption and decryption.

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Asymmetric Keys

• Each of them holding two keys, private and public
  keys from http//eiche.theoinf.tu-ilmenau.de/aa
  ver/lehre/hs_ws97/indians/fig6.gif

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Common Symmetric Algorithms (1)

• DES Data Encryption Standard by US national
  institute of standards. It uses 56-bit key. How
  many combination? 256 1017
• Triple DES an enhancement of DES (three times of
  DES, encrypt, encrypt and encrypt) with an
  effective length of 168. 2168(3x56) 1051.
• RC2, RC4 and RC5 By RSA data security up to 2048
  bits, 22048 10612, used by web browser for key
  up to 40 bits

Encryption
Encryption
Encryption
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Common Symmetric Algorithms (2)

• IDEA International Data Encryption Algorithm
  (IDEA) by AscomTech AG. It uses 128-bit key and
  is popular in Europe.
• Blowfish by BruceSchneier with a key length up
  to 448 bits 2448 10134
• You must use the same algorithm. For example, you
  cannot use DES to encrypt but use RC2 to decrypt.
  What you can change is the KEY? For example, this
  time, you use 56rt4565489 .., next time, you can
  change to 78654Rtgfcb

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Symmetric algorithm is it reliable?

• No, as the same key is used in both. This key is
  sent over the Internet and might be captured by
  the hacker.
• For example, you apply for an account over the
  Internet and the password is also sent over the
  Internet. You have the feeling that it might not
  be safe. It is better the password will be
  delivered through other means such as ordinary
  mail or DHL. (That is why the password is usually
  sent to you by ordinary mail.)

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Asymmetric Cryptography- your Hong Kong Permanent
Identity Card uses this method

• It is also called public key cryptography.
• It uses two keys separately to encrypt and
  decrypt message which is safer than symmetric
  cryptography, as the key cannot be reproduced.
• There are two common algorithms
• RSA by Ronald Rivest Adi Shamir with variable
  key length from 512 to 1024 bits
• EIGamal by Taher EIGamal uses key length from
  512 to 1024.

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Digital Signature

• The reversal of public key encryption
• It uses senders private and public key rather
  than recipients public and private key. It is
  used for verification. (the document you sent
  belongs to you.)

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Summary

• Cryptography secret word
• Four basic parts plaintext, ciphertext,
  algorithms, Key
• Symmetric cryptography use the same key
• Asymmetric cryptography use two keys, private
  and public keys
• Encryption (asymmetric) use recipient's public
  key to encrypt the message you are sending,
  recipient uses his/her private key to decrypt the
  message.
• Digital signature uses senders private key to
  encrypt message you are sending. Recipient uses
  senders public key to verify the message.

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Chinese Terms

• ??????? (Password Cracking)
• ????(Packets Sniffing)
• ARP ?? (ARP Spoofing)
• DNS ?? (DNS Spoofing)
• ???? (Session Hijacking)
• ???? (Remote Scaning)

???? (Backdoors Creating) ???? (Tracks
Covering) ?????? (Network Scanning) ???(firewall)
??(Back Doors) ????(Trojan Horses) ??????(Honeypot
)