python : caesar cipher

序

对python不熟，找到一本用python实现古典密码学算法的书.
对密码算法的应用很感兴趣. 将密码算法实验做完，python也该熟悉了:)
边用边学，学习效率是最高的。有些细节问题，只有在用的时候才会发现。

实验

@echo offrem @file run.batpython main.pypause

@echo offrem @file debug.batpython -m pdb main.pypause

# @file main.py# @brief caesar cipher## for debug : python -m pdb main.py# debug same to gdb, ref : https://docs.python.org/2/library/pdb.html#   n is step over#   s is step in#   view val is type the val name and ENTER#   ENTER is execute last debug command#   q is quit python debug"""run resultPython 3.6.2cipher text = guvF vF CynvA GrKG:)plain text = This is plain text:)END"""from subprocess import call# @fn caesar_cipher# @brief caesar cipher encrypt or decrypt# @param b_encryp, True = encrypt; False = decrypt# @param msg,#   when (b_encryp == True), msg is plain text#   when (b_encryp == False), msg is cipher text# @param cipher_key, the cipher key by caesar cipher# @return string, the result string#   when (b_encryp == True), return cipher text#   when (b_encryp == False), return plain textdef caesar_cipher(b_encryp, msg, cipher_key):    sz_cipher_charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"    i_len_cipher_charset = len(sz_cipher_charset)    sz_cipher_msg = ""    c_letter = ''    i_pos = 0    i_num = 0    for c_letter in msg:        if c_letter in sz_cipher_charset:            # 求字符在字符集数组中的索引            i_pos = sz_cipher_charset.find(c_letter)            i_num = i_pos            # print("i_pos = ", i_pos, "c_letter = ", c_letter)            if b_encryp == True:                i_num += cipher_key            else:                i_num -= cipher_key            if i_num >= i_len_cipher_charset:                i_num -= i_len_cipher_charset            elif i_num < 0:                i_num += i_len_cipher_charset            sz_cipher_msg += sz_cipher_charset[i_num]        else:            # if letter not on charset, don't cipher            sz_cipher_msg += c_letter    return sz_cipher_msgif __name__ == "__main__":    cipher_key = 13    sz_cipher_msg = ""    # print python version    call(["python", "-V"])    # caesar encrypt    sz_cipher_msg = caesar_cipher(True, "This is plain text:)", cipher_key)    print("cipher text =", sz_cipher_msg)    # caesar decrypt    sz_cipher_msg = caesar_cipher(False, "guvF vF CynvA GrKG:)", cipher_key)    print("plain text =", sz_cipher_msg)    print("END")