/*! \file xtea.c
\author Thomas.Hoehenleitner [at] seerose.net
*******************************************************************************/

#include "xtea.h"
#include "trice.h"

#if ((TRICE_DIRECT_XTEA_ENCRYPT == 1) || (TRICE_DEFERRED_XTEA_ENCRYPT == 1)) && TRICE_OFF == 0

//! golang XTEA works with 64 rounds
static const unsigned int numRounds = 64;

//! 128 bit static key
static const uint32_t k[4] = XTEA_ENCRYPT_KEY;

//! internal constant
static const uint32_t delta = 0x9E3779B9;

//! precomputed values for faster execution
static uint32_t table[64];

//! XTEAInitTable precalculates the table.
//! It is possible to put this table completely into FLASH by precomputing it during compile time.
void XTEAInitTable(void) {
	uint32_t sum = 0;
	unsigned i;
	// Two rounds of XTEA applied per loop
	for (i = 0; i < numRounds;) {
		table[i] = sum + k[sum & 3];
		i++;
		sum += delta;
		table[i] = sum + k[(sum >> 11) & 3]; // lint !e661 Warning 661: Possible access of out-of-bounds pointer (1 beyond end of data) by operator '['
		i++;
	}
}

// encipher converts 64 bits.
//! Code taken and adapted from xtea\block.go
//!\param v 64 bits of data in v[0] and v[1] are encoded in place
static void encipher(uint32_t v[2]) {
	uint32_t v0 = v[0], v1 = v[1];
	unsigned i;
	for (i = 0; i < numRounds;) {
		v0 += (((v1 << 4) ^ (v1 >> 5)) + v1) ^ table[i];
		i++;
		v1 += (((v0 << 4) ^ (v0 >> 5)) + v0) ^ table[i]; // lint !e661 Warning 661: Possible access of out-of-bounds pointer (1 beyond end of data) by operator '['
		i++;
	}
	v[0] = v0;
	v[1] = v1;
}

#if XTEA_DECRYPT == 1
//! decipher reverses encipher action.
//! Code taken and adapted from xtea\block.go
//!\param v 64 bits of data in v[0] and v[1] are decoded in place
static void decipher(uint32_t v[2]) {
	uint32_t v0 = v[0], v1 = v[1];
	for (int i = numRounds; i > 0;) {
		i--;
		v1 -= (((v0 << 4) ^ (v0 >> 5)) + v0) ^ table[i];
		i--;
		v0 -= (((v1 << 4) ^ (v1 >> 5)) + v1) ^ table[i];
	}
	v[0] = v0;
	v[1] = v1;
}

//! XTEADecrypt re-converts from xtea cipher.
//! \param p pointer to 8 byte buffer
//! count is expected to be an even number.
void XTEADecrypt(uint32_t* p, unsigned count) {
	for (int i = 0; i < count; i += 2) {
		decipher(&p[i]); // byte swapping is done inside receiver according to endianness.
	}
}
#endif // #if XTEA_DECRYPT == 1

//! XTEAEncrypt converts to xtea cipher.
//! \param p pointer to 8 byte buffer.
//! count is expected to be an even number.
void XTEAEncrypt(uint32_t* p, unsigned count) {
	unsigned i;
	for (i = 0; i < count; i += 2) {
		encipher(&p[i]); // byte swap is done inside receiver
	}
}

#endif // #if ((TRICE_DIRECT_XTEA_ENCRYPT == 1) || (TRICE_DEFERRED_XTEA_ENCRYPT == 1)) && TRICE_OFF == 0