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Reverse simple decryption.
https://zenhax.com/viewtopic.php?f=4&t=15202
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Author:  chrrox [ Wed Apr 28, 2021 5:51 pm ]
Post subject:  Reverse simple decryption.

We will look at the game Valkyrie Crusade.
ID com.nubee.valkyriecrusade
we look at the games files and they are engrypted.
Image

So 1st step is load it up in IDA pro or ghidra.
This is a custom game engine so a good place to start is with a file open instruction
if we search for that we find nb::File::openRead.
Code:

/* nb::File::openRead(char const*, unsigned int*, nb::Drive::Base, nb::File::Filter) */

void * nb::File::openRead(char *param_1,uint *param_2,Base param_3,Filter param_4)

{
  void *pvVar1;
  int iVar2;
  void *__ptr;
  void *local_50;
  Info aIStack76 [8];
  void *local_44;
  File aFStack64 [8];
  int local_38;
  void *local_34;
 
  local_50 = (void *)0x0;
  File(aFStack64);
  open(aFStack64,param_1,1,param_3);
  if (local_38 == 0) {
    __ptr = (void *)0x0;
  }
  else {
    local_50 = local_34;
    pvVar1 = malloc((size_t)local_34);
    if (pvVar1 == (void *)0x0) {
      iVar2 = Compress::getInfo((void *)0x0,(uint)local_34,aIStack76);
    }
    else {
      read(aFStack64,pvVar1,(uint)local_34);
      iVar2 = Compress::getInfo(pvVar1,(uint)local_50,aIStack76);
    }
    __ptr = pvVar1;
    if (iVar2 != 0) {
      __ptr = (void *)Compress::uncompress(aIStack76);
      local_50 = local_44;
      free(pvVar1);
    }
    if ((param_4 == 1) || ((param_4 == 2 && (iVar2 = Coder::isCode(__ptr), iVar2 != 0)))) {
      pvVar1 = (void *)Coder::decode(__ptr,(uint)local_50,(uint *)&local_50);
      free(__ptr);
      __ptr = pvVar1;
      if (pvVar1 == (void *)0x0) {
        local_50 = pvVar1;
      }
    }
  }
  if (param_2 != (uint *)0x0) {
    *param_2 = (uint)local_50;
  }
  ~File(aFStack64);
  return __ptr;
}


So if we look at this we notice something interesting.
Code:
pvVar1 = (void *)Coder::decode(__ptr,(uint)local_50,(uint *)&local_50);


So in our decompiler lets look at Coder::decode function.
Code:

/* nb::Coder::decode(void const*, unsigned int, unsigned int*) */

int * nb::Coder::decode(void *param_1,uint param_2,uint *param_3)

{
  uint *puVar1;
  int iVar2;
  uint uVar3;
  uint __size;
  uint *__src;
  int *piVar4;
  uint uVar5;
  int *local_2c;
 
  iVar2 = isCode(param_1);
  if (iVar2 == 0) {
    return (int *)0x0;
  }
  __size = param_2 - 0x10;
  local_2c = (int *)malloc(__size);
  __src = (uint *)((int)param_1 + 0x10);
  memcpy(local_2c,__src,__size);
  iVar2 = *(int *)((int)param_1 + 0xc);
  uVar3 = 4;
  if (3 < __size) {
    uVar5 = (param_2 - 0x11) * 0x8000000 >> 0x1d;
    piVar4 = local_2c;
    if (uVar5 != 0) {
      *local_2c = (*__src ^ 0x45af6e5d) - iVar2;
      piVar4 = local_2c + 1;
      __src = (uint *)((int)param_1 + 0x14);
      if ((__size < 5) || (uVar3 = 8, __size < 8)) goto LAB_005fe4fe;
      if (uVar5 != 1) {
        if (uVar5 != 2) {
          if (uVar5 != 3) {
            if (uVar5 != 4) {
              if (uVar5 != 5) {
                if (uVar5 != 6) {
                  uVar5 = *__src;
                  __src = (uint *)((int)param_1 + 0x18);
                  uVar3 = 0xc;
                  *piVar4 = (uVar5 ^ 0x45af6e5d) - iVar2;
                  piVar4 = local_2c + 2;
                  if (__size < 0xc) goto LAB_005fe4fe;
                }
                uVar5 = *__src;
                __src = __src + 1;
                uVar3 = uVar3 + 4;
                *piVar4 = (uVar5 ^ 0x45af6e5d) - iVar2;
                piVar4 = piVar4 + 1;
                if (__size < uVar3) goto LAB_005fe4fe;
              }
              uVar5 = *__src;
              __src = __src + 1;
              uVar3 = uVar3 + 4;
              *piVar4 = (uVar5 ^ 0x45af6e5d) - iVar2;
              piVar4 = piVar4 + 1;
              if (__size < uVar3) goto LAB_005fe4fe;
            }
            uVar5 = *__src;
            __src = __src + 1;
            uVar3 = uVar3 + 4;
            *piVar4 = (uVar5 ^ 0x45af6e5d) - iVar2;
            piVar4 = piVar4 + 1;
            if (__size < uVar3) goto LAB_005fe4fe;
          }
          uVar5 = *__src;
          __src = __src + 1;
          uVar3 = uVar3 + 4;
          *piVar4 = (uVar5 ^ 0x45af6e5d) - iVar2;
          piVar4 = piVar4 + 1;
          if (__size < uVar3) goto LAB_005fe4fe;
        }
        uVar5 = *__src;
        __src = __src + 1;
        uVar3 = uVar3 + 4;
        *piVar4 = (uVar5 ^ 0x45af6e5d) - iVar2;
        piVar4 = piVar4 + 1;
        if (__size < uVar3) goto LAB_005fe4fe;
      }
    }
    do {
      *piVar4 = (*__src ^ 0x45af6e5d) - iVar2;
      if (((((__size <= uVar3) || (__size < uVar3 + 4)) ||
           (piVar4[1] = (__src[1] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 8)) ||
          ((piVar4[2] = (__src[2] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0xc ||
           (piVar4[3] = (__src[3] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0x10)))) ||
         ((piVar4[4] = (__src[4] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0x14 ||
          ((piVar4[5] = (__src[5] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0x18 ||
           (piVar4[6] = (__src[6] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0x1c)))))) break;
      puVar1 = __src + 7;
      uVar3 = uVar3 + 0x20;
      __src = __src + 8;
      piVar4[7] = (*puVar1 ^ 0x45af6e5d) - iVar2;
      piVar4 = piVar4 + 8;
    } while (uVar3 <= __size);
  }
LAB_005fe4fe:
  iVar2 = makeCheckSum((uchar *)local_2c,__size);
  if (*(int *)((int)param_1 + 8) != iVar2) {
    free(local_2c);
    local_2c = (int *)0x0;
    __size = 0;
  }
  if (param_3 != (uint *)0x0) {
    *param_3 = __size;
  }
  return local_2c;
}


This look very long and complicated but its not too bad if we break it down.

The start

Code:
  iVar2 = isCode(param_1);
  if (iVar2 == 0) {
    return (int *)0x0;
  }

the isCode function checks for the valid file header.
Image
So as we can see its checking if it starts with CODE
then 0x100 as a short.
Code:
43 4F 44 45 00 01


Then the next few variables are defined
Code:
  __size = param_2 - 0x10;
- sets __size to file size - 16
Code:
  local_2c = (int *)malloc(__size);
- creates a buffer with the size of __size
Code:
  __src = (uint *)((int)param_1 + 0x10);
sets __src to the input file starting at offset 16
Code:
  memcpy(local_2c,__src,__size);
copies __src into local_2c buffer
Code:
iVar2 = *(int *)((int)param_1 + 0xc);
read an int from offset 0xC in the file

We can skip all the checks in the middle for file size and just go do the actual decryption portion.
Code:
    do {
      *piVar4 = (*__src ^ 0x45af6e5d) - iVar2;
      if (((((__size <= uVar3) || (__size < uVar3 + 4)) ||
           (piVar4[1] = (__src[1] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 8)) ||
          ((piVar4[2] = (__src[2] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0xc ||
           (piVar4[3] = (__src[3] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0x10)))) ||
         ((piVar4[4] = (__src[4] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0x14 ||
          ((piVar4[5] = (__src[5] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0x18 ||
           (piVar4[6] = (__src[6] ^ 0x45af6e5d) - iVar2, __size < uVar3 + 0x1c)))))) break;
      puVar1 = __src + 7;
      uVar3 = uVar3 + 0x20;
      __src = __src + 8;
      piVar4[7] = (*puVar1 ^ 0x45af6e5d) - iVar2;
      piVar4 = piVar4 + 8;
    } while (uVar3 <= __size);

This can be simplified down all this code is doing is xoring 4 bytes with 0x45af6e5d then subtracting the seed value iVar2
from the result the program is just doing it 0x20 bytes at a time but we can just do it 4 bytes at a time.
so we only need
Quote:
*piVar4 = (*__src ^ 0x45af6e5d) - iVar2;

which is basically
Code:
buffer[i] = (buffer[i] ^ secretKey) - seed;


working bms script to show the code in action.
viewtopic.php?f=9&t=15201

The file header is.
magic 4 : CODE
short 0x100
short 0x7755
u32 CRC32
u32 random seed & 0xFFFF

(obtained with encrypt function that is also in the executable)
Code:
  puVar1 = (undefined4 *)malloc(param_2 + 0x10);
  *puVar1 = 0x45444f43;
  *(undefined2 *)((int)puVar1 + 6) = 0x7755;
  *(undefined2 *)(puVar1 + 1) = 0x100;
  __dest = puVar1 + 4;
  memcpy(__dest,param_1,param_2);
  lVar2 = lrand48();
  __aeabi_idivmod(lVar2,0xffff);


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