Function EmeTransform

libcppcryptfs/crypt/eme.cpp:219–299

Transform - EME-encrypt or EME-decrypt, according to "direction" (defined in the constants directionEncrypt and directionDecrypt). The data in "P" is en- or decrypted with the block ciper "bc" under tweak "T". The result is returned in a freshly allocated slice.

// The data in "P" is en- or decrypted with the block ciper "bc" under tweak "T".
// The result is returned in a freshly allocated slice.
bool EmeTransform(const EmeCryptContext *eme_context, const BYTE *T, const BYTE *P, int len, bool direction, EmeBuffer_t& buffer)  {

	BYTE *C = NULL;

	bool error = false;

	try {
		if (len % 16 != 0) {
			panic(L"Data length is not a multiple of 16");
		}
		int m = len / 16;
		if (m == 0 || m > 16 * 8) {
			panic(L"EME operates on 1-128 block-cipher blocks");
		}

		C = buffer.get(len+1); // +1 so caller can add a null terminator if necessary without any trouble

		BYTE **LTable = eme_context->m_LTable;

		BYTE PPj[16];

		for (int j = 0; j < m; j++) {
			BYTE Pj[16];
			memcpy(Pj, P + j * 16, 16);
			/* PPj = 2**(j-1)*L xor Pj */
			xorBlocks(PPj, Pj, LTable[j], 16);
			/* PPPj = AESenc(K; PPj) */
			aesTransform(C + j * 16, PPj, direction, 16, eme_context);
		}

		/* MP =(xorSum PPPj) xor T */
		BYTE MP[16];
		xorBlocks(MP, C, T, 16);
		for (int j = 1; j < m; j++) {
			xorBlocks(MP, MP, C + j * 16, 16);
		}

		/* MC = AESenc(K; MP) */
		BYTE MC[16];
		aesTransform(MC, MP, direction, 16, eme_context);

		/* M = MP xor MC */
		BYTE M[16];
		xorBlocks(M, MP, MC, 16);
		BYTE CCCj[16];
		for (int j = 1; j < m; j++) {
			multByTwo(M, M, 16);
			/* CCCj = 2**(j-1)*M xor PPPj */
			xorBlocks(CCCj, C + j * 16, M, 16);
			memcpy(C + j * 16, CCCj, 16);
		}

		/* CCC1 = (xorSum CCCj) xor T xor MC */

		BYTE CCC1[16];
		xorBlocks(CCC1, MC, T, 16);
		for (int j = 1; j < m; j++) {
			xorBlocks(CCC1, CCC1, C + j * 16, 16);