初识RC4加密逆向

笔记

RC4_decrypt.py

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import re
import base64
def rc4_decrypt(ciphertext,key):
    S=list(range(256))
    j=0
    res=[]
    #初始化S
    for i in range(256):
        j=(j+S[i]+key[i%len(key)])%256
        S[i],S[j]=S[j],S[i]
    #解密
    i=j=0
    for char in ciphertext:
        i=(i+1)%256
        j=(j+S[i])%256
        S[i],S[j]=S[j],S[i]
        res.append(char^S[(S[i]+S[j])%256])
    return bytes(res)
data='wr3ClVcSw7nCmMOcHcKgacOtMkvDjxZ6asKWw4nChMK8IsK7KMOOasOrdgbDlx3DqcKqwr0hw701Ly57w63CtcOl'
data=base64.b64decode(data.encode()).decode()
data_list=[ord(j) for j in data]
key='As_we_do_as_you_know'
key_list=[ord(c) for c in key]
flag=rc4_decrypt(data_list,key_list)
print(flag)

RC4_endecrypto.py

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# RC4加密和解密
def KSA(key):
    key_length = len(key)
    # 初始化S盒
    S = list(range(256))
    j = 0
    for i in range(256):
        j = (j + S[i] + key[i % key_length]) % 256
        # 交换S[i]和S[j]
        S[i], S[j] = S[j], S[i]
    return S
def PRGA(S):
    i = 0
    j = 0
    while True:
        i = (i + 1) % 256
        j = (j + S[i]) % 256
        # 交换S[i]和S[j]
        S[i], S[j] = S[j], S[i]
        K = S[(S[i] + S[j]) % 256]
        yield K
def RC4(key):
    # 加密函数，返回加密后的字节流
    def encrypt(data):
        data_length = len(data)
        keystream = PRGA(KSA(key))
        res = []
        for i in range(data_length):
            res.append(next(keystream) ^ data[i])
        return bytes(res)
    # 解密函数，返回解密后的字节流
    def decrypt(data):
        return encrypt(data)
    return encrypt, decrypt
# 示例：
key = b'secret_key'
encrypt_func, decrypt_func = RC4(key)
data_to_encrypt=b'Hello World'
encrypted_data=encrypt_func(data_to_encrypt)
print('Encrypted Data:', encrypted_data)
decrypted_data=decrypt_func(encrypted_data)
print('Decrypted Data:', decrypted_data)

RC4_cversion.cpp

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#include<iostream>
#include<cstring>
#include<string>
using namespace std;
unsigned char s[256];
void rc4_start( char key[],int n){
	int j=0;
	for(int i=0;i<256;i++){
		s[i]=-1-i;     //这里是改动的地方
	j=(j+s[i]+key[i%n])%256;
		swap(s[i],s[j]);
	}
	
}
unsigned char secr[256];
void rc4_enc( unsigned char pub[],int n){
	int i=0,j=0;
	for(int k=0;k<n;k++){
		i=(i+1)%256;
		j=(j+s[i])%256;
		swap(s[i],s[j]);
		unsigned char temp=s[(s[i]+s[j])%256]^pub[k];
		secr[k]=temp;
		
	}
}
int main(){
	 char key[]="THISISAFAKEFLAG";
	rc4_start(key,strlen(key));
	 unsigned char pub[] =
{
  0x44, 0x3F, 0x53, 0x2F, 0x73, 0x86, 0x3E, 0xAE, 0x55, 0xBE, 
  0x18, 0x5F, 0x74, 0x68, 0x33, 0x5F, 0xF2, 0x06, 0x6D, 0x62}; 
	rc4_enc(pub,20);
	for(int i=0;i<20;i++)cout<<secr[i];
	return 0;
}

做题

[SWPUCTF 2022 新生赛]pypy

rc4加密，原文随便塞个长的，拿生成的密钥流解密，exp就不放了…

rc4加密流程长这样：

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初始化S和T：
for i=0 to do
	S[i]=i;
	T[i]=K[i mod keylen];

初始排列S：
j=0;
for i=0 to 255 do
	j = (j+S[i]+T[i]mod256;
	swap(S[i],S[j];
	
生成密钥流，利用密钥流和明文进行加密：
i,j=0;
for r=0 to len do //r为明文长度，r字节
	i=(i+1)mod 256;
	j=(j+S[i])mod 256;
	swap(S[i],S[j]);
	t=(S[i]+S[j])mod 256;
	K[r]=S[t];
	data[r]^=K[r];

[长城杯 2021 政企组]魔鬼凯撒的RC4茶室

第一部分：自定义凯撒加密

第二部分：需要从给定的两个文件中通过凯撒和rc4恢复key，发现key找不到定义…

放弃rc4逆向，直接调试得key[0]地址esp+f4h，发现为0xde，exp：

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key=0xde
enc=[0x0d,0x0a,0xb2,0xbf,0xb9,0xa5,0xa6,0xef,0xbf,0xb1,0xb3,0xef,0xb0,0xb9,0x81,0xef,0xad,0x81,0xed,0xb1,0x81,0xbb,0xbf,0xad,0xa7,0x81]
for i in enc:
    print(chr((i^key)&0xff),end='')

[SWPUCTF 2021 新生赛]PYRE

RC4、MD5、Base64，exp：

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import re
import base64
def rc4_decrypt(ciphertext,key):
    S=list(range(256))
    j=0
    res=[]
    #初始化S
    for i in range(256):
        j=(j+S[i]+key[i%len(key)])%256
        S[i],S[j]=S[j],S[i]
    #解密
    i=j=0
    for char in ciphertext:
        i=(i+1)%256
        j=(j+S[i])%256
        S[i],S[j]=S[j],S[i]
        res.append(char^S[(S[i]+S[j])%256])
    return bytes(res)
s1="w4s1PUYsJ8OYwpRXVjvDkVPCgzIEJ27Dt2I="
s2=base64.b64decode(s1.encode()).decode()
data_list=[ord(i) for i in s2]
key='e3d68926fe0ec451105275d838847bd1'
key_list=[ord(c) for c in key]
flag=rc4_decrypt(data_list,key_list)
print(flag)

[HGAME 2023 week2]stream

rc4，没意思

[HGAME 2022 week1]flagchecker

Base64+RC4解密即可。

[FSCTF 2023]EZRC4

RC4

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From_Hex('Auto')
RC4({'option':'UTF8','string':'wanyuanshenwande'},'Latin1','Latin1')
Input:EB 0D 61 29 BF 9B 05 22 F3 32 28 97 E3 86 4D 2D 5A 2A A3 55 AA D5 B4 6C 8B 51 B1

[MoeCTF 2021]RedC4Bomb

一大堆各种花指令，还有些混淆。加密算法是个魔改的RC4：

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#include<iostream>
#include<cstring>
#include<string>
using namespace std;
unsigned char s[256];
void rc4_start( char key[],int n){
	int j=0;
	for(int i=0;i<256;i++){
		s[i]=-1-i;
		j=(j+s[i]+key[i%n])%256;
		swap(s[i],s[j]);
	}
}
unsigned char secr[256];
void rc4_enc( unsigned char pub[],int n){
	int i=0,j=0;
	for(int k=0;k<n;k++){
		i=(i+1)%256;
		j=(j+s[i])%256;
		swap(s[i],s[j]);
		unsigned char temp=s[(s[i]+s[j])%256]^pub[k];
		secr[k]=temp;
	}
}
int main(){
	char key[]="THISISAFAKEFLAG";
	rc4_start(key,strlen(key));
	unsigned char pub[] ={
        0x44, 0x3F, 0x53, 0x2F, 0x73, 0x86, 0x3E, 0xAE, 0x55, 0xBE, 
        0x18, 0x5F, 0x74, 0x68, 0x33, 0x5F, 0xF2, 0x06, 0x6D, 0x62
    }; 
	rc4_enc(pub,20);
	for(int i=0;i<20;i++)cout<<secr[i];
	return 0;
}