decrypt.c

#include <stdio.h>
#include <string.h>
#include "api.h"
#include "isap.h"

int crypto_aead_decrypt(
	unsigned char *m, unsigned long long *mlen,
	unsigned char *nsec,
	const unsigned char *c, unsigned long long clen,
	const unsigned char *ad, unsigned long long adlen,
	const unsigned char *npub,
	const unsigned char *k
){
    (void)nsec;

	// Plaintext length is clen - tag length
	*mlen = clen-ISAP_TAG_SZ;

	// Generate tag
	unsigned char tag[ISAP_TAG_SZ];
	isap_mac(k,npub,ad,adlen,c,*mlen,tag);

	// Compare tag
	unsigned long eq_cnt = 0;
	for(unsigned int i = 0; i < ISAP_TAG_SZ; i++) {
		eq_cnt += (tag[i] == c[(*mlen)+i]);
	}

	// Perform decryption if tag is correct
	if(eq_cnt == (unsigned long)ISAP_TAG_SZ){
		if (*mlen > 0) {
			isap_enc(k,npub,c,*mlen,m);
		}
		return 0;
	} else {
		return -1;
	}
}