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Chapter 2

Basic Terminology

1. plaintext(明文) - the original message.
2. ciphertext(密文) - the coded message.
3. cipher(加密演算法)– cryptographic systems. algorithm for transforming
                                plaintext to ciphertext
4. key(金鑰)– secret info used in cipher known only to sender/receiver
5. encipher(加密) - converting plaintext to ciphertext
6. decipher(解密) – restoring plaintext from ciphertext
7. cryptography(密碼規則) - study of encryption principles/methods
8. cryptanalysis(密碼分析) - the study of principles/ methods of deciphering
                                        ciphertext without knowing key
9. cryptology(密碼學) - the field of both cryptography and cryptanalysis

Types of attacks on encrypted message

1. ciphertext only
        only know algorithm / ciphertext, statistical, can identify plaintext.
2. known plaintext
        know/suspect plaintext-ciphertext pairs to attack cipher.
3. chosen plaintext
        select plaintext and obtain ciphertext to attack cipher
4. chosen ciphertext
        select ciphertext and obtain plaintext to attack cipher
5. chosen text
        select either plaintext or ciphertext to en/decrypt to attack cipher

Cryptography systems

1. Type of encryption operations used
        (1). Substitution
                each element mapped into another element.
        (2). Transposition
                rearrange elements.

2. Number of keys used
        (1). Symmetric, single-key, secret-key, or conventional encryption
                sender and receiver share the same key.
        (2). Asymmetric, two-key, or public-key encryption
                sender and receiver use the different key.

3. The way in which plaintext is processed
        (1). Block cipher
                Input one block of element at a time.
        (2). Stream cipher
                Input elements continuous.

4. Unconditional secure
        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。

        scheme: one-time pad

5. Computational secure
        (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。

6. Substition Techniques
        where letters of plaintext are replaced by other letters or by numbers
        or symbols.
        or if plaintext is viewed as a sequence of bits, then substitution
        involves replacing plaintext bit patterns with ciphertext bit patterns
        用其他字母取代原先字母。

(1). Caser Cipher
        Replaces each letter by krd letter on.
        C = E(P) = (P+k) mod (26)
        P = D(C) = (C-k) mod (26)

        Problems:
        (1). Only 25 keys (easily attacked by brute-force search)
        (2). Language is known and easily recognizable.

(2). Monoalphabetic Ciphers
        Each plaintext letter maps to a different random ciphertext letter.

        Problem: relative frequency of letters is not change.
        字母的使用頻率並未改變。

(3). Playfair Cipher
        Based on the use of a 5X5 matrix of letters constructed using
        a keyword.
        建構一個5X5的矩陣，由左至右由上至下，先填入keywords(重複字母
        捨去)，再填入剩餘的字母。

        Encrypted rules:
                 一次編譯兩個字(pair)。
                 a). pair字為重複時，用任意字母取代其中之一。
                        ex: ballon→ ba lx lo on
                 b). 同一個row的pair，用pair字右邊的letter取代。
                        ex: row: MONAR，AR→RM。
                 c). 同一個column的pair，用pair字下方的letter取代。
                        ex: column: MCELU，MU→CM。
                 d). 一對pair用彼此的row與column交會處的letter取代。
                        ex: H------B, HS→BP.
                              |        |
                              |        |
                              P------S

(4). Polyalphabetic Ciphers and Vigenere Cipher
        p. 40, Table 2.3
        Key決定orw，plaintext決定column，兩者交會處為ciphertext。

(5). One-Time Pad(p. 43~44)
        Algorithm:
        Ci = Pi ⊕ Ki
        Pi = Ci ⊕ Ki

        Problem:
         a). problem of making large amount of random keys required
                資料size多大，key就需要多大。
         b). problem of key distribution and protection
                運送資料時，需運送同等大小的key，保護不易。

Transposition Techniques
        Hide the message by rearranging the letter order.
        更改字母的排列方式。

(1). Rail fence technique(p. 44)
        Write message letters out diagonally over a number of rows
        then read off cipher row by row
        依對角線寫下明文然後逐排閱讀。

(2). Row Transposition Ciphers(p. 45)
        Write letters of message out in rows over a specified number
        of columns then reorder the columns according to some key
        before reading off the rows
        將明文逐排寫下，接著亂數產生一組key，依照key的編號逐行組成密文。

3. Rotor Machines(p. 47)
        Figure 2.7
        machine內包含許多獨立的cylinders，每個cylinder都有26個inputs及26個outputs，
        而input跟output的編號為隨機排列。letter依照input的編號尋找對應的output，
        並成為下一個cylinder的input，反覆運作直到結束。

4. Steganography(資訊隱藏) (p. 47)
        Conceal the existence of the message.
        將訊息隱藏在圖畫、文字或物體之中。

        (1). using only a subset of letters/words in a longer message
               marked in some way.
               使用文字片段阻合成有意義的訊息。ex: 每句開頭第一個字的組合。
         (2). Character marking
                Selected letters of printed or typewritten text are overwritten in pencil
         (3). Invisible ink
                使用特殊墨水。
         (4). Pin punctures
                在字上刺穿以作辨別。
         (5). Typewritter correction ribbon

         Drawback:
         (1). High overhead to hide relatively few info bits.
         (2). Once the system is discovered, it becomes virtually worthless.