Information Security CS 526

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Information Security CS 526

• HW2

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HW2 Review

• See the partial solution emailed out for more
  answers.

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HW2 Problem 6. Exercise 3.4

• Problem 6. Exercise 3.4. Given an encryption
  scheme ? defined for messages of length at most
  l(n) such that ? satisfies Definition 3.8.
  Construct an encryption scheme such that it
  satisfies Definition 3.8 even when the adversary
  is not restricted to output equal-length
  messages.
• Define a new encryption for messages of length at
  most l(n)- ?log l(n)?. For each message m,
  insert in the beginning ?log l(n)? bits to encode
  the length of m, then pad the message up to l(n).
  Then use ? to encrypt.

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HW2. Problem 9. Exercise 3.15

• Given a PRF F, and a PRG G with expansion factor
  n1. Which is CPA-secure.
• mm1,m2. G(k)?m1,G(k1) ?m2
• Insecure. Why?
• Use ltr,G(r) ?mgt
• Secure.
• Use m ? Fk(0)
• Insecure
• mm1,m2. Use (r,m1 ? Fk(r), m2 ? Fk(r1))
• Secure. Counter mode.