Introduction to conversion between python bases

This article mainly summarizes the relevant information for python to implement various base conversions, including string and hexadecimal conversion, built-in function hex() and base conversion, etc. Friends who need it can For reference, let’s take a look below.

Preface

We often encounter the problem of base conversion when playing CTF, so I just made a summary of base conversion and shared it for your reference. To learn, let’s take a look at the detailed introduction:

String and hexadecimal conversion

For example, Baidu ctf 12 The first misc of the second game of the month

666C61677B65633862326565302D336165392D346332312D613031322D3038616135666137626536377D

A simpler way is to directly call the string's .decode('hex') Just decrypt it, but how would you decrypt it if you didn't use this function?

One idea is to group 2 first, solve the ascii value of each group, and merge the strings to get it. The specific code is as follows

import re
s='666C61677B65633862326565302D336165392D346332312D613031322D3038616135666137626536377D'
s = re.findall(r'.{2}',s)
s = map(lambda x:chr(int(x,16)),s)
print ''.join(s)
>>>
flag{ec8b2ee0-3ae9-4c21-a012-08aa5fa7be67}

I mentioned the decode('hex') function of strings before, and there are also two functions for converting to hexadecimal. Here is a summary of the

built-in functions hex()

can only convert decimal integers to hexadecimal, not strings

hexlify() of binascii library and b2a_hex()

The function of these two functions is to convert the string into hexadecimal. The corresponding decryption functions are unhexlify() and# respectively. ##a2b_hex()

Conversion between decimals

Convert binary, octal, hexadecimal to decimal The system is relatively simple, just call the

int function

int(str,base) //返回十进制整数,但注意此时第一个参数为字符串

and the corresponding decryption functions are

bin() //10进制转二进制

oct() //十进制转八进制

hex() //十进制转十六进制

But to directly convert binary to hexadecimal, you need to take one more step. First use int to convert to decimal, and then use the

hex() function mentioned above to convert decimal to hexadecimal. The simpler way to write it is

map(lambda x:hex(int(x,2)),['0011']) //lambda表达式

or

##

[hex(int(x,2)) for x in ['0011']] //列表解析

. The corresponding decryption function is

map(lambda x:bin(int(x,16)),['ef'])

Finally, I attach a small hex conversion tool I wrote in python. Its main function is to convert a set of binary, or ascii, or hexadecimal into strings. It must be on ctf We often encounter this type of question

# make by 江sir
#coding:utf-8
import re
import argparse
 
def bintostr(text):
 text = text.replace(' ','')
 text = re.findall(r'.{8}',text)
 s = map(lambda x:chr(int(x,2)),text) #批量二进制转十进制
 flag = ''.join(s)
 return flag
 
def asciitostr(text):
 if ' ' in text:
 text = text.split(' ')
 elif ',' in text:
 text = text.split(',')
 s = map(lambda x:chr(int(x)),text)
 flag = ''.join(s)
 return flag
 
def hextostr(text):
 text = re.findall(r'.{2}',text)
 #print text
 s = map(lambda x:chr(int(x,16)),text)
 #print s
 flag = ''.join(s)
 return flag
 
 
if __name__ == '__main__':
 parser = argparse.ArgumentParser()
 parser.add_argument("-b")
 parser.add_argument("-a")
 parser.add_argument("-x")
 argv = parser.parse_args()
 #print argv
 if argv.b:
 res = bintostr(argv.b)
 elif argv.a:
 res = asciitostr(argv.a)
 elif argv.x:
 res = hextostr(argv.x)
 print res

Usage:

Hexadecimal to string ：

##666C61677B65633862326565302D336165392D346332312D613031322D3038616135666137626536377D

##

bintostr.py -x "666C61677B65633862326565302D336165392D346332312D613031322D3038616135666137626536377D"
flag{ec8b2ee0-3ae9-4c21-a012-08aa5fa7be67}

Binary conversion String:

Can have spaces or no spaces
00101111 01100110 00110110 00110111 00110011 00110010 00110100 00110001 00110000 01100001 01100001 0110010 0 01100011 00110000 00110011 00110111 01100110 01100010 00110000 01100011 01100010 01100001 01100001 00110000 00110000 01100 011 00110111 00110101 00110011 00110001 00110011 00110111 00110011 00101110 01110100 01111000 01110100

bintostr.py -b "00101111 01100110 00110110 00110111 00110011 00110010 00110100 00110001 00110000 01100001 01100001 01100100 01100011 00110000 00110011 00110111 01100110 01100010 00110000 01100011 01100010 01100001 01100001 00110000 00110000 01100011 00110111 00110101 00110011 00110001 00110011 00110111 00110011 00101110 01110100 01111000 01110100"
/f6732410aadc037fb0cbaa00c7531373.txt

ascii to string

can be separated by spaces or separated by
s='45 46 45 46 32 45 32 46 46 45 46 32 46 45 46 46 32 46 46 46 32 45 46 46 46 32 46 46 45 45 46 45 32 45 46 46 46 32 46 4 6 46 32 46 45 46 46 32'

bintostr.py -a "45 46 45 46 32 45 32 46 46 45 46 32 46 45 46 46 32 46 46 46 32 45 46 46 46 32 46 46 45 45 46 45 32 45 46 46 46 32 46 46 46 32 46 45 46 46 32"
-.-. - ..-. .-.. ... -... ..--.- -... ... .-..

The above examples are all from some ctf competition questions

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