网络安全 L3 Symmetric cryptography 对称加密

nformation is stored information is stored digitally - just 0s and 1s, so how is text represented?
        1. ASCII codes.

                1. 0 to 255 decimal or 0 to FF hexadecimal.
                2. 8 bits per ASCII character.
                3. ASCII table readily available on internet.
        2. 2 Hex characters used to represent each of 256 (FF).
        3. For example:
                1. lowercase ’a’ is 6116 or 011000012 in binary.
                2. uppercase ’A’ is 4116 or 010000012 in binary.

Block ciphers
1. A block cipher is a symmetric-key cipher that breaks up the plaintext message
into blocks of a fixed length and encrypts one block at a time.
2. Break the plaintext P into successive blocks:
        1. P1, P2, . . . of size n bits each
3. Encrypt each Pi with the same key K of size k bits:
        1. EK(P) = EK(P1)EK(P2) . . .

Main types of block ciphers
1. substitution cipher: replace symbols (or groups of symbols) by other symbols
(or groups of symbols).

• 示例：凯撒密码是一种简单的替换密码，其中每个字母在字母表中向前或向后移动固定数量的位置。

2. transposition cipher: permutes the symbols within the block.

• 示例：列转置密码是一种转置密码，其中消息的字母被写入一个矩阵