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2002    Fall Network Security Midterm Exam

1. [Public-Key Cryptosystem]
        Draw the public-key cryptosystem with secrecy and authentication. (20%)
                ans: Fgure 9.4 (p. 265)

2. [Trapdoor One-Way Function]
        Use function ｆk to describe what trapdoor one-way function is. (15%)
                Not in range.

3. [Public-Key Certificates]
        Draw the diagram for distributing public keys using public-key certificates
        distribution scenario. (15%)
                Not in range(p. 214)

4. [DES]
        (a) Explain what Avalanche Effect is. (5%)
                ans: A smallchange in either the plaintext or the key should produce
                       a significant change in the ciphertext.
                       key或plaintext的細微改變會使ciphertext明顯變化。(p. 80)
        (b) Draw the encryption and decryption diagram of DES Cipher Block Chaining
              (CBC) Mode. (20%)
                ans: Figure 3.12(p. 93)
        (c) What is the advantage of CBC mode over ECB mode? (5%)
                ans: CBC mode中每一個block的ciphertext都會彼此影響，避免了
                       ECB mode中相同message產生相同ciphertext的可能性，
                       oppornent便無法從中推測規則。

5. [Cryptanalysis]
        (a) Explain what unconditionally secure is. (5%)
                ans: the ciphertext provides insufficient information to uniquely determine
                         the corresponding plaintext.
                        ciphertext的資訊不足，無法決定相對唯一的一組plaintext。
        (b) What is the only unconditionally secure scheme? (No need to explain)(5%)
                ans: one-time pad.
        (c) Explain what computationally secure is. (10%)
                ans:(1). The cost of breaking the cipher exceeds the value of the
                            encrypted information.
                            破解密文的成本大於資訊的價值。
                       (2). The time required to break the cipher exceeds the useful
                             lifetime of the information
                             破解密文的時間超過明文的資訊有效期。