fr = open('letter.txt',mode='r',encoding='utf-8')
plaincode = fr.read()
print('明文:' + plaincode)
 
 
print('密文:',end='')
for c in plaincode:
    print(chr(ord(c)+3),end='')

fr = open('letter.txt',mode='r',encoding='utf-8')
plaincode = fr.read()
fr.close


fw = open('c_letter.txt',mode='a',encoding='utf-8')
for c in plaincode:
    fw.write(chr(ord(c)+3))
fw.close