slitheen/server/crypto.c

#include <openssl/evp.h>
#include <openssl/dh.h>
#include <openssl/bn.h>
#include <openssl/err.h>
#include <openssl/rand.h>
#include <openssl/ssl.h>
#include "rserv.h"
#include "crypto.h"
#include "flow.h"
#include "slitheen.h"

/** Updates the hash of all TLS handshake messages upon the
 *  receipt of a new message. This hash is eventually used
 *  to verify the TLS Finished message
 *
 *  Inputs:
 *  	f: the tagged flow
 *  	hs: A pointer to the start of the handshake message
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int update_finish_hash(flow *f, uint8_t *hs){
	//find handshake length
	const struct handshake_header *hs_hdr;
	uint8_t *p = hs;
	hs_hdr = (struct handshake_header*) p;
	uint32_t hs_len = HANDSHAKE_MESSAGE_LEN(hs_hdr);
	
	EVP_DigestUpdate(f->finish_md_ctx, hs, hs_len+4);

	return 0;
}

/** Extracts the server parameters from the server key
 *  exchange message
 *
 *  Inputs:
 *  	f: the tagged flow
 *  	hs: the beginning of the server key exchange
 *  		handshake message
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int extract_parameters(flow *f, uint8_t *hs){
	DH *dh;
	uint8_t *p;
	long i;

	p = hs + HANDSHAKE_HEADER_LEN;

	if((dh = DH_new()) == NULL){
		return 1;
	}

	/* Extract prime modulus */
	n2s(p,i);

	if(!(dh->p = BN_bin2bn(p,i,NULL))){
		return 1;
	}
	p += i;

	/* Extract generator */
	n2s(p,i);

	if(!(dh->g = BN_bin2bn(p,i,NULL))){
		return 1;
	}
	p += i;

	/* Extract server public value */
	n2s(p,i);

	if(!(dh->pub_key = BN_bin2bn(p,i,NULL))){
		return 1;
	}

	f->dh = dh;
	return 0;
}

/** MAC a message
 * TODO: look at tls1_mac in t1_enc.c
 * For now, only goes one way (macs message to be written)
 *
int32_t mac(flow *f, uint8_t *input, uint8_t *output, int32_t len, int32_t incoming, int32_t type, int32_t enc){
	uint8_t header[13];
	int32_t md_size;

    header[8] = type;
    header[9] = 0x03;//TODO: update for different versions
    header[10] = 0x03;
    header[11] = (len) >> 8;
    header[12] = (len) & 0xff;

	EVP_DigestSignUpdate(f->read_mac_ctx, header, sizeof(header));
	EVP_DigestSignUpdate(f->read_mac_ctx, input, len);
	EVP_DigestSignFinal(f->read_mac_ctx, output, &md_size);

	return md_size;

}*/

/* Encrypt/Decrypt a TLS record
 *
 *  Inputs:
 * 		f: the tagged flow
 * 		input: a pointer to the data that is to be encrypted/
 * 			   decrypted
 * 		output: a pointer to where the data should be written
 * 				after it is encrypted or decrypted
 * 		len: the length of the data
 * 		incoming: the direction of the record
 * 		type: the type of the TLS record
 * 		enc: 1 for encryption, 0 for decryption
 *
 * 	Output:
 * 		length of the output data
 */
int encrypt(flow *f, uint8_t *input, uint8_t *output, int32_t len, int32_t incoming, int32_t type, int32_t enc){
	uint8_t *p = input;
	
	EVP_CIPHER_CTX *ds = (incoming) ? ((enc) ? f->srvr_write_ctx : f->clnt_read_ctx) : ((enc) ? f->clnt_write_ctx : f->srvr_read_ctx) ;
	if(ds == NULL){
		printf("FAIL\n");
		return 1;
	}

	uint8_t *seq;
	seq = (incoming) ? f->read_seq : f->write_seq;

	if(f->application && (ds->iv[EVP_GCM_TLS_FIXED_IV_LEN] == 0)){
		//fill in rest of iv
		for(int i = EVP_GCM_TLS_FIXED_IV_LEN; i< ds->cipher->iv_len; i++){
			ds->iv[i] = p[i- EVP_GCM_TLS_FIXED_IV_LEN];
		}
	}

#ifdef DEBUG
	printf("\t\tiv: ");
	for(int i=0; i<ds->cipher->iv_len; i++){
		printf("%02X ", ds->iv[i]);
	}
	printf("\n");
#endif

	uint8_t buf[13];
	memcpy(buf, seq, 8);

	for(int i=7; i>=0; i--){
		++seq[i];
		if(seq[i] != 0)
			break;
	}
	
	buf[8] = type;
	buf[9] = 0x03;
	buf[10] = 0x03;
	buf[11] = len >> 8; //len >> 8;
	buf[12] = len & 0xff;//len *0xff;
	int32_t pad = EVP_CIPHER_CTX_ctrl(ds, EVP_CTRL_AEAD_TLS1_AAD,
			13, buf); // = int32_t pad?
	printf("pad: %d\n", pad);

	if(enc)
		len += pad;

	int32_t n = EVP_Cipher(ds, p, p, len); //decrypt in place
	if(n<0) return 0;

#ifdef DEBUG
	printf("decrypted data:\n");
	for(int i=0; i< len; i++){
		printf("%02x ", p[EVP_GCM_TLS_EXPLICIT_IV_LEN+i]);
	}
	printf("\n");
#endif

	if(!enc)
		p[EVP_GCM_TLS_EXPLICIT_IV_LEN+n] = '\0';

	return n;
}

/** Verifies the hash in a TLS finished message
 *
 * 	Inputs:
 * 		f: the tagged flow
 * 		p: a pointer to the TLS Finished handshake message
 * 		incoming: the direction of the flow
 *
 * 	Output:
 * 		0 on success, 1 on failure
 */
int verify_finish_hash(flow *f, uint8_t *p, int32_t incoming){
	EVP_MD_CTX ctx;
	uint8_t hash[EVP_MAX_MD_SIZE];
	uint32_t hash_len;

	EVP_MD_CTX_init(&ctx);
	
	//get header length
	struct handshake_header *hs_hdr;
	hs_hdr = (struct handshake_header*) p;
	uint32_t fin_length = HANDSHAKE_MESSAGE_LEN(hs_hdr);
	p += HANDSHAKE_HEADER_LEN;

	//finalize hash of handshake msgs
	EVP_MD_CTX_copy_ex(&ctx, f->finish_md_ctx);
	EVP_DigestFinal_ex(&ctx, hash, &hash_len);

	//now use pseudorandom function
	uint8_t *output = calloc(1, fin_length);
	if(incoming){
		PRF(f->master_secret, SSL3_MASTER_SECRET_SIZE, (uint8_t *) TLS_MD_SERVER_FINISH_CONST, TLS_MD_SERVER_FINISH_CONST_SIZE , hash, hash_len, NULL, 0, NULL, 0, output, fin_length);
	} else {
		PRF(f->master_secret, SSL3_MASTER_SECRET_SIZE, (uint8_t *) TLS_MD_CLIENT_FINISH_CONST, TLS_MD_CLIENT_FINISH_CONST_SIZE , hash, hash_len, NULL, 0, NULL, 0, output, fin_length);
	}

	//now compare
	if(CRYPTO_memcmp(p, output, fin_length) != 0){
	//	printf("VERIFY FAILED\n");
		return 1;
	} else {
		printf("VERIFY PASSED\n");
	}

	free(output);
	EVP_MD_CTX_cleanup(&ctx);
	return 0;
}

/** Computes the TLS master secret from the decoy server's
 *  public key parameters and the leaked secret from the
 *  extracted Slitheen tag
 *
 *  Input:
 *  	f: the tagged flow
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int compute_master_secret(flow *f){
	printf("Computing master secret (%x:%d -> %x:%d)...\n", f->src_ip.s_addr, f->src_port, f->dst_ip.s_addr, f->dst_port);
	DH *dh_srvr = NULL;
	DH *dh_clnt = NULL;
	BN_CTX *ctx;
	BN_MONT_CTX *mont = NULL;
	BIGNUM *pub_key = NULL, *priv_key = NULL;

	ctx = BN_CTX_new();

	dh_srvr = f->dh;
	dh_clnt = DHparams_dup(dh_srvr);

    uint32_t l = dh_clnt->length ? dh_clnt->length : BN_num_bits(dh_clnt->p) - 1;
    int32_t bytes = (l+7) / 8;
	printf("length of dh param: %d\n", bytes);

    uint8_t *buf = (uint8_t *)OPENSSL_malloc(bytes);
    if (buf == NULL){
        BNerr(BN_F_BNRAND, ERR_R_MALLOC_FAILURE);
		goto err;
    }

	pub_key = BN_new();
	priv_key = BN_new();

	printf("key =");
	for(int i=0; i< 16; i++)
		printf(" %02x", f->key[i]);
	printf("\n");

	PRF(f->key, 16,
		(uint8_t *) SLITHEEN_KEYGEN_CONST, SLITHEEN_KEYGEN_CONST_SIZE,
		NULL, 0, NULL, 0, NULL, 0,
		buf, bytes);

//#ifdef DEBUG
	printf("Generated the following rand bytes: ");
	for(int i=0; i< bytes; i++){
		printf(" %02x ", buf[i]);
	}
	printf("\n");
//#endif

    if (!BN_bin2bn(buf, bytes, priv_key))
		goto err;

    {
        BIGNUM *prk;

		prk = priv_key;

        if (!dh_clnt->meth->bn_mod_exp(dh_clnt, pub_key, dh_clnt->g, prk, dh_clnt->p, ctx, mont)){
			goto err;
		}
    }

    dh_clnt->pub_key = pub_key;
    dh_clnt->priv_key = priv_key;

	// Compute master key 
	uint8_t *pre_master_secret = calloc(1, PRE_MASTER_MAX_LEN);//TODO: find right length

	int32_t pre_master_len = DH_compute_key(pre_master_secret, dh_srvr->pub_key, dh_clnt);
	
	PRF(pre_master_secret, pre_master_len, (uint8_t *) TLS_MD_MASTER_SECRET_CONST, TLS_MD_MASTER_SECRET_CONST_SIZE, f->client_random, SSL3_RANDOM_SIZE, f->server_random, SSL3_RANDOM_SIZE, NULL, 0, f->master_secret, SSL3_MASTER_SECRET_SIZE);

	if(f->current_session != NULL)
		memcpy(f->current_session, f->master_secret, SSL3_MASTER_SECRET_SIZE);

#ifdef DEBUG
    fprintf(stdout, "Premaster Secret:\n");
    BIO_dump_fp(stdout, (char *)pre_master_secret, pre_master_len);
    fprintf(stdout, "Client Random:\n");
    BIO_dump_fp(stdout, (char *)f->client_random, SSL3_RANDOM_SIZE);
    fprintf(stdout, "Server Random:\n");
    BIO_dump_fp(stdout, (char *)f->server_random, SSL3_RANDOM_SIZE);
    fprintf(stdout, "Master Secret:\n");
    BIO_dump_fp(stdout, (char *)f->master_secret, SSL3_MASTER_SECRET_SIZE);
#endif

	//remove pre_master_secret from memory
	memset(pre_master_secret, 0, PRE_MASTER_MAX_LEN);

err:
	if((pub_key != NULL) && (dh_srvr == NULL)){
		BN_free(pub_key);
	}
	if((priv_key != NULL) && (dh_clnt == NULL)){
		BN_free(priv_key);
	}
	BN_CTX_free(ctx);

	OPENSSL_free(buf);
	free(pre_master_secret);
	DH_free(dh_srvr);
	DH_free(dh_clnt);
	return 0;
}

/** Saves the random none from the server hello message
 *
 *  Inputs:
 *  	f: the tagged flow
 *  	hs: a pointer to the beginning of the server hello msg
 *  
 *  Output:
 *  	none
 */
void extract_server_random(flow *f, uint8_t *hs){

	uint8_t *p;

	p = hs + HANDSHAKE_HEADER_LEN;

	p+=2; //skip version

	memcpy(f->server_random, p, SSL3_RANDOM_SIZE);

}

/** PRF using sha384, as defined in RFC 5246
 *  
 *  Inputs:
 *  	secret: the master secret used to sign the hash
 *  	secret_len: the length of the master secret
 *  	seed{1, ..., 4}: seed values that are virtually
 *  		concatenated
 *  	seed{1,...4}_len: length of the seeds
 *  	output: a pointer to the output of the PRF
 *  	output_len: the number of desired bytes
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int PRF(uint8_t *secret, int32_t secret_len,
		uint8_t *seed1, int32_t seed1_len,
		uint8_t *seed2, int32_t seed2_len,
		uint8_t *seed3, int32_t seed3_len,
		uint8_t *seed4, int32_t seed4_len,
		uint8_t *output, int32_t output_len){

	EVP_MD_CTX ctx, ctx_tmp, ctx_init;
	EVP_PKEY *mac_key;
	const EVP_MD *md = EVP_sha384();

	uint8_t A[EVP_MAX_MD_SIZE];
	size_t len, A_len;
	int chunk = EVP_MD_size(md);
	int remaining = output_len;

	uint8_t *out = output;

	EVP_MD_CTX_init(&ctx);
	EVP_MD_CTX_init(&ctx_tmp);
	EVP_MD_CTX_init(&ctx_init);
	EVP_MD_CTX_set_flags(&ctx_init, EVP_MD_CTX_FLAG_NON_FIPS_ALLOW);

	mac_key = EVP_PKEY_new_mac_key(EVP_PKEY_HMAC, NULL, secret, secret_len);

	/* Calculate first A value */
	EVP_DigestSignInit(&ctx_init, NULL, md, NULL, mac_key);
	EVP_MD_CTX_copy_ex(&ctx, &ctx_init);
	if(seed1 != NULL && seed1_len > 0){
		EVP_DigestSignUpdate(&ctx, seed1, seed1_len);
	}
	if(seed2 != NULL && seed2_len > 0){
		EVP_DigestSignUpdate(&ctx, seed2, seed2_len);
	}
	if(seed3 != NULL && seed3_len > 0){
		EVP_DigestSignUpdate(&ctx, seed3, seed3_len);
	}
	if(seed4 != NULL && seed4_len > 0){
		EVP_DigestSignUpdate(&ctx, seed4, seed4_len);
	}
	EVP_DigestSignFinal(&ctx, A, &A_len);

	//iterate until desired length is achieved
	while(remaining > 0){
		/* Now compute SHA384(secret, A+seed) */
		EVP_MD_CTX_copy_ex(&ctx, &ctx_init);
		EVP_DigestSignUpdate(&ctx, A, A_len);
		EVP_MD_CTX_copy_ex(&ctx_tmp, &ctx);
		if(seed1 != NULL && seed1_len > 0){
			EVP_DigestSignUpdate(&ctx, seed1, seed1_len);
		}
		if(seed2 != NULL && seed2_len > 0){
			EVP_DigestSignUpdate(&ctx, seed2, seed2_len);
		}
		if(seed3 != NULL && seed3_len > 0){
			EVP_DigestSignUpdate(&ctx, seed3, seed3_len);
		}
		if(seed4 != NULL && seed4_len > 0){
			EVP_DigestSignUpdate(&ctx, seed4, seed4_len);
		}
		
		if(remaining > chunk){
			EVP_DigestSignFinal(&ctx, out, &len);
			out += len;
			remaining -= len;

			/* Next A value */
			EVP_DigestSignFinal(&ctx_tmp, A, &A_len);
		} else {
			EVP_DigestSignFinal(&ctx, A, &A_len);
			memcpy(out, A, remaining);
			remaining -= remaining;
		}
	}

	EVP_PKEY_free(mac_key);
	EVP_MD_CTX_cleanup(&ctx);
	EVP_MD_CTX_cleanup(&ctx_tmp);
	EVP_MD_CTX_cleanup(&ctx_init);
	OPENSSL_cleanse(A, sizeof(A));
	return 0;
}

/** After receiving change cipher spec, calculate keys from master secret
 *  
 *  Input:
 *  	f: the tagged flow
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int init_ciphers(flow *f){

	EVP_CIPHER_CTX *r_ctx;
	EVP_CIPHER_CTX *w_ctx;
	EVP_CIPHER_CTX *w_ctx_srvr;
	EVP_CIPHER_CTX *r_ctx_srvr;
	const EVP_CIPHER *c = EVP_aes_256_gcm();

	/* Generate Keys */
	uint8_t *write_key, *write_iv;
	uint8_t *read_key, *read_iv;
	int32_t mac_len, key_len, iv_len;

	key_len = EVP_CIPHER_key_length(c);
	iv_len = EVP_CIPHER_iv_length(c); //EVP_GCM_TLS_FIXED_IV_LEN;
	mac_len = EVP_MD_size(EVP_sha384());
	int32_t total_len = key_len + iv_len + mac_len;
	total_len *= 2;
	uint8_t *key_block = calloc(1, total_len);

	PRF(f->master_secret, SSL3_MASTER_SECRET_SIZE,
			(uint8_t *) TLS_MD_KEY_EXPANSION_CONST, TLS_MD_KEY_EXPANSION_CONST_SIZE,
			f->server_random, SSL3_RANDOM_SIZE,
			f->client_random, SSL3_RANDOM_SIZE,
			NULL, 0,
			key_block, total_len);

#ifdef DEBUG
	printf("master secret: (%x:%d -> %x:%d)\n", f->src_ip.s_addr, f->src_port, f->dst_ip.s_addr, f->dst_port);
	for(int i=0; i< SSL3_MASTER_SECRET_SIZE; i++){
		printf("%02x ", f->master_secret[i]);
	}
	printf("\n");

	printf("client random: (%x:%d -> %x:%d)\n", f->src_ip.s_addr, f->src_port, f->dst_ip.s_addr, f->dst_port);
	for(int i=0; i< SSL3_RANDOM_SIZE; i++){
		printf("%02x ", f->client_random[i]);
	}
	printf("\n");

	printf("server random: (%x:%d -> %x:%d)\n", f->src_ip.s_addr, f->src_port, f->dst_ip.s_addr, f->dst_port);
	for(int i=0; i< SSL3_RANDOM_SIZE; i++){
		printf("%02x ", f->server_random[i]);
	}
	printf("\n");

	printf("keyblock: (%x:%d -> %x:%d)\n", f->src_ip.s_addr, f->src_port, f->dst_ip.s_addr, f->dst_port);
	for(int i=0; i< total_len; i++){
		printf("%02x ", key_block[i]);
	}
	printf("\n");
#endif

	iv_len = EVP_GCM_TLS_FIXED_IV_LEN;
	
	write_key = key_block;
	read_key = key_block + key_len;
	write_iv = key_block + 2*key_len;
	read_iv = key_block + 2*key_len + iv_len;

	/* Initialize Cipher Contexts */
	r_ctx = EVP_CIPHER_CTX_new();
	w_ctx = EVP_CIPHER_CTX_new();
	EVP_CIPHER_CTX_init(r_ctx);
	EVP_CIPHER_CTX_init(w_ctx);
	w_ctx_srvr = EVP_CIPHER_CTX_new();
	r_ctx_srvr = EVP_CIPHER_CTX_new();
	EVP_CIPHER_CTX_init(w_ctx_srvr);
	EVP_CIPHER_CTX_init(r_ctx_srvr);

	/* Initialize MACs --- not needed for aes_256_gcm
	write_mac = key_block + 2*key_len + 2*iv_len;
	read_mac = key_block + 2*key_len + 2*iv_len + mac_len;
	read_mac_ctx = EVP_MD_CTX_create();
	write_mac_ctx = EVP_MD_CTX_create();
	read_mac_key =EVP_PKEY_new_mac_key(EVP_PKEY_HMAC, NULL, read_mac, mac_len);
	write_mac_key =EVP_PKEY_new_mac_key(EVP_PKEY_HMAC, NULL, write_mac, mac_len);
	EVP_DigestSignInit(read_mac_ctx, NULL, EVP_sha384(), NULL, read_mac_key);
	EVP_DigestSignInit(write_mac_ctx, NULL, EVP_sha384(), NULL, write_mac_key);
	EVP_PKEY_free(read_mac_key);
	EVP_PKEY_free(write_mac_key);*/


#ifdef DEBUG
    {
        int i;
        fprintf(stderr, "EVP_CipherInit_ex(r_ctx,c,key=,iv=,which)\n");
        fprintf(stderr, "\tkey= ");
        for (i = 0; i < c->key_len; i++)
            fprintf(stderr, "%02x", read_key[i]);
        fprintf(stderr, "\n");
        fprintf(stderr, "\t iv= ");
        for (i = 0; i < c->iv_len; i++)
            fprintf(stderr, "%02x", read_iv[i]);
        fprintf(stderr, "\n");
    }
    
	{
        int i;
        fprintf(stderr, "EVP_CipherInit_ex(w_ctx,c,key=,iv=,which)\n");
        fprintf(stderr, "\tkey= ");
        for (i = 0; i < c->key_len; i++)
            fprintf(stderr, "%02x", write_key[i]);
        fprintf(stderr, "\n");
        fprintf(stderr, "\t iv= ");
        for (i = 0; i < c->iv_len; i++)
            fprintf(stderr, "%02x", write_iv[i]);
        fprintf(stderr, "\n");
    }
#endif 

	EVP_CipherInit_ex(r_ctx, c, NULL, read_key, NULL, 0);
	EVP_CipherInit_ex(w_ctx, c, NULL, write_key, NULL, 1);
	EVP_CipherInit_ex(w_ctx_srvr, c, NULL, read_key, NULL, 1);
	EVP_CipherInit_ex(r_ctx_srvr, c, NULL, write_key, NULL, 0);
	EVP_CIPHER_CTX_ctrl(r_ctx, EVP_CTRL_GCM_SET_IV_FIXED, EVP_GCM_TLS_FIXED_IV_LEN, read_iv);
	EVP_CIPHER_CTX_ctrl(w_ctx, EVP_CTRL_GCM_SET_IV_FIXED, EVP_GCM_TLS_FIXED_IV_LEN, write_iv);
	EVP_CIPHER_CTX_ctrl(w_ctx_srvr, EVP_CTRL_GCM_SET_IV_FIXED, EVP_GCM_TLS_FIXED_IV_LEN, read_iv);
	EVP_CIPHER_CTX_ctrl(r_ctx_srvr, EVP_CTRL_GCM_SET_IV_FIXED, EVP_GCM_TLS_FIXED_IV_LEN, write_iv);

	f->clnt_read_ctx = r_ctx;
	f->clnt_write_ctx = w_ctx;
	f->srvr_read_ctx = r_ctx_srvr;
	f->srvr_write_ctx = w_ctx_srvr;

	free(key_block);
	return 0;
}

// To avoid warnings about MAC paddings, use this to update contexts
void update_context(flow *f, uint8_t *input, int32_t len, int32_t incoming, int32_t type, int32_t enc){

	uint8_t *output = calloc(1, len+16+8);
	memcpy(output + EVP_GCM_TLS_EXPLICIT_IV_LEN, input, len);

	//If the original message was a decryption, this will be an necryption.
	//Incoming field stays the same
	encrypt(f, output, output, len+8, incoming, type, !enc);

	//revert the sequence number
	uint8_t *seq = incoming ? f->read_seq : f->write_seq;
	for(int i=7; i>=0; i--){
		--seq[i];
		if(seq[i] >= 0)
			break;
		else
			seq[i] = 0;
	}

	free(output);
}

/** Checks a handshake message to see if it is tagged or a
 *  recognized flow. If the client random nonce is tagged,
 *  adds the flow to the flow table to be tracked.
 *
 *  Inputs:
 *  	info: the processed packet
 *  	f: the tagged flow
 *
 *  Output:
 *  	none
 */
void check_handshake(struct packet_info *info, flow f){

	FILE *fp;
	int res, i, code;
	uint8_t *hello_rand;
	const struct handshake_header *handshake_hdr;

    byte privkey[PTWIST_BYTES];
	byte key[16];

	uint8_t *p = info->app_data + RECORD_HEADER_LEN;
	handshake_hdr = (struct handshake_header*) p;

	code = handshake_hdr->type;

	if (code == 0x01){
		p += CLIENT_HELLO_HEADER_LEN;
		//now pointing to hello random :D
		hello_rand = p;
		p += 4; //skipping time bytes
		/* Load the private key */
		fp = fopen("privkey", "rb");
		if (fp == NULL) {
			perror("fopen");
			exit(1);
		}
		res = fread(privkey, PTWIST_BYTES, 1, fp);
		if (res < 1) {
			perror("fread");
			exit(1);
		}
		fclose(fp);

		/* check tag*/ 
		res = check_tag(key, privkey, p, (const byte *)"context", 7);
		if (res) {
			printf("Untagged\n");
		} else {
			fp = fopen("tags", "ab");
			if (fp == NULL) {
				perror("fopen");
				exit(1);
			}
			//Write tag to file
			for(i=0; i< 28; i++){
				fprintf(fp, "%02x ", p[i]);
			}
			fprintf(fp, "\n");
			fclose(fp);

			//Write key to file
			fp = fopen("sharedkey", "ab");
			if (fp == NULL) {
				perror("fopen");
				exit(1);
			}
			for(i=0; i<16;i++){
			    fprintf(fp, "%02x", key[i]);
			}
			fprintf(fp, "\n");
			fclose(fp);
			printf("Received tagged flow! (key =");
			for(i=0; i<16;i++){
			    printf(" %02x", key[i]);
			}
			printf(")\n");

			/* Save flow in table */
			flow *flow_ptr = add_flow(f);
			for(int i=0; i<16; i++){
				flow_ptr->key[i] = key[i];
			}

			memcpy(flow_ptr->client_random, hello_rand, SSL3_RANDOM_SIZE);
			for(int i=0; i< SSL3_RANDOM_SIZE; i++){
				printf("%02x ", hello_rand[i]);
			}
			printf("\n");
			
			printf("Saved new flow\n");

		}
	}

}