nsis_calc_hash.cpp

int __stdcall calc_hash(char *name)
 {
   int next_chunk;
   int hash;
   for ( hash = HASH_INIT; ; hash = next_chunk + 33 * hash )
   {
     next_chunk = *name++;
     if ( !next_chunk )
       break;
   }
   return hash;
 }

nsis_decrypt.cpp

void __stdcall decrypt_buf(BYTE *data, BYTE *key, unsigned int size)
{
  BYTE key_stream[512];
  int j;
  char next;
  int i;

  int v6 = 0;
  int v4 = 0;
  for ( i = 0; i < 256; ++i )
  {
    key_stream[i + 256] = i;
    key_stream[i] = key[i % size];
  }
  for ( i = 0; i < 256; ++i )
  {
    v6 = (key_stream[i] + v6 + key_stream[i + 256]) % 256;
    next = key_stream[v6 + 256];
    key_stream[v6 + 256] = key_stream[i + 256];
    key_stream[i + 256] = next;
  }
  v6 = 0;
  for ( j = 0; j < DATA_SIZE; ++j )
  {
    i = (i + 1) % 256;
    v6 = (v6 + key_stream[i + 256]) % 256;
    next = key_stream[v6 + 256];
    key_stream[v6 + 256] = key_stream[i + 256];
    key_stream[i + 256] = next;
    v4 = (key_stream[v6 + 256] + key_stream[i + 256]) % 256;
    data[j] ^= key[j % size];
    data[j] ^= key_stream[v4 + 256];
  }
}