C#

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void DecryptInstID(unsigned char *InstID)
{
	unsigned char *Left, *Right;
	unsigned char Aux[8];
	int i, k;

	Left = InstID;
	Right = InstID + 8;

	for (i = 0; i < 4; i++)
	{
		KeyedHash(Left, Aux);

		for (k = 0; k < 8; k++)
			Aux[k] ^= Right[k];

		memcpy(Right, Left, 8);
		memcpy(Left, Aux, 8);
	}
}

void KeyedHash(unsigned char *Data, unsigned char *Result)
{
	SHA_CTX Context;
	unsigned char Digest[20];
	static unsigned char Key[4] =
	{
          // The key has been removed from the source code.
	};

	SHA1_Init(&Context);
	SHA1_Update(&Context, Data, 8);
	SHA1_Update(&Context, Key, 4);
	SHA1_Final(Digest, &Context);

	memcpy(Result, Digest, 8);
}