Cracking Linksys “Encryption”

Perusing the release notes for the latest Linksys WRT120N firmware, one of the more interesting comments reads:

Firmware 1.0.07 (Build 01)

- Encrypts the configuration file.

Having previously reversed their firmware obfuscation and patched their code to re-enable JTAG debugging, I thought that surely I would be able to use this access to reverse the new encryption algorithm used to secure their backup configuration files.

Boy was I giving them way too much credit.

Here’s a diff of two backup configuration files from the WRT120N. The only change made between backups was that the administrator password was changed from “admin” in backup_config_1.bin to “aa” in backup_config_2.bin:

OFFSET        backup_config_1.bin              backup_config_2.bin

----------------------------------------------------------------------------------------

0x00001468    9E 9B 92 96 91 FF FF FF |........| / 9E 9E FF FF FF FF FF FF |........|

Two things to note here:

• The first letter of each password (“a”) is encrypted to the same value (0x9E)
• The same letter (“a”) is encrypted to the same value (0x9E), regardless of its position in the password

I immediately suspected some sort of simple single-byte XOR encryption. If true, then XORing the known plain text (“a”, aka, 0×61) with the known cipher text (0x9E) should produce the XOR key:

0x61 ^ 0x9E = 0xFF

Applying the XOR key of 0xFF to the other characters in the password gives us:

0x9E ^ 0xFF = a
0x9B ^ 0xFF = d

0x96 ^ 0xFF = i
0x92 ^ 0xFF = m


0x91 ^ 0xFF = n

And XORing every byte in the config file with 0xFF gives us a decrypted config file:

00000000  33 34 35 36 00 01 df 60  00 00 46 ec 76 31 2e 30  |3456...`..F.v1.0|
00000010  2e 30 37 00 00 00 00 00  00 00 00 00 00 00 00 00  |.07.............|

00000030  32 30 4e 00 00 00 00 00  00 00 00 00 00 00 00 00  |20N.............|
00000020  00 00 00 00 00 00 00 00  00 00 00 00 57 52 54 31  |............WRT1|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

|
*
00000080  61 64 6d 69 6e 00 00 00  00 00 00 00 00 00 00 00  |admin...........|
00000090  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |...............
.
000000a0  00 00 00 00 00 00 00 00  61 64 6d 69 6e 00 00 00  |........admin...|
000000b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

0
*
00000100  00 00 00 00 00 00 00 00  30 2e 30 2e 30 2e 30 00  |........0.0.0.0.|
00000110  00 00 00 00 00 00 00 00  01 01 01 00 00 00 00 01  |................|

00140  2e 31 2e 31 00 00 00 00  00 32 35 35 2e 32 35 35  |.1.1.....255.255|
0
0000120  00 00 00 01 00 00 00 00  00 00 00 08 32 39 34 38  |............2948|
00000130  33 31 30 35 00 01 00 00  00 31 39 32 2e 31 36 38  |3105.....192.168|
00
00000150  2e 32 35 35 2e 30 00 00  00 00 00 00 04 00 02 00  |.255.0..........|
00000160  01 00 00 00 00 00 00 00  00 00 00 00 00 00 4c 4f  |..............LO|
00000170  4f 50 42 41 43 4b 00 00  00 00 31 32 37 2e 30 2e  |OPBACK....127.0.|

000001b0  00 00 00 00 49 52 51 3d  30 20 50 4f 52 54 3d 30  |....IRQ=0 PORT=0|
00000180  30 2e 31 00 00 00 00 00  00 00 32 35 35 2e 32 35  |0.1.......255.25|
00000190  35 2e 32 35 35 2e 32 35  35 00 00 00 00 00 00 00  |5.255.255.......|
000001a0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|


...

This is truly atrocious. Given that “encrypting” the backup configuration files is done presumably to protect end users, expecting this to thwart any attacker and touting it as a product feature is unforgivable.

OK, I don’t really care that much. I’m just disappointed that it took longer to write this blog post than it did to break their “crypto”.