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Ciphers

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Ciphers

  1. 1. Bishop: Chapter 11 An Overview of Cipher Techniques (in the context of networks) (11.1-11.3)
  2. 2. Topics <ul><li>Problems with Design of Ciphers </li></ul><ul><li>Stream and Block Ciphers </li></ul><ul><li>A Brief Overview of Network Security </li></ul><ul><ul><li> more in Chapter 26 </li></ul></ul><ul><li>Privacy-enhanced E-Mails (PEM) </li></ul><ul><li>Advanced Topics: SSL, IPsec </li></ul><ul><ul><li> next course ( Web Security ) </li></ul></ul>
  3. 3. Context-related Vulnerability <ul><li>Key point: How a crypto system is used will affect its strength. </li></ul><ul><li>Sample Problems </li></ul><ul><ul><li>Pre-computing the possible messages </li></ul></ul><ul><ul><ul><li>Assumption: The plaintext corresponding to intercepted ciphertext is drawn from a relatively small set of possible plaintexts. </li></ul></ul></ul><ul><ul><ul><li>The cryptanalyst can encipher the set of possible plaintexts and simply search that set for the intercepted ciphertext. </li></ul></ul></ul>
  4. 4. Context-related Vulnerability <ul><li>Sample Problems (cont.) </li></ul><ul><ul><li>Polluted Blocks </li></ul></ul><ul><ul><ul><li>Parts of a ciphertext message may be deleted, replayed, or reordered. </li></ul></ul></ul><ul><ul><ul><li>Unless different parts are bound together, their order may be changed by the attacker, without being detected by the receiver. </li></ul></ul></ul><ul><ul><ul><li>Example: Reordered RSA data blocks </li></ul></ul></ul><ul><ul><ul><ul><li>‘LIVE’ can be reordered to ‘EVIL’. </li></ul></ul></ul></ul><ul><ul><ul><li>Source of problem: Each block is independently enciphered, so integrity of each part does not guarantee the integrity of the whole. </li></ul></ul></ul><ul><ul><ul><li>Solution? ‘binding’ of blocks + digital signature </li></ul></ul></ul>
  5. 5. Context-related Vulnerability <ul><li>Sample Problems (cont.) </li></ul><ul><ul><li>Statistical Regularities </li></ul></ul><ul><ul><ul><li>Such regularities may exist when each part of the ciphertext was generated from independent part of the plaintext. </li></ul></ul></ul><ul><ul><ul><li>Example: DES in ECB mode </li></ul></ul></ul><ul><ul><ul><li>Solution? </li></ul></ul></ul>
  6. 6. Stream vs Block Ciphers <ul><li>Block ciphers : Plaintexts are encoded into ciphertexts block-by-block . </li></ul><ul><ul><li>Each block is encrypted by the same key. </li></ul></ul><ul><ul><li>See definition 11-1. </li></ul></ul><ul><ul><li>Example: DES </li></ul></ul><ul><li>Stream ciphers : The plaintext characters are encoded by the sender unit-by-unit , usually with different key for each unit. </li></ul><ul><ul><li>Each letter may be encrypted by different key. (See definition 11-2) </li></ul></ul><ul><ul><ul><li>Example: one-time pad, where a random, infinitely long key is used. </li></ul></ul></ul><ul><ul><ul><li>If the key stream repeats itself  periodic cipher </li></ul></ul></ul><ul><li>Questions: Is Vigen è re cipher a block or stream cipher? How about RSA ? </li></ul>
  7. 7. Stream Ciphers <ul><li>Approaches in simulating a random, infinitely long key </li></ul><ul><ul><li>Synchronous Stream Ciphers </li></ul></ul><ul><ul><li>Generates bits (of the key) from a source other than the message itself. </li></ul></ul><ul><ul><li>See definition 11-3: LFSR ( n-stage linear feedback shift register ) </li></ul></ul><ul><ul><ul><li>Example on p.278 </li></ul></ul></ul><ul><ul><li>Definition 11-4: NLFSR ( n-stage nonlinear feedback shift register ) </li></ul></ul><ul><ul><ul><li>Example on p.279 </li></ul></ul></ul><ul><ul><ul><li>Purpose? To eliminate lineality </li></ul></ul></ul><ul><ul><li>c.f., LFSR vs NLFSR: How the new bit is inserted into the register r. </li></ul></ul>
  8. 8. Stream Ciphers <ul><li>Alternative approaches in eliminating linearity : </li></ul><ul><ul><li>Output Feedback Mode (OFM) </li></ul></ul><ul><ul><ul><li>The register, r, is never shifted. It is repeatedly enciphered. </li></ul></ul></ul><ul><ul><li>Counter Method: a variant of OFM </li></ul></ul>
  9. 9. Stream Ciphers <ul><ul><li>Self-Synchronous Stream Ciphers </li></ul></ul><ul><ul><li>The key is obtained from the message itself. </li></ul></ul><ul><ul><li>Example: autokey cipher (p.280) </li></ul></ul><ul><ul><ul><li>Problems? The selection of the key. </li></ul></ul></ul><ul><ul><ul><li>Statistical regularities in the plaintext show up in the key. </li></ul></ul></ul><ul><ul><li>An alternative: Use the ciphertext as the key stream </li></ul></ul><ul><ul><ul><li>Problems? Weak cipher, because plaintext can be deducted from the ciphertext </li></ul></ul></ul><ul><ul><li>Another alternative: CFM (cipher feedback mode) </li></ul></ul><ul><ul><ul><li>See Fig. 11-1, p.281 </li></ul></ul></ul>
  10. 10. Block Ciphers <ul><ul><li>A block of multiple bits are enciphered each time. </li></ul></ul><ul><ul><li>Faster than stream cipher (?). </li></ul></ul><ul><ul><li>Problem? Encipherment of the same plaintexts result in the same ciphertexts (because the same key is used for each block). </li></ul></ul><ul><ul><li>Solution: Cipher block chaining (CBC) </li></ul></ul><ul><ul><ul><li>IV is needed for the first block encipherment </li></ul></ul></ul>
  11. 11. Block Ciphers <ul><ul><li>Multiple Encryption </li></ul></ul><ul><ul><ul><li>e.g., c = E k’ (E k (m)) </li></ul></ul></ul><ul><ul><ul><li>Suppose the length of k and k’ are both n. </li></ul></ul></ul><ul><ul><ul><li>[Merkle/Hellman, 1981] The effective strenghth of the above encryption is 2 n+1 , not 2 2n . </li></ul></ul></ul><ul><ul><ul><li>EDE </li></ul></ul></ul><ul><ul><ul><li>Triple encryption mode </li></ul></ul></ul>
  12. 12. Next <ul><li>A Brief Overview of Network Security </li></ul><ul><li>Privacy-enhanced E-Mails (PEM) </li></ul>

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