Tags: crypto
Rating: 4.6
# Volga CTF Quals 2017 PyCrypto
### Category: Crypto, 150 points
>This crypto algorithm uses a huge key and it's implementation is not so trivial to reverse engineer. Isn't it wonderful?
### Write-up
We take a peek in encrypt.py -> 160 bit key, 20 bytes.
A team mate noticed that when using a secret of multiple A's, we can see a repetition in the cipher text. Probably Xor Then.
```
$ echo 'flag="AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"' > secret.py
$ ./encrypt.py
$ cat flag.enc
G��%�a�����H�M�MG��%�a�����H�M�MG��%�a�����H�M�MG��%�a�����H�M�MG��%�a�����H�M
```
We toss the provided flag to https://wiremask.eu/tools/xor-cracker/
We know the key is 20 bytes and we get two suggested keys. The unencrypted text becomes the following with the top suggested key.
> key = d1 ff 63 f7 c8 75 d8 c4 1a 84 ca 24 5b 66 0c 1f c6 e2 cc ea
> ?ol3$CTF{?@m _is_Pad?Ma:<_Tim s_?@d_Mi$$?mek8
> Gil'er1 Vernamewa'ean A?&TeBell La's 1+gine r 2ho, in t91ci inv nt d an ad!it=3e po)ya)phabeti& ...
Not quite right. But we learn some key things.
The plaintext after the flag is about Gilbert Vernam, looking at his wikipedia page, we figure out po)ya)phabeti&
should be polyalphabetic.
We do some Xor math and calculate the key should be
> key = 94 ff 63 a3 8d 75 d8 c4 1a c1 ca 24 1e 66 0c 1f c6 e2 cc ea
We use this following code to decrypt the text
```
int main()
{
char key[20] = { 0x94, 0xff, 0x63, 0xa3, 0x8d, 0x75, 0xd8, 0xc4, 0x1a, 0xc1, 0xca, 0x24, 0x1e, 0x66, 0x0c, 0x1f, 0xc6, 0xe2, 0xcc, 0xea };
FILE *fileptr;
char *buffer;
long filelen;
fileptr = fopen("flag.enc", "rb");
fseek(fileptr, 0, SEEK_END);
filelen = ftell(fileptr);
rewind(fileptr);
buffer = (char *)malloc((filelen+1)*sizeof(char));
fread(buffer, filelen, 1, fileptr);
fclose(fileptr);
int i;
for(i = 0; i < filelen; i++) {
printf("%c", buffer[i] ^ key[i%20]);
}
return 0;
}
```
We now get this plaintext.
> VolgaCTF{N@me_is_Pad_Many_Times_P@d_Mi$$_me?}
> Gilbert Vernam was an AT&T Bell Labs engineer who, in 1917, invented an additive polyalphabetic ...
Bingo.
