/*
 decrypt.c - EPOC-1 decryption function

 Copyright NTT MCL, 2000.

 Duncan S Wong
 Security Group, NTT MCL
 July 2000
*/

#include <stdio.h>
#include <gmp.h>
#include "epoc1.h"
#include "hash.h"
#include "utils.h"

/*
 EPOC-1 dercyption

 Return: TRUE if succeed; otherwise FALSE
*/
BYTE EPOC1_Decryption (
	BYTE             *ciphertext,
   EPOC1_PRIV_KEY   *privateKey,
   EPOC1_PUB_KEY    *publicKey,
	BYTE             *message
)
{
mpz_t C, verC, comp1, comp2, p2, p_1, c_p, L_c_p, g_p_1, L_g_p, X, r;
BYTE *H_in, *H_out;
WORD H_in_len;
long i;

	mpz_init_set_str(C, ciphertext, 0);
	/*
	printf("EPOC1_Decryption: C =\n");
	printf("%s\n\n", mpz_get_str(NULL,16, C));
	*/
	mpz_init(verC);
	mpz_init(comp1);
	mpz_init(comp2);
	mpz_init(p2);
	mpz_init(p_1);
	mpz_init(c_p);
	mpz_init(L_c_p);
	mpz_init(g_p_1);
	mpz_init(L_g_p);
	mpz_init(X);
	mpz_init(r);

	/* compute c_p */
	mpz_mul(p2, privateKey->p, privateKey->p);

	mpz_sub_ui(p_1, privateKey->p, 1);

	mpz_powm(c_p, C, p_1, p2);

	/* compute L(c_p) and L(g_p) */
	mpz_sub_ui(c_p, c_p, 1);
	mpz_divexact(L_c_p, c_p, privateKey->p);
	/*
	printf("EPOC1_Decryption: L_c_p =\n");
	printf("%s\n\n", mpz_get_str(NULL, 16, L_c_p));
	*/

	mpz_sub_ui(g_p_1, privateKey->g_p, 1);
	mpz_divexact(L_g_p, g_p_1, privateKey->p);
	/*
	printf("EPOC1_Decryption: L_g_p =\n");
	printf("%s\n\n", mpz_get_str(NULL, 16, L_g_p));
	*/

	/* compute X */
	mpz_invert(L_g_p, L_g_p, privateKey->p);
	mpz_mul(X, L_c_p, L_g_p);
	mpz_mod(X, X, privateKey->p);
	/*
	printf("EPOC1_Decryption: X =\n");
	printf("%s\n\n", mpz_get_str(NULL, 16, X));
	*/

	H_in_len = (publicKey->mLen + publicKey->rLen) / 8;
	H_in = (BYTE *) malloc (H_in_len);
	memset(H_in, 0, H_in_len);
	WORD2BYTE(H_in+(H_in_len - ABS(X->_mp_size)*4), X->_mp_d, ABS(X->_mp_size));
	/*
	printf("EPOC1_Decryption: H_in =\n");
	for(i=0; i<H_in_len; i++)
		printf("%02x ", H_in[i]);
	printf("\n\n");
	*/

	H_out = (BYTE *) malloc (publicKey->hLen/8);
	
	indexedSHA (H_out, publicKey->hLen/8, H_in, H_in_len);
	/*
	printf("EPOC1_Decryption: H_out =\n");
	for(i=0; i<publicKey->hLen/8; i++)
		printf("%02x ", H_out[i]);
	printf("\n\n");
	*/

	BYTE2WORD(r, H_out, publicKey->hLen/8);
	/*
	printf("EPOC1_Decryption: r=\n");
	printf("%s\n\n", mpz_get_str(NULL, 16, r));
	*/

	/* compute verC = g^X h^H(X) mod n */
   mpz_powm(comp1, publicKey->g, X, publicKey->n);

   mpz_powm(comp2, publicKey->h, r, publicKey->n);

   mpz_mul(verC, comp1, comp2);
   mpz_mod(verC, verC, publicKey->n);
	/*
   printf("EPOC1_Encryption: C =\n");
   printf("%s\n\n", mpz_get_str(NULL, 16, C));
   printf("EPOC1_Encryption: verC =\n");
   printf("%s\n\n", mpz_get_str(NULL, 16, verC));
	*/

	if(mpz_cmp(C, verC) == 0) {
		mpz_tdiv_q_2exp(X, X, publicKey->rLen);
		sprintf(message, "0x%s", mpz_get_str(NULL, 16, X));
	} else {
		printf("checking failed!\n");
		message[0] = '\0';
	}

	/* clean up */
	mpz_clear(C);
	mpz_clear(verC);
	mpz_clear(comp1);
	mpz_clear(comp2);
	mpz_clear(p2);
	mpz_clear(p_1);
	mpz_clear(c_p);
	mpz_clear(L_c_p);
	mpz_clear(g_p_1);
	mpz_clear(L_g_p);
	mpz_clear(X);
	mpz_clear(r);

	memset(H_in, 0, H_in_len);
	memset(H_out, 0, publicKey->hLen/8);
	free(H_out);
	free(H_in);

	H_in_len = 0;

	return TRUE;
}