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 off
rem @file run.bat

python main.py
pause

@echo off
rem @file debug.bat

python -m pdb main.py
pause

# @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 result

Python 3.6.2
cipher 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 text
def 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_msg

if __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")