通过编程实现替代密码算法和置换密码算法，加深对古典密码体制的了解

替代密码算法：

# 替代密码算法
def substitution_cipher(plaintext, key):
    # 构建密文字典
    ciphertext_dict = {}
    for i in range(len(key)):
        ciphertext_dict[chr(97+i)] = key[i]
    # 加密明文
    ciphertext = ''
    for c in plaintext:
        if c.isalpha():
            if c.isupper():
                ciphertext += ciphertext_dict[c.lower()].upper()
            else:
                ciphertext += ciphertext_dict[c]
        else:
            ciphertext += c
    return ciphertext

示例：

plaintext = 'hello world'
key = 'qwertyuiopasdfghjklzxcvbnm'
ciphertext = substitution_cipher(plaintext, key)
print(ciphertext)  # lazzq zphqw

置换密码算法：

# 置换密码算法
def transposition_cipher(plaintext, key):
    # 根据密钥计算列数
    col_num = len(key)
    # 根据明文长度计算行数
    row_num = (len(plaintext) + col_num - 1) // col_num
    # 在明文末尾填充空格
    plaintext += ' ' * (row_num * col_num - len(plaintext))
    # 构建密文矩阵
    matrix = []
    for i in range(row_num):
        row = [plaintext[i*col_num+j] for j in range(col_num)]
        matrix.append(row)
    # 按照密钥的字母顺序排序列
    sorted_index = sorted(range(col_num), key=lambda k: key[k])
    # 按照排序后的列顺序输出密文
    ciphertext = ''
    for j in sorted_index:
        for i in range(row_num):
            ciphertext += matrix[i][j]
    return ciphertext

示例：

plaintext = 'hello world'
key = 'helo wrd'
ciphertext = transposition_cipher(plaintext, key)
print(ciphertext)  # hleolwolrd