; CS Special Topics on Network Security
Documents
User Generated
Resources
Learning Center
Your Federal Quarterly Tax Payments are due April 15th

# CS Special Topics on Network Security

VIEWS: 3 PAGES: 19

• pg 1
```									          Cryptography Overview
Cryptography is a strong tool
against many kinds of security threats.

Encryption
Message Authentication Code
Digital Signature

@Yuan Xue (yuan.xue@vanderbilt.edu)
Cryptography vs. Cryptanalysis
Cryptography
   Studies the encryption and decryption schemes.
Cryptanalysis
   studies how to “break the code”, i.e., how to decrypt
the ciphertext without the knowledge of the encryption
details.

@Yuan Xue (yuan.xue@vanderbilt.edu)
Attacking Encryption Scheme
Brute-force attack tires every possible key on a
piece of cihphertext until an intelligible translation
into plaintext is obtained.
Cryptanalysis exploits the characteristics of the
algorithm and the traces of structure or pattern in
the plaintext that survive encryption
   break a single message
   deduce the key in order to break the subsequent
messages.
   How ? -- Using statistical tools and properties of
languages.
@Yuan Xue (yuan.xue@vanderbilt.edu)
Cryptanalytic Attack Classification
Ciphertext only attack
   encryption algorithm and ciphertext
Known plaintext attack
   encryption algorithm, ciphertext, and one or more plaintext-ciphertext
pairs formed with the secret key.
Chosen plaintext attack
   encryption algorithm, ciphertext, and chosen plaintext and its
corresponding ciphertext generated with the secret key.
Chosen ciphertext attack
   encryption algorithm, ciphertext, and chosen ciphertext and its
corresponding decrypted plaintext with the secret key.
Chosen text attack
   chosen plaintext attack + chosen ciphertext attack
Encryption algorithm is designed to withstand
a known plaintext attack.
@Yuan Xue (yuan.xue@vanderbilt.edu)
Cryptanalytic Attack Classification
Unconditionally secure
   If the ciphertext generated by the algorithm does not contain
enough information to determine uniquely the corresponding
plaintext, no matter how much ciphertext is available, and how
much time an opponents has.
Computationally secure
   The cost of breaking the cipher exceeds the value of the encrypted
information
   The time required to break the cipher exceeds the useful lifetime of
the information

@Yuan Xue (yuan.xue@vanderbilt.edu)
CS 285 Network Security

Classical Encryption Algorithm

Fall 2008

@Yuan Xue (yuan.xue@vanderbilt.edu)
Mono-Alphabetic Ciphers
Caeser cipher
   How many possible substitutions?
   Bruce-force attack
 Enumerate all possible keys

Allow arbitrary substitution
   How many possible substitutions?
   How to represent the key?

@Yuan Xue (yuan.xue@vanderbilt.edu)
Frequency Analysis

@Yuan Xue (yuan.xue@vanderbilt.edu)
Multiple-letter Ciphers
Key idea: Treat consecutive letters in the plaintext
as a single unit (block) and translate the unit into
ciphertext.
Benefit: Lessen the extent to which the structure
of the plaintext survives in the ciphertext, thus are
stronger against frequency analysis.
Issue: How to specify a mapping (substitution)?

@Yuan Xue (yuan.xue@vanderbilt.edu)
Playfair Cipher

- Use a table

@Yuan Xue (yuan.xue@vanderbilt.edu)
Playfair Cipher
Playfair cipher was invented in 1854 by Charles
Wheatstone, but bears the name of Lord Playfair
who promoted the use of the cipher.
Playfair cipher takes two-letter combinations
(digrams) as single units for encryption.
The encryption algorithm takes a 5*5 matrix of
letters as a key table to translate digrams.
The key table is constructed by a keyword.

@Yuan Xue (yuan.xue@vanderbilt.edu)
Playfair Cipher
To encrypt a message,
1. If both letters are the same (or only one letter is left), add an
``x'' (any uncommon letter will do) after the first letter. For example,
``balloon'' would be treated as ``ba lx lo on''.

2. If the letters appear on the same row of the table, replace them with
the letters to their immediate right respectively (the table wraps around).
For example, ar is encrypted as RM.

3. If the letters appear on the same column of the table, replace them
with the letters immediately below respectively. For example, mu is
encrypted as CM.

4. If the letters are not on the same row or column, replace them with
the letters on the same row respectively but at the other pair of corners
of the rectangle defined by the original pair. For example, hs is encrypted
as BP, and ea is encrypted as IM (or JM).

@Yuan Xue (yuan.xue@vanderbilt.edu)
Hill Cipher
Use a mathematical function to specify a mapping

@Yuan Xue (yuan.xue@vanderbilt.edu)
Hill Cipher
Hill cipher was invented by Lester Hill in 1929. In the
encryption algorithm, n successive letters in plaintext are
considered as a n-dimension vector P.
The algorithm takes a n*n matrix K as a key.
The ciphertext C of P is also a n-dimension vector derived
by multiplying P by K, modulo 26.

Issue
   Known plaintext attack

@Yuan Xue (yuan.xue@vanderbilt.edu)
Hill Cipher Example
Key

P: pay

@Yuan Xue (yuan.xue@vanderbilt.edu)
Multiple-letter Ciphers
Benefit: Reduce the extent to which the structure
of the plaintext survives in the ciphertext, thus are
stronger against frequency analysis.
How to design a stronger cipher?

Larger block size is stronger

@Yuan Xue (yuan.xue@vanderbilt.edu)
Polyalphabetic Ciphers
Polyalphabetic cipher uses different
monoalphabetic substitution as it proceeds
through the plaintext.
   A set of monoalphabetic substitution rules;
   A key that determines which particular rule is chosen for
a given transformation.
Vigenere cipher

@Yuan Xue (yuan.xue@vanderbilt.edu)
Transposition
Transposition
   Transposition technique changes the order of the letters
in a message.
   Columnar Transposition Cipher

   Permutation Cipher

@Yuan Xue (yuan.xue@vanderbilt.edu)
Summary
Two types of ciphers
   Stream cipher
 E.g. Vigenere cipher
 encrypts one letter at a time.
   Block cipher
 E.g. Hill cipher
 treats a n-letter block of plaintext as a whole and produce a ciphertext
block of equal length.

Two basic building components for block ciphers
   Substitution
   Transposition

@Yuan Xue (yuan.xue@vanderbilt.edu)

```
To top
;