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# Obfuscated filters
## Resource location
Obfuscated standalone filters:
- Windows resource: RCDATA\16001\0
- MacOS resource: 'DATA' 16001
Normal standalone filters:
- Windows resource: PARM\16000\0
- MacOS resource: 'PARM' 16000
## Implementation
Defined in **ff.h**, implemented in **obfusc.c**:
// Implements Obfusc V5 on Windows and Obfusc V4 on MacOS.
// Returns a seed that needs to be stored in the executable code.
uint32_t obfusc(PARM_T* pparm);
// In V1+V2: Seed is hardcoded
// In V3: Seed is in PARM (field "unknown2")
// In V4+V5: Seed is in the program code and will me modified with a binary search+replace
void deobfusc(PARM_T* pparm);
### Obfuscation "Version 5"
Introduced in **Filter Foundry 1.7.0.8**
Obfuscation version 5 is the same as version 4, but there is a constraint
that the seed must be equal to the CRC32b checksum of the deobfuscated PARM.
This is done to check the integrity of the deobfuscation.
Also, the xor-shifting is intentionally incompatible with version 4
(to avoid downgrade-attacks) by XORing the initial seed with 0xFFFFFFFF.
The DWORD value `0x00000005` will be stored at field `unknown2`
(byte 0x30..0x33; the field is not used in the `PARM` resource).
While generating and applying the random data stream, the bytes
0x30..0x33 (the location where the version info is stored) are skipped,
like in version 3.
### Obfuscation "Version 4"
Introduced in **Filter Foundry 1.7.0.7**
It is not compiler-dependant, but different between every standalone filter.
Windows version:
The binary code of the 8BF file will be manipulated during building
to store the seed into the `deobfusc()` function.
The placeholder value is `OBFUSC_V4_DEFAULT_SEED 0x52830517`
This allows that 32-bit and 64-bit filters are "cross built".
(Theoretical) Macintosh version:
Obfuscation and deobfuscation has the seed `0x52830517`, since the
manipulation of the binary code is not implemented.
Algorithm: XOR-Shift like in version 2, but the seed is individual for
each individual built standalone filter.
The DWORD value `0x00000004` will be stored at field `unknown2`
(byte 0x30..0x33; the field is not used in the `PARM` resource).
While generating and applying the random data stream, the bytes
0x30..0x33 (the location where the version info is stored) are skipped,
like in version 3.
### Obfuscation "Version 3"
Introduced in **Filter Foundry 1.7.0.5**
A random seed is chosen and written to field `unknown2` (byte 0x30..0x33).
Then, the `PARM` resource will be obfuscated by applying an XOR operation to a random data stream:
unsigned char *p;
*p++ ^= (int)(rand() * 1.0 / (RAND_MAX + 1) * 256);
Bytes 0x30..0x33 (the location where the seed is stored) are skipped.
The `rand()` operation is compiler-dependant, and therefore the resource cannot be exchanged between plugins.
32 bit plugin is built with OpenWatcom (for Win95 compatibility) which has following formula:
int rand_openwatcom(unsigned int* seed) {
*seed = *seed * 1103515245L + 12345L;
return (*seed >> 16) & 0x7fff; /* Scale between 0 and RAND_MAX */
}
64 bit plugin is built with Visual C++ which has following formula:
int rand_msvcc(unsigned int* seed) {
*seed = *seed * 214013L + 2531011L;
return (*seed >> 16) & 0x7fff; /* Scale between 0 and RAND_MAX */
}
### Obfuscation "Version 2"
Introduced in **Filter Foundry 1.7b1**
It is compiler-independent!
Algorithm: [XOR-Shift](https://de.wikipedia.org/wiki/Xorshift "XOR-Shift") with hardcoded seed `0x95d4a68f`.
x32 = 0x95d4a68f;
for(i = size, p = pparm; i--;) {
x32 ^= x32 << 13;
x32 ^= x32 >> 17;
x32 ^= x32 << 5;
*p++ ^= x32;
}
### Obfuscation "Version 1"
Introduced in **Filter Foundry 1.4b8,9,10**
It is compiler-dependant, and therefore the resource cannot be exchanged between plugins!
Algorithm: XOR with `rand()`-stream with hardcoded seed `0xdc43df3c`.
srand(0xdc43df3c);
for(i = size, p = pparm; i--;) {
*p++ ^= rand();
}
The plugin is built with Visual C++ which has following formula:
int rand_msvcc(unsigned int* seed) {
*seed = *seed * 214013L + 2531011L;
return (*seed >> 16) & 0x7fff; /* Scale between 0 and RAND_MAX */
}