crypto加密类型简介

ASCII

ASCII is a 7-bit encoding standard which allows the representation of text using the integers 0-127.

我们可以通过python的chr和ord对其在ASCII和密钥之间转化

Hex

对于一串16进制编码，it is something more user-friendly and portable across different systems.

我们可以通过bytes.fromhex()和hex()对其在HEX和密钥之间转化

Base64

One character of a Base64 string encodes 6 bits, and so 4 characters of Base64 encode three 8-bit bytes.

python输入指令前需要import base64

我们可以通过base64.b64encode()和base64.b64decode()之间转化

例：

给出一串 72bca9b68fc16ac7beeb8f849dca1d8a783e8acf9679bf9269f7bf

先对其进行bytes.fromhex()转化，得到

b’r\xbc\xa9\xb6\x8f\xc1j\xc7\xbe\xeb\x8f\x84\x9d\xca\x1d\x8ax>\x8a\xcf\x96y\xbf\x92i\xf7\xbf’

再输入base64.b64encode()

得到最终的flag为 b’crypto/Base+64+Encoding+is+Web+Safe/‘

Bytes and Big Integers

image-20211003202156335

对于RSA加密，习惯上先对一段密文进行bytes_to_long()加密，得到一串数字

python使用指令前需要from Crypto.Util.number import *

我们通过long_to_bytes()和bytes_to_long()在数字和密钥之间转化

XOR Starter

For longer binary numbers we XOR bit by bit

我们可以通过首先将每个字符转换为代表 Unicode 字符的整数来 XOR 字符串

XOR在python中表示为 ^

下图为XOR的几条运算律

image-20211003225400497

Quadratic Residues

We say that an integer x is a Quadratic Residue if there exists an a such that a2 = x mod p. If there is no such solution, then the integer is a Quadratic Non-Residue.

If a2 = x then (-a)2 = x. So if x is a quadratic residue in some finite field, then there are always two solutions for a.