Introduction to Cryptography Techniques

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Introduction to Cryptography Techniques

• How secure is that banking network traffic?

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Social and ComputingImplications of Cryptography

• CSCI 365 Information Security is about creating
  good doors. But what if a door lock gets picked
  and someone can see your encrypted data, or your
  data is intercepted traveling between two doors?
• The internet is a collection of networks designed
  to deliver data packets.
• Packets are easy to sniff.
• The internet is not secure, but is used to
  connect banks, the power grid, pipelines,
  transportation systems, etc.

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Terms

• Plaintext the readable message
• Ciphertext the coded message

key
key
plaintext ciphertext
plaintext
Encryption
Decryption
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Types of Attacks

• Ciphertext Only adversary uses just the
  ciphertext to gain either the key or the
  plaintext (really bad encryption)
• Known Plaintext adversary gets the key using
  some ciphertext and its plaintext
• Chosen Plaintext adversary introduces some
  plaintext to generate some ciphertext

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Symmetric Key Encryption

• Both parties share a single secret key
• The key is used for both encryption and
  decryption
• Encryption and decryption are equal efforts

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Shift Ciphers

• key amount to shift each character
• Example Rotate13
• A 13 1 13 14 N
• So, the message aardvark becomes
• nneqinex.

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Shift Ciphers

• Advantage of Rot13
• Easy to implement.
• Rot13('A') 'N' (1 13)26 14
• Rot13('N') 'A' (14 13)26 1
• So, one function does both encoding and decoding.
• Disadvantage of Any Rotation
• Very easy to break just try all 26
  possibilities.
• aka - Brute Force attack.

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Substitution Cipher

• Key list of character substitutions
• Example Key Chair
• 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
• Y Z c h a i r B D E F G J K L M N O P Q S T U V W
  X
• Disadvantage
• Susceptible to Character Frequency Analysis

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Character Frequencies
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Character Frequencies
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Polyalphbetic Ciphers

• Key is repeated and used to shift characters.
• Example
• plaintext now is the time for all
• key aar dv ark aard var kaa
• Ciphertext opo mo uzp ujei bpj lmm

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Polyalphbetic Ciphers

• Advantage Thwarts character frequency analysis.
  For example, an e will encrypt to several
  different letters.
• Disadvantage Statistics can still be used to
  break the code.

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Polyalphbetic Ciphers

• How to Break Them
• 1 - Look for repeated strings.
• For example, if the characters thi appear
  together frequently, then it could be because the
  key is hitting a common word.
• Text and we need to test and retest
• Key ste ve stev es teve ste vestev
• Sum thi sj gyjz yh njoy thi njmyxp

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Polyalphbetic Ciphers

• How to Break Them
• 2 Determine Probable Key Length
• The start of strings thi are frequently
  separated by distances that are multiples of 5.
  So, key length is probably five.
• 3A Try keys of that length.
• 3B Use CharFreqAnal on characters separated by
  that length.

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One-Time Pad

• Key is used to shift the plaintext.
• Key is used only once.
• Key has same length as the message.
• Advantage Unbreakable!
• Disadvantage Requires lots of keys.

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DES History

• Data Encryption Standard
• Solicited in 1973 by the National Bureau of
  Standards (National Institute of Standards and
  Technology)
• Developed by IBM and the NSA
• Adopted in 1977

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DES Design Principles

• Confusion complicate the relationship between
  key and ciphertext
• Diffusion spread structure of plaintext around
  the ciphertext

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DES Design Overview

• http//www.itl.nist.gov/fipspubs/fip46-2.htm
• Key 56 bits plus 8 parity bits
• 70,000,000,000,000,000 possible keys of 56 bits
• Key generates 16 subkeys
• 16 rounds of functions

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Breaking DES

• 1993 design of 1M machine to search entire key
  space in one day
• 1997 design of 1M machine to search entire key
  space in one hour
• 1999 - DES Challenge prize claimed in 22 hours
  by distributed.net
• 2006 - University of Bochum and Kiel, Germany,
  uses 10,000 hardware cost to get average time of
  6.4 days.
• triple DES is much less breakable

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Unix Crypt

• man 3 crypt
• include ltunistd.hgt
• char crypt(const char key, const char salt)
• crypt is the password encryption function.
• It is based on the Data Encryption Standard
  algorithm with variations intended (among other
  things) to discourage use of hardware
  implementations of a key search.

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Password Salt

• Based on time when password created
• First two letters in the passwd field
• Used to discourage a brute force attack
• Encrypting every dictionary word then comparing
  that list to passwd entries will not work since
  every dictionary word can yield 4096 different
  possibilities.
• Even if my password is the same for two systems,
  they have different salts so they look different

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Public Key Encryption

• Two Keys encryption and decryption
• Encryption key is public
• Decryption key is private
• Once sender encrypts a message, even they cant
  decrypt it

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Public Key Encryption

1. Receiver sends their public key to the sender
2. Sender encrypts message using that public key
3. Sender sends encrypted message
4. Receiver decrypts message using their private key

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Summary

• Nothing on a public network is completely safe.