trivial

题目

An unlocked terminal is displaying the following:

Encryption complete, ENC(???,T0pS3cre7key) = Bot kmws mikferuigmzf rmfrxrwqe abs perudsf! Nvm kda ut ab8bv_w4ue0_ab8v_DDU

You poke around and find this interesting file.

题目链接

http://www.shiyanbar.com/ctf/1980

题解

题目给了一个加密的python代码

import sys

alphaL = "abcdefghijklnmopqrstuvqxyz"
alphaU = "ABCDEFGHIJKLMNOPQRSTUVQXYZ"
num    = "0123456789"
keychars = num+alphaL+alphaU

if len(sys.argv) != 3:
  print "Usage: %s SECRET_KEY PLAINTEXT"%(sys.argv[0])
  sys.exit()

key = sys.argv[1]
if not key.isalnum():
  print "Your key is invalid, it may only be alphanumeric characters"
  sys.exit()

plaintext = sys.argv[2]

ciphertext = ""
for i in range(len(plaintext)):
  rotate_amount = keychars.index(key[i%len(key)])
  if plaintext[i] in alphaL:
    enc_char = ord('a') + (ord(plaintext[i])-ord('a')+rotate_amount)%26
  elif plaintext[i] in alphaU:
    enc_char = ord('A') + (ord(plaintext[i])-ord('A')+rotate_amount)%26
  elif plaintext[i] in num:
    enc_char = ord('0') + (ord(plaintext[i])-ord('0')+rotate_amount)%10
  else:
    enc_char = ord(plaintext[i])
  ciphertext = ciphertext + chr(enc_char)

print "Encryption complete, ENC(%s,%s) = %s"%(plaintext,key,ciphertext)

其实就是个移位加密，看懂了加密之后稍微修改一下就可以解密了：

import sys

alphaL = "abcdefghijklnmopqrstuvqxyz"
alphaU = "ABCDEFGHIJKLMNOPQRSTUVQXYZ"
num    = "0123456789"
keychars = num+alphaL+alphaU


key = "T0pS3cre7key"

plaintext = "Bot kmws mikferuigmzf rmfrxrwqe abs perudsf! Nvm kda ut ab8bv_w4ue0_ab8v_DDU"

ciphertext = ""
for i in range(len(plaintext)):
    rotate_amount = keychars.index(key[i%len(key)])
    if plaintext[i] in alphaL:
        enc_char = ord('a') + (ord(plaintext[i])-ord('a')-rotate_amount)%26
    elif plaintext[i] in alphaU:
        enc_char = ord('A') + (ord(plaintext[i])-ord('A')-rotate_amount)%26
    elif plaintext[i] in num:
        enc_char = ord('0') + (ord(plaintext[i])-ord('0')-rotate_amount)%10
    else:
        enc_char = ord(plaintext[i])
    ciphertext = ciphertext + chr(enc_char)

print("Encryption complete, ENC(%s,%s) = %s"%(plaintext,key,ciphertext))

得到：

Encryption complete, ENC(Bot kmws mikferuigmzf rmfrxrwqe abs perudsf! Nvm kda ut ab8bv_w4ue0_ab8v_DDU,T0pS3cre7key) = You hawe successfully decrypwed the message! The key is th4ts_w0rs3_th4n_DES