Subversion Repositories filter_foundry

# 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.

### 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 position 0x30 (this field is not used in the `PARM` resource).

### Obfuscation "Version 3"

Introduced in **Filter Foundry 1.7.0.5**

It is compiler-dependant, therefore the resource cannot be exchanged between plugins!

Algorithm: XOR with a modified `rand()`-stream with seed that is stored at position 0x30
(this field is not used in the `PARM` resource).

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-independant!

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, 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 */
    }