slitheen/relay_station/flow.c

/* Name: flow.c
 *
 * This file contains functions for manipulating tagged flows. 
 *
 * Slitheen - a decoy routing system for censorship resistance
 * Copyright (C) 2017 Cecylia Bocovich (cbocovic@uwaterloo.ca)
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, version 3.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * Additional permission under GNU GPL version 3 section 7
 * 
 * If you modify this Program, or any covered work, by linking or combining
 * it with the OpenSSL library (or a modified version of that library), 
 * containing parts covered by the terms of [name of library's license],
 * the licensors of this Program grant you additional permission to convey
 * the resulting work. {Corresponding Source for a non-source form of such
 * a combination shall include the source code for the parts of the OpenSSL
 * library used as well as that of the covered work.}
 */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <pthread.h>
#include <errno.h>
#include <semaphore.h>

#include "flow.h"
#include "crypto.h"
#include "slitheen.h"
#include "relay.h"
#include "util.h"

#define DEBUG_HS

static flow_table *table;
static session_cache *sessions;
client_table *clients;

sem_t flow_table_lock;

/* Initialize the table of tagged flows */
int init_tables(void) {

	table = emalloc(sizeof(flow_table));
	table->first_entry = NULL;
	table->len = 0;

	sem_init(&flow_table_lock, 0, 1);

	clients = emalloc(sizeof(client_table));
	clients->first = NULL;
	printf("initialized downstream queue\n");

	return 0;
}


/* Add a new flow to the tagged flow table */
flow *add_flow(struct packet_info *info) {
	flow_entry *entry = emalloc(sizeof(flow_entry));

	flow *new_flow = emalloc(sizeof(flow));

	entry->f = new_flow;
	entry->next = NULL;

	new_flow->src_ip = info->ip_hdr->src;
	new_flow->dst_ip = info->ip_hdr->dst;
	new_flow->src_port = info->tcp_hdr->src_port;
	new_flow->dst_port = info->tcp_hdr->dst_port;

	new_flow->ref_ctr = 1;
        printf("Adding new flow (%p ref ctr %d)\n", new_flow, 1);
	new_flow->removed = 0;

	new_flow->upstream_app_data = emalloc(sizeof(app_data_queue));
	new_flow->upstream_app_data->first_packet = NULL;
	new_flow->downstream_app_data = emalloc(sizeof(app_data_queue));
	new_flow->downstream_app_data->first_packet = NULL;

	new_flow->upstream_seq_num = ntohl(info->tcp_hdr->sequence_num);
	new_flow->downstream_seq_num = ntohl(info->tcp_hdr->ack_num);

    new_flow->us_frame_queue = emalloc(sizeof(frame_queue));
    new_flow->us_frame_queue->first_frame = NULL;
    new_flow->ds_frame_queue = emalloc(sizeof(frame_queue));
    new_flow->ds_frame_queue->first_frame = NULL;

	new_flow->streams=NULL;
	new_flow->downstream_queue=NULL;
	new_flow->client_ptr=NULL;

	sem_init(&(new_flow->flow_lock), 0, 1);
	new_flow->state = TLS_CLNT_HELLO;
	new_flow->in_encrypted = 0;
	new_flow->out_encrypted = 0;
	new_flow->application = 0;
	new_flow->stall = 0;
	new_flow->resume_session = 0;
	new_flow->current_session = NULL;

        new_flow->us_hs_queue = init_queue();
        new_flow->ds_hs_queue = init_queue();

	new_flow->us_packet_chain = emalloc(sizeof(packet_chain));
	new_flow->us_packet_chain->expected_seq_num = ntohl(info->tcp_hdr->sequence_num);
	new_flow->us_packet_chain->record_len = 0;
	new_flow->us_packet_chain->remaining_record_len = 0;
	new_flow->us_packet_chain->first_packet = NULL;

	new_flow->ds_packet_chain = emalloc(sizeof(packet_chain));
	new_flow->ds_packet_chain->expected_seq_num = ntohl(info->tcp_hdr->ack_num);
	new_flow->ds_packet_chain->record_len = 0;
	new_flow->ds_packet_chain->remaining_record_len = 0;
	new_flow->ds_packet_chain->first_packet = NULL;
	sem_init(&(new_flow->packet_chain_lock), 0, 1);
	
	new_flow->upstream_queue = NULL;
	new_flow->upstream_remaining = 0;
	sem_init(&(new_flow->upstream_queue_lock), 0, 1);
	new_flow->outbox = NULL;
	new_flow->outbox_len = 0;
	new_flow->outbox_offset = 0;
	new_flow->partial_record_header = NULL;
	new_flow->partial_record_header_len = 0;
	new_flow->remaining_record_len = 0;
	new_flow->remaining_response_len = 0;
	new_flow->httpstate = PARSE_HEADER;
	new_flow->replace_response = 0;

	new_flow->ecdh = NULL;
	new_flow->dh = NULL;

	new_flow->finish_md_ctx = EVP_MD_CTX_create();
	const EVP_MD *md = EVP_sha384();
	EVP_DigestInit_ex(new_flow->finish_md_ctx, md, NULL);

	new_flow->cipher = NULL;
	new_flow->clnt_read_ctx = NULL;
	new_flow->clnt_write_ctx = NULL;
	new_flow->srvr_read_ctx = NULL;
	new_flow->srvr_write_ctx = NULL;

	memset(new_flow->read_seq, 0, 8);
	memset(new_flow->write_seq, 0, 8);


	sem_wait(&flow_table_lock);
	flow_entry *last = table->first_entry;
	if(last == NULL){
		table->first_entry = entry;
	} else {
		for(int i=0; i< table->len-1; i++){
			last = last->next;
		}
		last->next = entry;
	}
	table->len ++;
	sem_post(&flow_table_lock);

	return new_flow;

}

/** Observes TLS handshake messages and updates the state of
 *  the flow
 *
 *  Inputs:
 *  	f: the tagged flow
 *  	record: a complete TLS record
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int update_flow(flow *f, uint8_t *record, uint8_t incoming) {
	const struct record_header *record_hdr;
	const struct handshake_header *handshake_hdr;
	uint8_t *p;

	record_hdr = (struct record_header*) record;
	int record_len;

	record_len = RECORD_LEN(record_hdr)+RECORD_HEADER_LEN;

	switch(record_hdr->type){
		case HS:
			p = record;
			p += RECORD_HEADER_LEN;

			if((incoming && f->in_encrypted) || (!incoming && f->out_encrypted)){
#ifdef DEBUG_HS
				printf("Decrypting finished (%d bytes) (%x:%d -> %x:%d)\n", record_len - RECORD_HEADER_LEN, f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
				printf("Finished ciphertext:\n");
				for(int i=0; i< record_len; i++){
					printf("%02x ", record[i]);
				}
				printf("\n");
#endif
				int32_t n = encrypt(f, p, p, record_len - RECORD_HEADER_LEN, incoming, 0x16, 0, 0);
				if(n<=0){
					printf("Error decrypting finished  (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
				}
#ifdef DEBUG_HS
				printf("Finished decrypted: (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
				p += EVP_GCM_TLS_EXPLICIT_IV_LEN;
				
#ifdef DEBUG_HS
				printf("record:\n");
				for(int i=0; i< n; i++){
					printf("%02x ", p[i]);
				}
				printf("\n");
#endif
				if(p[0] != 0x14){
					p[0] = 0x20; //trigger error
				}

				if(incoming){
					f->in_encrypted = 2;
				} else {
					f->out_encrypted = 2;
				}

			}
			handshake_hdr = (struct handshake_header*) p;
			f->state = handshake_hdr->type;

			switch(f->state){
				case TLS_CLNT_HELLO: 
#ifdef DEBUG_HS
					printf("Received tagged client hello (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish has with CLNT_HELLO msg\n");
						remove_flow(f);
						goto err;
					}
					if(check_session(f, p, HANDSHAKE_MESSAGE_LEN(handshake_hdr))){
						fprintf(stderr, "Error checking session, might cause problems\n");
					}

					break;
				case TLS_SERV_HELLO:
#ifdef DEBUG_HS
					printf("Received server hello (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
					if(f->resume_session){
						if(verify_session_id(f,p)){
							fprintf(stderr, "Failed to verify session id\n");
						}
					} else {
						if(save_session_id(f,p)){
							fprintf(stderr, "Failed to save session id\n");
						}
					}
					if(extract_server_random(f, p)){
						fprintf(stderr, "Failed to extract server random nonce\n");
						remove_flow(f);
						goto err;
					}

					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with SRVR_HELLO msg\n");
						remove_flow(f);
						goto err;
					}
					break;
				case TLS_NEW_SESS:
#ifdef DEBUG_HS
					printf("Received new session\n");
#endif
					if(save_session_ticket(f, p, HANDSHAKE_MESSAGE_LEN(handshake_hdr))){
						fprintf(stderr, "Failed to save session ticket\n");
					}
					
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with NEW_SESS msg\n");
						remove_flow(f);
						goto err;
					}
					
					break;
				case TLS_CERT:
#ifdef DEBUG_HS
					printf("Received cert\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with CERT msg\n");
						remove_flow(f);
						goto err;
					}

					break;
				case TLS_SRVR_KEYEX:
#ifdef DEBUG_HS
					printf("Received server keyex\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with SRVR_KEYEX msg\n");
						remove_flow(f);
						goto err;
					}

					if(extract_parameters(f, p)){
						printf("Error extracting params\n");
						remove_flow(f);
						goto err;
					}

					if(compute_master_secret(f)){
						printf("Error computing master secret\n");
						remove_flow(f);
						goto err;

					}

					break;

				case TLS_CERT_REQ:

					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with CERT_REQ msg\n");
						remove_flow(f);
						goto err;
					}

					break;
				case TLS_SRVR_HELLO_DONE:
#ifdef DEBUG_HS
					printf("Received server hello done\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with HELLO_DONE msg\n");
						remove_flow(f);
						goto err;
					}

					break;
				case TLS_CERT_VERIFY:
#ifdef DEBUG_HS
					printf("received cert verify\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with CERT_VERIFY msg\n");
						remove_flow(f);
						goto err;
					}

					break;

				case TLS_CLNT_KEYEX:
#ifdef DEBUG_HS
					printf("Received client key exchange\n");
#endif
					if(update_finish_hash(f, p)){
						fprintf(stderr, "Error updating finish hash with CLNT_KEYEX msg\n");
						remove_flow(f);
						goto err;
					}

					break;
				case TLS_FINISHED:
#ifdef DEBUG_HS
					printf("Received finished (%d) (%x:%d -> %x:%d)\n", incoming, f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
					if(verify_finish_hash(f,p, incoming)){
						fprintf(stderr, "Error verifying finished hash\n");
						remove_flow(f);
						goto err;
					}
					
					//re-encrypt finished message
					int32_t n =  encrypt(f, record+RECORD_HEADER_LEN, record+RECORD_HEADER_LEN, record_len - (RECORD_HEADER_LEN+16), incoming, 0x16, 1, 1);

#ifdef HS_DEBUG
					printf("New finished ciphertext:\n");
					for(int i=0; i< record_len; i++){
						printf("%02x ", record[i]);
					}
					printf("\n");
#endif

					if(n<=0){
						printf("Error re-encrypting finished  (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port),
								f->dst_ip.s_addr, ntohs(f->dst_port));
					}

					if((f->in_encrypted == 2) && (f->out_encrypted == 2)){
						f->application = 1;
                                                printf("Handshake complete!\n");
					}

					break;
				default:
					printf("Error? (%x:%d -> %x:%d)...\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
					remove_flow(f);
					goto err;
			}
			break;
		case APP:
			printf("Application Data (%x:%d -> %x:%d)...\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
			break;
		case CCS:
#ifdef DEBUG_HS
			printf("CCS (%x:%d -> %x:%d) \n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
#endif
			/*Initialize ciphers */
			if ((!f->in_encrypted) && (!f->out_encrypted)){
				if(init_ciphers(f)){
					remove_flow(f);
					goto err;
				}
			}

			if(incoming){
				f->in_encrypted = 1;
			} else {
				f->out_encrypted = 1;
			}
			
			break;
		case ALERT:
			p = record;
			p += RECORD_HEADER_LEN;
			if(((incoming) && (f->in_encrypted > 0)) || ((!incoming) && (f->out_encrypted > 0))){
				//decrypt alert
				encrypt(f, p, p, record_len - RECORD_HEADER_LEN, incoming, 0x16, 0, 0);
				p += EVP_GCM_TLS_EXPLICIT_IV_LEN;
			}
			printf("Alert (%x:%d -> %x:%d) %02x %02x \n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port), p[0], p[1]);
			fflush(stdout);
			
			//re-encrypt alert
			if(((incoming) && (f->in_encrypted > 0)) || ((!incoming) && (f->out_encrypted > 0))){
				int32_t n =  encrypt(f, record+RECORD_HEADER_LEN, record+RECORD_HEADER_LEN, record_len - (RECORD_HEADER_LEN+16), incoming, 0x16, 1, 1);
				if(n <= 0){
					printf("Error re-encrypting alert\n");
				}
			}

			break;
		case HB:
			printf("Heartbeat\n");
			break;
		default:
			printf("Error: Not a Record (%x:%d -> %x:%d)\n", f->src_ip.s_addr, ntohs(f->src_port), f->dst_ip.s_addr, ntohs(f->dst_port));
			fflush(stdout);
			remove_flow(f);
			goto err;
	}
	return 0;

err:
	return 1;
}

/** Removes the tagged flow from the flow table: happens when
 *  the station receives a TCP RST or FIN packet
 *
 *  Input:
 *  	index: the index into the flow table of the tagged flow
 *
 *  Output:
 *  	0 on success, 1 on failure
 */
int remove_flow(flow *f) {

    sem_wait(&flow_table_lock);
    //decrement reference counter
    f->ref_ctr--;
    if(f->ref_ctr){ //if there are still references to f, wait to free it
        printf("Cannot free %p, still %d reference(s)\n", f, f->ref_ctr);
        f->removed = 1; 
        sem_post(&flow_table_lock);
        return 0;
    }

    if(f->removed)
        printf("Trying again to free\n");

    frame *first_frame = f->us_frame_queue->first_frame;
    while(first_frame != NULL){
        printf("Injecting delayed frame (seq = %u )\n", first_frame->seq_num);
        inject_packet(first_frame->iargs, first_frame->header, first_frame->packet);
        frame *tmp = first_frame->next;
        free(first_frame);
        first_frame = tmp;
    }
    free(f->us_frame_queue);

    first_frame = f->ds_frame_queue->first_frame;
    while(first_frame != NULL){
        printf("Injecting delayed frame (seq = %u )\n", first_frame->seq_num);
        inject_packet(first_frame->iargs, first_frame->header, first_frame->packet);
        frame *tmp = first_frame->next;
        free(first_frame);
        first_frame = tmp;
    }
    free(f->ds_frame_queue);

	//Empty application data queues
	packet *tmp = f->upstream_app_data->first_packet;
	while(tmp != NULL){
		f->upstream_app_data->first_packet = tmp->next;
		free(tmp->data);
		free(tmp);
		tmp = f->upstream_app_data->first_packet;
	}
	free(f->upstream_app_data);

	tmp = f->downstream_app_data->first_packet;
	while(tmp != NULL){
		f->downstream_app_data->first_packet = tmp->next;
		free(tmp->data);
		free(tmp);
		tmp = f->downstream_app_data->first_packet;
	}
	free(f->downstream_app_data);

    if(f->ds_hs_queue != NULL){
        remove_queue(f->ds_hs_queue);
    }
    if(f->us_hs_queue != NULL){
        remove_queue(f->us_hs_queue);
    }

    //free partial record headers
    if(f->partial_record_header_len > 0){
        f->partial_record_header_len = 0;
        free(f->partial_record_header);
    }

	//Clean up cipher ctxs
	EVP_MD_CTX_cleanup(f->finish_md_ctx);
	if(f->finish_md_ctx != NULL){
		EVP_MD_CTX_destroy(f->finish_md_ctx);
	}
	if(f->clnt_read_ctx != NULL){
		EVP_CIPHER_CTX_cleanup(f->clnt_read_ctx);
		OPENSSL_free(f->clnt_read_ctx);
		f->clnt_read_ctx = NULL;
	}
	if(f->clnt_write_ctx != NULL){
		EVP_CIPHER_CTX_cleanup(f->clnt_write_ctx);
		OPENSSL_free(f->clnt_write_ctx);
		f->clnt_write_ctx = NULL;
	}
	if(f->srvr_read_ctx != NULL){
		EVP_CIPHER_CTX_free(f->srvr_read_ctx);
	}
	if(f->srvr_write_ctx != NULL){
		EVP_CIPHER_CTX_free(f->srvr_write_ctx);
	}

	if(f->ecdh != NULL){
		EC_KEY_free(f->ecdh);
	}

    if(f->dh != NULL){
        DH_free(f->dh);
    }

	if(f->current_session != NULL && f->resume_session == 1){
		if( f->current_session->session_ticket != NULL){
			free(f->current_session->session_ticket);
		}
		free(f->current_session);
	}

	if(f->ds_packet_chain != NULL){
		packet *tmp = f->ds_packet_chain->first_packet;
		while(tmp != NULL){
			f->ds_packet_chain->first_packet = tmp->next;
			printf("Freed data %p\n", tmp->data);
			printf("Freed packet %p\n", tmp);
			free(tmp->data);
			free(tmp);
			tmp = f->ds_packet_chain->first_packet;
		}
	}
	free(f->ds_packet_chain);

	if(f->us_packet_chain != NULL){
		packet *tmp = f->us_packet_chain->first_packet;
		while(tmp != NULL){
			f->us_packet_chain->first_packet = tmp->next;
			printf("Freed data %p\n", tmp->data);
			printf("Freed packet %p\n", tmp);
			free(tmp->data);
			free(tmp);
			tmp = f->us_packet_chain->first_packet;
		}
	}
	free(f->us_packet_chain);
		
	if(f->upstream_queue != NULL){
		queue_block *tmp = f->upstream_queue;
		while(tmp != NULL){
			f->upstream_queue = tmp->next;
			printf("Freed data %p\n", tmp->data);
			printf("Freed packet %p\n", tmp);
			free(tmp->data);
			free(tmp);
			tmp = f->upstream_queue;
		}
	}

	flow_entry *entry = table->first_entry;
	if(entry->f == f){
		table->first_entry = entry->next;
		free(entry->f);
		free(entry);
		table->len --;
	} else {

		flow_entry *next;
		for(int i=0; i< table->len; i++){
			if(entry->next != NULL){
				next = entry->next;
			} else {
				printf("Flow not in table\n");
				break;
			}

			if(next->f == f){
				entry->next = next->next;
				free(next->f);
				free(next);
				table->len --;
				break;
			}

			entry = next;
		}
	}
	sem_post(&flow_table_lock);

	return 1;
}

/** Returns the index of a flow in the flow table if
 *  it exists, returns 0 if it is not present.
 *
 *  Inputs:
 *  	observed: details for the observed flow
 *
 *  Output:
 *  	flow struct from table or NULL if it doesn't exist
 */
flow *check_flow(struct packet_info *info){
	/* Loop through flows in table and see if it exists */
	int i;
	flow_entry *entry = table->first_entry;
	flow *candidate;
	flow *found = NULL;
	if(entry == NULL)
		return NULL;

	sem_wait(&flow_table_lock);
	/* Check first in this direction */
	for(i=0; i<table->len; i++){
		if(entry == NULL){
			printf("Error: entry is null\n");
			break;
		}
		candidate = entry->f;
		if(candidate->src_ip.s_addr == info->ip_hdr->src.s_addr){
			if(candidate->dst_ip.s_addr == info->ip_hdr->dst.s_addr){
				if(candidate->src_port == info->tcp_hdr->src_port){
					if(candidate->dst_port == info->tcp_hdr->dst_port){
						found = candidate;
					}
				}
			}
		}
		entry = entry->next;
	}


	entry = table->first_entry;
	/* Then in the other direction */
	for(i=0; i<table->len; i++){
		if(entry == NULL){
			printf("Error: entry is null\n");
			break;
		}
		candidate = entry->f;
		if(candidate->src_ip.s_addr == info->ip_hdr->dst.s_addr){
			if(candidate->dst_ip.s_addr == info->ip_hdr->src.s_addr){
				if(candidate->src_port == info->tcp_hdr->dst_port){
					if(candidate->dst_port == info->tcp_hdr->src_port){
						found = candidate;
					}
				}
			}
		}
		entry = entry->next;
	}

	if(found != NULL){
            found->ref_ctr++;
            printf("Acquiring flow (%p ref ctr %d)\n", found, found->ref_ctr);
	}

	sem_post(&flow_table_lock);

	if(found != NULL && found->removed){
		remove_flow(found);
		found=NULL;
	}

	return found;
}

int init_session_cache(void){
	sessions = emalloc(sizeof(session_cache));

	sessions->length = 0;
	sessions->first_session = NULL;

	return 0;
}

/** Called from ServerHello, verifies that the session id returned matches
 *  the session id requested from the client hello
 *
 *  Input:
 *  	f: the tagged flow
 *  	hs: a pointer to the ServerHello message
 *
 *  Output:
 *  	0 if success, 1 if failed
 */
int verify_session_id(flow *f, uint8_t *hs){
	
	if (f->current_session == NULL)
		return 1;

	//increment pointer to point to sessionid
	uint8_t *p = hs + HANDSHAKE_HEADER_LEN;
	p += 2; //skip version
	p += SSL3_RANDOM_SIZE; //skip random

	uint8_t id_len = (uint8_t) p[0];
	p ++;
	
	//check to see if it matches flow's session id set by ClientHello
	if(f->current_session->session_id_len > 0 && !memcmp(f->current_session->session_id, p, id_len)){
		//if it matched, update flow with master secret :D
#ifdef DEBUG_HS
		printf("Session id matched!\n");
		printf("First session id (%p->%p):", sessions, sessions->first_session);
#endif
		session *last = sessions->first_session;
		int found = 0;
		for(int i=0; ((i<sessions->length) && (!found)); i++){
#ifdef DEBUG_HS
			printf("Checking saved session id: ");
			for (int j=0; j< last->session_id_len; j++){
				printf("%02x ", last->session_id[j]);
			}
			printf("\n");
#endif
			if(!memcmp(last->session_id, f->current_session->session_id, id_len)){
				memcpy(f->master_secret, last->master_secret, SSL3_MASTER_SECRET_SIZE);
				found = 1;
			}
			last = last->next;
		}
		if((!found) && (f->current_session->session_ticket_len > 0)){
			last = sessions->first_session;
			for(int i=0; ((i<sessions->length) && (!found)); i++){
				if( (last->session_ticket != NULL) && (last->session_ticket_len == f->current_session->session_ticket_len)){
					if(!memcmp(last->session_ticket, f->current_session->session_ticket, f->current_session->session_ticket_len)){
						memcpy(f->master_secret, last->master_secret, SSL3_MASTER_SECRET_SIZE);
						found = 1;
	#ifdef DEBUG_HS
						printf("Found new session ticket (%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< last->session_ticket_len; i++){
							printf("%02x ", last->session_ticket[i]);
						}
						printf("\n");
	#endif
					}
				}
				last = last->next;
			}
		}

	} else if (f->current_session->session_id_len == 0){
		//search for session ticket in session cache
        printf("clnt session id was empty, looking for ticket\n");
		session *last = sessions->first_session;
		if(f->current_session->session_ticket_len > 0){
			last = sessions->first_session;
			for(int i=0; i<sessions->length; i++){
				if(last->session_ticket_len == f->current_session->session_ticket_len){
					if(!memcmp(last->session_ticket, f->current_session->session_ticket, f->current_session->session_ticket_len)){
						memcpy(f->master_secret, last->master_secret, SSL3_MASTER_SECRET_SIZE);
	#ifdef DEBUG_HS
						printf("Found new session ticket (%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< last->session_ticket_len; i++){
							printf("%02x ", last->session_ticket[i]);
						}
						printf("\n");
						break;
	#endif
					}
				}
				last = last->next;
			}
		}

	} else if (f->current_session->session_id_len > 0){
		//server refused resumption, save new session id
        printf("session ids did not match, saving new id\n");
		save_session_id(f, p);
	}

	return 0;

}

/* Called from ClientHello. Checks to see if the session id len is > 0. If so,
 * saves sessionid for later verification. Also checks to see if a session
 * ticket is included as an extension.
 *
 *  Input:
 *  	f: the tagged flow
 *  	hs: a pointer to the ServerHello message
 *
 *  Output:
 *  	0 if success, 1 if failed
 */
int check_session(flow *f, uint8_t *hs, uint32_t len){

	uint8_t *p = hs + HANDSHAKE_HEADER_LEN;
	p += 2; //skip version
	p += SSL3_RANDOM_SIZE; //skip random

	session *new_session = emalloc(sizeof(session));
	new_session->session_id_len = (uint8_t) p[0];
	new_session->session_ticket_len = 0;
	new_session->session_ticket = NULL;
	p  ++;

	if(new_session->session_id_len > 0){
		f->resume_session = 1;
		memcpy(new_session->session_id, p, new_session->session_id_len);
		new_session->next = NULL;
#ifdef DEBUG_HS
		printf("Requested new session (%x:%d -> %x:%d)\n", f->src_ip.s_addr, f->src_port, f->dst_ip.s_addr, f->dst_port);
		printf("session id: \n");
		for(int i=0; i< new_session->session_id_len; i++){
			printf("%02x ", p[i]);
		}
		printf("\n");
#endif

		f->current_session = new_session;
	}

	p += new_session->session_id_len;
	
	//check to see if there is a session ticket included

	//skip to extensions
	uint16_t ciphersuite_len = (p[0] << 8) + p[1];
	p += 2 + ciphersuite_len;
	uint8_t compress_meth_len = p[0];
	p += 1 + compress_meth_len;
	
	//search for SessionTicket TLS extension
	if(2 + SSL3_RANDOM_SIZE + new_session->session_id_len + 1 + 2 + ciphersuite_len + 1 + compress_meth_len > len){
		//no extension
		if(f->current_session == NULL)
			free(new_session);
		return 0;
	}
	uint16_t extensions_len = (p[0] << 8) + p[1];
	p += 2;
	while(extensions_len > 0){
		uint16_t type = (p[0] << 8) + p[1];
		p += 2;
		uint16_t ext_len = (p[0] << 8) + p[1];
		p += 2;
		if(type == 0x23){
			if(ext_len > 0){
				f->resume_session = 1;
				new_session->session_ticket_len = ext_len;
				new_session->session_ticket = ecalloc(1, ext_len);
				memcpy(new_session->session_ticket, p, ext_len);
				f->current_session = new_session;
			}
		}
		p += ext_len;
		extensions_len -= (4 + ext_len);
	}

	if(!f->resume_session){
		free(new_session);
		f->stall = 0; //unstall the next packet
	}

	return 0;
}
	

/* Called from ServerHello during full handshake. Adds the session id to the
 * cache for later resumptions
 *
 *  Input:
 *  	f: the tagged flow
 *  	hs: a pointer to the ServerHello message
 *
 *  Output:
 *  	0 if success, 1 if failed
 */
int save_session_id(flow *f, uint8_t *hs){

	//increment pointer to point to sessionid
	uint8_t *p = hs + HANDSHAKE_HEADER_LEN;
	p += 2; //skip version
	p += SSL3_RANDOM_SIZE; //skip random
	
	session *new_session = emalloc(sizeof(session));

	new_session->session_id_len = (uint8_t) p[0];
	if((new_session->session_id_len <= 0) || (new_session->session_id_len > SSL_MAX_SSL_SESSION_ID_LENGTH)){
		//if this value is zero, the session is non-resumable or the
		//server will issue a NewSessionTicket handshake message
		free(new_session);
		return 0;
	}
	p++;
	memcpy(new_session->session_id, p, new_session->session_id_len);
    new_session->session_ticket_len = 0;
	new_session->session_ticket = NULL;
	new_session->next = NULL;

	if(f->current_session != NULL){
		free(f->current_session);
	}
	f->resume_session = 0;
	f->current_session = new_session;

	if(sessions->first_session == NULL){
		sessions->first_session = new_session;
		printf("First session id (%p->%p):", sessions, sessions->first_session);
		for(int i=0; i< new_session->session_id_len; i++){
			printf(" %02x", sessions->first_session->session_id[i]);
		}
		printf("\n");
	} else {
		session *last = sessions->first_session;

		for(int i=0; i< sessions->length -1; i++){
			if(last == NULL){
				printf("UH OH: last is null?\n");
				fflush(stdout);
			}
			last = last->next;
		}
		last->next = new_session;
	}

	sessions->length ++;

#ifdef DEBUG_HS
	printf("Saved session id:");
	for(int i=0; i< new_session->session_id_len; i++){
		printf(" %02x", new_session->session_id[i]);
	}
	printf("\n");

	printf("THERE ARE NOW %d saved sessions\n", sessions->length);

#endif

	return 0;

}

/* Called from NewSessionTicket. Adds the session ticket to the
 * cache for later resumptions
 *
 *  Input:
 *  	f: the tagged flow
 *  	hs: a pointer to the ServerHello message
 *
 *  Output:
 *  	0 if success, 1 if failed
 */
int save_session_ticket(flow *f, uint8_t *hs, uint32_t len){
#ifdef DEBUG_HS
	printf("TICKET HDR:");
	for(int i=0; i< HANDSHAKE_HEADER_LEN; i++){
		printf("%02x ", hs[i]);
	}
	printf("\n");
#endif
	uint8_t *p = hs + HANDSHAKE_HEADER_LEN;
	p += 4;
	session *new_session = ecalloc(1, sizeof(session));

	new_session->session_id_len = 0;
	
	new_session->session_ticket_len = (p[0] << 8) + p[1];
    new_session->next = NULL;
	p += 2;

	uint8_t *ticket = emalloc(new_session->session_ticket_len);

	memcpy(ticket, p, new_session->session_ticket_len);
	new_session->session_ticket = ticket;
	memcpy(new_session->master_secret, f->master_secret, SSL3_MASTER_SECRET_SIZE);

	if(sessions->first_session == NULL){
		sessions->first_session = new_session;
	} else {
		session *last = sessions->first_session;

		for(int i=0; i< (sessions->length-1); i++){
			if(last == NULL){
				printf("UH OH: last is null?\n");
				fflush(stdout);
			}
			last = last->next;
		}
		last->next = new_session;
	}

	sessions->length ++;

#ifdef DEBUG_HS
	printf("Saved session ticket:");
	for(int i=0; i< new_session->session_ticket_len; i++){
		printf(" %02x", p[i]);
	}
	printf("\n");
	fflush(stdout);

	printf("Saved session master secret:");
	for(int i=0; i< SSL3_MASTER_SECRET_SIZE; i++){
		printf(" %02x", new_session->master_secret[i]);
	}
	printf("\n");
	fflush(stdout);

	printf("THERE ARE NOW %d saved sessions (2)\n", sessions->length);
	fflush(stdout);

#endif
	return 0;
}

/* Adds a (handshake) packet to the flow's packet chain. If it can complete a record, passes
 * this record to update_flow
 *
 * Note: the code in slitheen-proxy.c should ensure that this function only ever gets the next
 * expected sequence number
 */
int add_packet(flow *f, struct packet_info *info){
    if (info->tcp_hdr == NULL || info->app_data_len <= 0){
        return 0;
    }

    packet *new_packet = emalloc(sizeof(packet));

    new_packet->seq_num = ntohl(info->tcp_hdr->sequence_num);
    new_packet->len = info->app_data_len;

    uint8_t *packet_data = emalloc(new_packet->len);
    memcpy(packet_data, info->app_data, new_packet->len);

    new_packet->data = packet_data;
    new_packet->next = NULL;
    uint8_t incoming = (info->ip_hdr->src.s_addr == f->src_ip.s_addr) ? 0 : 1;
    packet_chain *chain = (incoming) ? f->ds_packet_chain : f->us_packet_chain;
    queue *packet_queue = (incoming) ? f->ds_hs_queue : f->us_hs_queue;

    if(new_packet->seq_num < chain->expected_seq_num){
        //see if this packet contains any data we are missing
        //TODO: figure out how/why this happens and what should follow
        printf("ERROR: Received replayed packet O.o\n");

        free(new_packet->data);
        free(new_packet);
        remove_flow(f);
        return 1;

    }
        
    if(new_packet->seq_num > chain->expected_seq_num) {
        printf("ERROR: Received future packet O.o\n");
        free(new_packet->data);
        free(new_packet);
        remove_flow(f);
        return 1;
    }
    
    //temporary: see if it's the only packet, if so is new record
    if(peek(packet_queue, 0) == NULL){
        if(new_packet->seq_num == chain->expected_seq_num){
            const struct record_header *record_hdr = (struct record_header *) new_packet->data;
            chain->record_len = RECORD_LEN(record_hdr)+RECORD_HEADER_LEN;
            chain->remaining_record_len = chain->record_len;
        }
    }

    //append packet to queue
    enqueue(packet_queue, new_packet);


    chain->expected_seq_num += new_packet->len;

    uint32_t record_offset = 0; //offset into record for updating info with any changes
    uint32_t info_offset = 0; //offset into info for updating with changes
    uint32_t info_len = 0; //number of bytes that possibly changed

    //while there is still data left:
    uint32_t available_data = new_packet->len;

    while(available_data > 0){

        //if full record, give to update_flow
        if(chain->remaining_record_len <= new_packet->len){//we have enough to make a record
            chain->remaining_record_len = 0;
            uint8_t *record = emalloc(chain->record_len);
            uint32_t record_len = chain->record_len;
            uint32_t tmp_len = chain->record_len;

            packet *next = peek(packet_queue, 0);
            while(tmp_len > 0){
                if(tmp_len >= next->len){
                    memcpy(record+chain->record_len - tmp_len, next->data, next->len);
                    if(next == new_packet){
                        new_packet = NULL;//TODO: why?
                        record_offset = chain->record_len - tmp_len;
                        info_len = next->len;
                    }

                    tmp_len -= next->len;
                    //remove packet from queue
                    next = dequeue(packet_queue);
                    free(next->data);
                    free(next);
                    next = peek(packet_queue, 0); //TODO: Do we need this???
                    available_data = 0;

                } else { //didn't use up entire packet

                    memcpy(record+chain->record_len - tmp_len, next->data, tmp_len);
                    if(next == new_packet){//TODO: opposite shouldn't happen?
                        record_offset = chain->record_len - tmp_len;
                        info_len = tmp_len;
                    }

                    memmove(next->data, next->data+tmp_len, next->len - tmp_len);
                    next->len -= tmp_len;
                    available_data -= tmp_len;
                    tmp_len = 0;

                    //Last part of packet is a new record
                    const struct record_header *record_hdr = (struct record_header *) next->data;
                    chain->record_len = RECORD_LEN(record_hdr)+RECORD_HEADER_LEN;
                    chain->remaining_record_len = chain->record_len;
#ifdef DEBUG
                    printf("Found record of type %d\n", record_hdr->type);
                    fflush(stdout);
#endif

                }
            }

            //if handshake is complete, send to relay code
            if(f->application == 1){
                //update packet info and send to replace_packet
                struct packet_info *copy_info = copy_packet_info(info);
                copy_info->app_data = record;
                copy_info->app_data_len = record_len;
                replace_packet(f, copy_info);
                free(copy_info->app_data);
                free(copy_info);
            } else {
                if(update_flow(f, record, incoming)){
                    free(record);
                    return 1;//error occurred and flow was removed
                }

                if(f->in_encrypted ==2){
                    //if server finished message was received, copy changes back to packet

#ifdef DEBUG
                    printf("Replacing info->data with finished message (%d bytes).\n", info_len);

                    printf("Previous bytes:\n");
                    for(int i=0; i<info_len; i++){
                        printf("%02x ", info->app_data[info_offset+i]);
                    }
                    printf("\n");
                    printf("New bytes:\n");
                    for(int i=0; i<info_len; i++){
                        printf("%02x ", record[record_offset+i]);
                    }
                    printf("\n");
                    printf("SLITHEEN: Previous packet contents:\n");
                    for(int i=0; i< info->app_data_len; i++){
                        printf("%02x ", info->app_data[i]);
                    }
                    printf("\n");
#endif
                    memcpy(info->app_data+info_offset, record+record_offset, info_len);
#ifdef DEBUG
                    printf("SLITHEEN: Current packet contents:\n");
                    for(int i=0; i< info->app_data_len; i++){
                        printf("%02x ", info->app_data[i]);
                    }
                    printf("\n");
#endif
                    //update TCP checksum
                    tcp_checksum(info);
                }
                free(record);

                if(new_packet != NULL){
                    info_offset += info_len;
                }

            }
        } else {//can't make a full record yet
            chain->remaining_record_len -= new_packet->len;
            available_data = 0;
        }
    } //exhausted new packet len

    return 0;

}