ISM6223

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Anonymous
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116754
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ISM6223
Updated:
2011-11-14 16:59:10
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Exam 1
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  1. Crytographers
    invent crytographic algorithms (secret codes)
  2. Crytopanalysts
    find ways to break codes
  3. Decipher a message
    find the plaintext without being given the key or secret algorithm
  4. Break a code
    • find a systematic way to decipher ciphertext
    • created using the code with affordable resources.
  5. encryption scheme has five ingredients
    • 1. plaintext
    • 2. encryption algorithm
    • 3. secret key
    • 4. ciphertext
    • 5. decryption algorithm
  6. Secret Key (also "Conventional" or "Symmetric")
    • 1. Identical keys used to encrypt and decrypt data
    • 2. Ciphertext is same length as plaintext (+ padding)
    • 3.Used for transmission and storage for privacy
    • 4. Can be used for authentication
  7. Public Key Cryptography ("Public-Private", "Asymmetric")
    • 1.Invented in 1975 (RSA)
    • 2.Public Key can be used by anyone to send a message
    • 3.Private Key can be used for a "Digital Signature“
    • 4. Hash Algorithms ("Message Digest" or "1-Way Transform")
    • 5.Password hashing
  8. Feistel Cipher Structure
    • 1. Block size
    • 2. Key size
    • 3. Number of rounds
    • 4. Subkey generation algorithm
    • 5. Fast software encryption/decrytion
  9. DES
    Data Encryption Standard
  10. DES attributes
    • 1. The most widely used encryption scheme
    • 2. DES is a block cipher
    • 3. The plaintext is processed in 64-bit blocks
    • 4. The key is 56-bits in length
  11. Triple DEA
    Use three keys and three executions of the DES algorithm (encrypt-decrypt-encrypt)

    Effective key length of 168 bits
  12. Triple DEA algorithm
    C = EK3[DK2[EK1[P]]]
  13. Other Symmetric Block Ciphers
    • 1. International Data Encryption Algorithm (IDEA)
    • 2.Blowfish
    • 3. RC5
  14. IDEA
    • 128-bit
    • Used in PGP
  15. Blowfish
    • Easy to implement
    • High execution speed
    • Run in less than 5k of memory
  16. RC5
    • 1.Suitable for hardware and software
    • 2.Fast, simple
    • 3.Adaptable to processors of different word lengths
    • 4.Variable number of rounds
    • 5.Variable-length key
    • 6.Low memory requirement
    • 7.High security
    • 8.Data-dependent rotations
  17. Location of encryption device
    • 1. Link encryption
    • 2. End-to-end encryption
    • 3. High security
  18. Link encryption
    • 1. A lot of encryption devices
    • 2. High level of security
    • 3. Decrypt each packet at every switch
  19. End-to-end encryption
    • 1. the source encrypts and the receiver decrypts
    • 2. payload encrypted
    • 3. header in the clear
  20. high security
    both link and end-to-end encryption are needed
  21. the use of two keys has consequences i
    • 1. key
    • 2. distribution
    • 3. confidentiality
    • 4. authentication
  22. Public-key crypothography ingredients
    • 1. Plaintext
    • 2. Encryption algorithm
    • 3. Public key
    • 4. Private key
    • 5. Ciphertext
    • 6. Decryption algorithm
  23. Applications for Public-Key Cryptosystems
    • 1.Encryption/decryption: The sender encrypts a message with the recipient’s public key.
    • 2.Digital signature: The sender ”signs” a message with its private key.
    • 3.Key echange: Two sides cooperate to exhange a session key.
  24. Public-Key Cryptographic Algorithms
    • RSA
    • Diffie-Hellman
  25. RSA
    • 1. RSA is a block cipher
    • 2. The most widely implemented
  26. Diffie-Hellman
    • 1. Echange a secret key securely
    • 2. Compute discrete logarithms
  27. Authentication requirements
    • 1. Message came from apparent source or author
    • 2. Contents have not been altered
  28. Authentication
    protection against active attack (falsification of data and transactions)
  29. Authentication Using Conventional Encryption
    Only the sender and receiver should share a key
  30. Message Authentication without Message Encryption
    An authentication tag is generated and appended to each message
  31. Message Authentication Code
    Calculate the MAC as a function of the message and the key. MAC = F(K, M)
  32. One-way HASH function
    Secret value is added before the hash and removed before transmission.
  33. Secure HASH Functions
    Purpose of the HASH function is to produce a ”fingerprint”.
  34. Properties of a HASH function H
    1.H can be applied to a block of data of any size

    2.H produces a fixed length output

    3.H(x) is easy to compute for any given x.

    4.For any given code h, it is computationally infeasible to find x such that H(x) = h

    5.For any given block x, it is computationally infeasible to find with H(y) = H(x).

    6.It is computationally infeasible to find any pair (x, y) such that H(x) = H(y)
  35. Requirements for Public-Key Cryptography
    • 1.Computationally easy for a party B to
    • generate a pair (public key KUb, private key KRb)

    2.Easy for sender to generate ciphertext: C = Ekub(M)

    • 3.Easy for the receiver to decrypt ciphertect
    • using private key: M = Dkrb(C) = Dkrb[Ekub(M)]

    4.Computationally infeasible to determine private key (KRb) knowing public key (KUb)

    5.Computationally infeasible to recover message M, knowing KUb and ciphertext C

    • 6.Either of the two keys can be used for
    • encryption, with the other used for decryption:
    • M = Dkrb[Ekub(M)] = Dkub[Ekrb(M)]

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