readme.md

title	author	date
MD5 Collision Attack Lab	Xinyi Li	\today{}

Instruction: https://seedsecuritylabs.org/Labs_16.04/PDF/Crypto_MD5_Collision.pdf

Task 1

Generate a prefix.txt containing 64 '9':

python3 -c "print('9'*64,end='')" > prefix.txt
ls -ld prefix.txt

Use it to create 2 files with the same MD5 value:

md5collgen -p prefix.txt -o out1.bin out2.bin

Wait for seconds, check if the 2 files are different:

$ diff out1.bin out2.bin
Binary files out1.bin and out2.bin differ

Check if their MD5 values are the same:

$ md5sum out1.bin
b3d3a557001d81af72250f58b1cea4aa  out1.bin
$ md5sum out2.bin
b3d3a557001d81af72250f58b1cea4aa  out2.bin

Compare their differences:

$ hexdump out1.bin > hex1
$ hexdump out2.bin > hex2
$ diff hex1 hex2
4,6c4,6
< 0000050 3d59 83f8 861b 0b01 523e 813f a5f3 9c8c
< 0000060 0412 b9f9 0e70 0fbd 508b 4256 a7ee 3f38
< 0000070 d438 668f 4abb ed3e 9003 ab89 a859 58bd
---
> 0000050 3d59 03f8 861b 0b01 523e 813f a5f3 9c8c
> 0000060 0412 b9f9 0e70 0fbd 508b 4256 27ee 3f39
> 0000070 d438 668f 4abb ed3e 9003 2b89 a859 58bd
8,10c8,10
< 0000090 7dac 713e e61b 6a1e 1acb 3d6e 86ec e602
< 00000a0 7c98 5b26 ca06 3fde 4c8a 973c 2450 28ea
< 00000b0 f128 b84a 6c42 0ac2 db2d 0427 baa5 93b1
---
> 0000090 7dac f13e e61b 6a1e 1acb 3d6e 86ec e602
> 00000a0 7c98 5b26 ca06 3fde 4c8a 973c a450 28e9
> 00000b0 f128 b84a 6c42 0ac2 db2d 8427 baa5 93b1

Use the program diff_bytes.py to find out where they differs from each other in the last 128 bytes:

head -c 128 out1.bin > P
head -c 128 out2.bin > Q
diff_bytes.py P Q

The result:

different bytes in 0x53 :  0x2c vs 0xac
different bytes in 0x6d :  0xb4 vs 0x34
different bytes in 0x6e :  0x96 vs 0x97
different bytes in 0x7b :  0x77 vs 0xf7

We make the content in prefix.txt as '9'*27. When using:

md5collgen -p prefix.txt -o out1.bin out2.bin

It also produces 2 output files with 192 bytes, which means that the prefix is padded into a multiple of 64. (it is 64 itself here.) check it by

head -c 64 out1.bin | hexdump

We can see the end is padded as 0, which shows as an empty character if printed out.

Task 2

Use the 2 files generated in the previous task:

tail -c 128 out1.bin > P
tail -c 128 out2.bin > Q

Check if P and Q have the same MD5 hash values:

$ md5sum P
7553683527861d35d134ffde7863ec62  P
$ md5sum Q
9eff026b6557922e812d4d31240b236d  Q

Create a new suffix:

python3 -c "print('114514'*10,end='')" > suffix.txt

Concatenate 2 files that have the same MD5 with suffixes to generate 2 new files:

cat out1.bin suffix > s1.out
cat out2.bin suffix > s2.out

Their MD5 values are exactly the same:

$ md5sum s1.out
c4b50342bd2b771bfa696549d3dd0198  s1.out
$ md5sum s2.out
c4b50342bd2b771bfa696549d3dd0198  s2.out

Task 3

Initialize the xyz array as 'A'*200:

#include <stdio.h>
unsigned char xyz[200] = {
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"};
int main()
{
    int i;
    for (i = 0; i < 200; i++)
    {
        printf("%x", xyz[i]);
    }
    printf("\n");
}

Compile print_array.c and find the location of continous 0x41 in its executable output:

gcc print_array.c -o print_array
bless print_array

It starts from the offset: 0x1040. Since 0x1040 (4160 in decimal) can be divided by 64, we can use the first 0x1040 bytes as prefix and generate 128 bytes to fill the contents in xyz.

First, we truncate the prefix and the suffix (start from the offset:4160+128) from the file:

head -c 4160 print_array > prefix
tail -c +4288  print_array > suffix

Generate 2 files with prefix:

md5collgen -p prefix -o P Q

Append suffix to make them like normal programs:

cat P suffix > arr1.out
cat Q suffix > arr2.out

Make them executable:

chmod u+x arr1.out arr2.out

Verify:

Task 4

#include <stdio.h>
#define LEN 300

unsigned char X[LEN] = {
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"};

unsigned char Y[LEN] = {
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
    "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"};

int main()
{
    for (int i = 0; i < LEN; i++)
    {
        if (X[i] != Y[i])
        {
            printf("i = %d, X[i] = %.2x, Y[i] = %.2x\n", i, X[i], Y[i]);
            printf("Malicious\n");
            return 0;
        }
    }
    printf("Benign\n");
    return 0;
}

Compile benign_evil.c

gcc benign_evil.c -o benign_evil

Find where array X and Y start (continuous 0x41s occur):

bless benign_evil

X starts from offset 0x1040 and Y starts from offset 0x1180.

Obtain the prefix and suffix from benign_evil:

head -c 4160 benign_evil > prefix
tail -c +4288 benign_evil > suffix

md5collgen -p prefix -o s1 s2

tail -c 128 s1 > P
tail -c 128 s2 > Q

Y starts from 0xC1(193) in suffix, So if we want to make X=Y, we should replace offset [193,321) with the same P or Q generated above:

head -c 192 suffix > suffix_pre
tail -c +320 suffix > suffix_post

Construct the final executable programs and make them executable:

cat s1 suffix_pre P suffix_post > benign
cat s2 suffix_pre P suffix_post > evil
chmod u+x benign evil

They do have the same MD5 but perform very different behaviors: