Cryptography Lecture 3

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CryptographyLecture 3

• Cryptography
• Terminology
• Secret-Key Encryption
• Public-Key Encryption

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Reading Assignment

• Reading assignments for lecture 3
• Required
• Pfleeger Ch 2.1, 2.2
• Recommended
• U.S. Senate hearings on Internet Security B.
  Schenier http//www.senate.gov/commerce/hearings
  /071601Schneier.pdf
• Crypto-Gram Newsletters http//www.counterpane.com
  /crypto-gram.html
• Security Focus Online, cryptanalysis tools
  http//online.securityfocus.com/tools/category/54
  
• Reading assignments for next class
• Required
• Pfleeger Ch 2.3, 2.4

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Cryptography Tools

• Crypt Breaker's Workbench, http//wombat.doc.ic.ac
  .uk/foldoc/foldoc.cgi?CryptBreakersWorkbench
• PGP, http//web.mit.edu/network/pgp.html
• Crypto and Security, http//www.programmersheaven.
  com/zone16/cat731/index.htm

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Insecure communications
Confidential
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Cryptographic Protocols

• Messages should be transmitted to destination
• Only the recipient should see it
• Only the recipient should get it
• Proof of the senders identity
• Message shouldnt be corrupted in transit
• Message should be sent/received once only

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Terminology

• Plaintext (cleartext) a message in its original
  form
• Ciphertext (cyphertext) an encrypted message
• Encryption transformation of a message to hide
  its meaning
• Cipher cryptographic algorithm. A mathematical
  function used for encryption (encryption
  algorithm) and decryption (decryption algorithm).

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Terminology

• Decryption recovering meaning from ciphertext
• Cryptography art and science of keeping messages
  secure
• Cryptanalysis art and science of breaking
  ciphertext
• Cryptology study of both cryptography and
  cryptanalysis

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Encryption and Decryption
Plaintext
Ciphertext
Plaintext
Encryption
Decryption
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Conventional (Secret Key) Cryptosystem
Plaintext
Ciphertext
Plaintext
Encryption
Decryption
Sender
Recipient
K
CE(K,M) MD(K,C)
K needs secure channel
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Public Key Cryptosystem
Recipients public Key (Kpub)
Recipients private Key (Kpriv)
Plaintext
Ciphertext
Plaintext
Encryption
Decryption
Sender
Recipient
CE(Kpub,M) MD(Kpriv,C)
Kpub needs reliable channel
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Cryptanalysis

• Cryptanalysts goal
• Break message
• Break key
• Break algorithm

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Taxonomy of Attacks

• Ciphertext-only attack attacker has ciphertext
  for messages encrypted with E. Deduce keys
  and/or plaintext messages.
• Known plaintext attack attacker additionally
  knows the plaintext of the messages. Deduce keys
  or a decryption algorithm.
• Chosen plaintext attack attacker can obtain the
  ciphertext for selected plaintext messages.
  Deduce as above.
• Chosen ciphertext attack attacker can obtain
  decrypted (plaintext) versions of selected
  ciphertext. Deduce as above.

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Breakable versus Practically breakable

• Unconditionally secure impossible to
  decrypt. No amount of ciphertext will enable a
  cryptanalyst to obtain the plaintext
• Computationally secure an algorithm that is not
  breakable in practice based on worst case
  scenario
• Breakable all algorithms (except one-time pad)
  are theoretically breakable

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What makes a good cryptosystem?

• A good cryptosystem is one whose security does
  not depend upon the secrecy of the algorithm.
• From Bruce Schneier
• Good cryptographers rely on peer review to
  separate the good algorithms from the bad.''

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Secret Key Cryptosystem
Plaintext
Ciphertext
Plaintext
Encryption
Decryption
Sender
Recipient
K
CE(K,M) MD(K,C)
K needs secure channel
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Secret Key Cryptosystem Vulnerabilities (1

• Passive Attacker (Eavesdropper)
• Obtain and/or guess key and cryptosystem use
  these to decrypt messages
• Capture text in transit and try a ciphertext-only
  attack to obtain plaintext.

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Secret Key Cryptosystem Vulnerabilities

• Active Attacker
• Break communication channel (denial of service)
• Obtain and/or guess key and cryptosystem and use
  these to send fake messages

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Inherent Weaknesses of Symmetric Cryptography

• Key distribution must be done secretly (difficult
  when parties are geographically distant, or don't
  know each other)
• Need a key for each pair of users
• n users need n(n-1)/2 keys
• If the secret key (and cryptosystem) is
  compromised, the adversary will be able to
  decrypt all traffic and produce fake messages

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Basic Encryption Techniques

• Substitution
• Permutation
• Combinations and iterations of these

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Caesar cipher

• CE(K,M), e.g., C(Mn) mod 26
• plaintext placement A B C D E
• ciphertext placement A B C D E F
• e.g., MCAB
• C ECD
• Advantages simple to implement
• Disadvantages easy to break (25 possibilities
  for English alphabet)

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Simple Alphabetic Substitution

• Assign a new symbol to each plain text symbol
  randomly, e.g.,
• C ?K, A ?H, B ? L

• MCAB
• C KHL
• Advantages large key space 26!
• Disadvantages trivially broken for known
  plaintext attack

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One-Time Pad

• Perfect Secrecy!
• Large, non-repeating set of keys
• Key is larger than the message
• Advantages immune to most attacks
• Disadvantages
• Need total synchronization
• Need very long, non-repeating key
• Key cannot be reused

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Summary of Substitution

• Advantages
• Simple
• Easy to encrypt
• Disadvantages
• Easy to break!!!

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• Next class
• Transpositions
• Characterization of good encryption algorithms