Modern Cryptography Techniques

1
Modern Cryptography Techniques

• continuation of Crypto 101

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Review

• Symmetric Key Encryption
• Uses a key that is known by both parties.
• The one key is used for both encryption and
  decryption.
• Attacks are based on finding the key through
  analysis of patterns in the ciphertext.

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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

4
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
• 1998 distributed.net won contest by finding the
  key in 40 days (searching 88 of key space)
• 1999 - DES Challenge prize claimed in 22 hours
• 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

• Receiver sends their public key to the sender
• Sender encrypts message using that public key
• Sender sends encrypted message
• Receiver decrypts message using their private key

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Summary

• Nothing on a public network is completely safe.