Some time ago, we rechecked some Stuxnet code. Guess what have we learned: Kasperksy already published Flame-Stuxnet relationship, but on the encryption level, there is another similarity. In fact, this was found by Norman back in June , but they compared with soapr32’s encryption which is slightly more different than 4069.dll’s encryption E2.
Stuxnet PLC dll encryption code:
unsigned int __cdecl encryption_routine_sub_10010B26(int a1)
{
int v1; // eax@1
v1 = (a1 + 11) * (a1 + 17);
return (a1 + 11) * (a1 + 17) ^ (((a1 + 11) * (a1 + 17) & 0xFFFFFF00 ^
((((unsigned int)((a1 + 11) * (a1 + 17)) >> 8) ^ v1
& 0xFF0000) >> 8)) >> 8);
}
Flame 4069.dll:
unsigned int __cdecl encryptor_sub_4025C0(int a1)
{
return (a1 + 11) * (a1 + 17) ^ (((unsigned __int16)((a1 + 11) * (a1 + 17) & 0xFF00)
^ ((((unsigned int)((a1 + 11) * (a1 + 17)) >> 8) ^ (a1 + 11) * (a1 + 17) &
0xFF0000) >> 8)) >> 8);
But for what reason was this encryption (obfuscation) used in 4069?
Flame 4069 contains some strings like this:
5F 5F 73 73 5F 73 5F 5F 00 31 32 25 77 69 6E 64 ss_s.12%wind
69 72 25 5C 73 79 73 74 65 6D 33 32 5C 72 64 63 ir%\system32\rdc
76 6C 74 33 32 2E 65 78 65 00 5F 5F 73 73 5F 65 vlt32.exe.__ss_e
Basically ss_s is some kind of magic string where “ss” stands for string. then “00” is a placeholder for a length variable,
“12” is a magic string, and finally the encrypted string is put in the file. Oh, no. Wait a minute. This seems to be human readable?! Yes, basically 4069 is prepared to accept encrypted strings “if needed”, but the marker for doing that is the length field. If it is 00 (as above), then the string is unencrypted, and can be direcrtly read, otherwise it uses E2 to decrypt the string. Magic “12” is not part of the “real” string, this info is a fix for our Flame/Skywiper tech report.
All-in-all, the connection is not jut strange, but in 4069 this encryption routine is not even really used. Most likely, authors have made a postprocessor for the binary finding s_s strings, 12 magics and then encrypting strings and writing back length field into the file, but for some reason in the samples we saw, they did not use the post-processing tool.
.text:004025F1 mov ebx, [esp+4+length_arg_4]
.text:004025F5 push esi
.text:004025F6 xor esi, esi
.text:004025F8 test ebx, ebx
.text:004025FA jbe short loc_402618
If length is not set, then jump out of the decryption loop.