Decryptor/LocalAttestationCode/EnclaveMessageExchange.cpp

/*
 * Copyright (C) 2011-2017 Intel Corporation. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   * Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in
 *     the documentation and/or other materials provided with the
 *     distribution.
 *   * Neither the name of Intel Corporation nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */


#include "sgx_trts.h"
#include "sgx_utils.h"
#include "EnclaveMessageExchange.h"
#include "sgx_eid.h"
#include "error_codes.h"
#include "sgx_ecp_types.h"
#include "sgx_thread.h"
#include <map>
#include "dh_session_protocol.h"
#include "sgx_dh.h"
#include "sgx_tcrypto.h"
#include "LocalAttestationCode_t.h"
#include "sgx_tseal.h"

#ifdef __cplusplus
extern "C" {
#endif

uint32_t enclave_to_enclave_call_dispatcher(char* decrypted_data, size_t decrypted_data_length, char** resp_buffer, size_t* resp_length);
uint32_t message_exchange_response_generator(char* decrypted_data, char** resp_buffer, size_t* resp_length);
uint32_t verify_peer_enclave_trust(sgx_dh_session_enclave_identity_t* peer_enclave_identity);

#ifdef __cplusplus
}
#endif

#define MAX_SESSION_COUNT  16

//number of open sessions
// uint32_t g_session_count = 0;

ATTESTATION_STATUS generate_session_id(uint32_t *session_id);
ATTESTATION_STATUS end_session();
sgx_ec256_private_t signing_priv_key;
sgx_ecc_state_handle_t ecc_state;

 uint32_t session_ids[MAX_SESSION_COUNT];

// Our enclave will not be doing LA with more than 1 decryptor enclave at a time.
// We should not need this.
//std::map<int, dh_session_t>g_dest_session_info_map;
dh_session_t global_session_info;
// TODO: May be we need to store all previously assigned session IDs instead of just the counter; to prevent replay attacks -
uint32_t global_session_id=0;


//Handle the request from Source Enclave for a session
ATTESTATION_STATUS session_request(sgx_dh_msg1_t *dh_msg1,
                          uint32_t *session_id )
{
 //   dh_session_t session_info;
    sgx_dh_session_t sgx_dh_session;
    sgx_status_t status = SGX_SUCCESS;

    if(!session_id || !dh_msg1)
    {
        return INVALID_PARAMETER_ERROR;
    }

    //Intialize the session as a session responder
    status = sgx_dh_init_session(SGX_DH_SESSION_RESPONDER, &sgx_dh_session);
    if(SGX_SUCCESS != status)
    {
        return status;
    }

    *session_id=1;

    global_session_info.status = IN_PROGRESS;

    //Generate Message1 that will be returned to Source Enclave
    status = sgx_dh_responder_gen_msg1((sgx_dh_msg1_t*)dh_msg1, &sgx_dh_session);
    if(SGX_SUCCESS != status)
    {
        global_session_id--;
        // SAFE_FREE(g_session_id_tracker[*session_id]);
        return status;
    }
    memcpy(&global_session_info.in_progress.dh_session, &sgx_dh_session, sizeof(sgx_dh_session_t));
    //return sgx_seal_data(0, NULL, 0, NULL, 0, NULL); 
    //Store the session information under the correspoding source enlave id key
    //    g_dest_session_info_map.insert(std::pair<sgx_enclave_id_t, dh_session_t>(src_enclave_id, session_info));

    return status;
}

//Verify Message 2, generate Message3 and exchange Message 3 with Source Enclave
ATTESTATION_STATUS exchange_report(
                          sgx_dh_msg2_t *dh_msg2,
                          sgx_dh_msg3_t *dh_msg3,
                          uint32_t* session_id)
{

    sgx_key_128bit_t dh_aek;   // Session key
//    dh_session_t session_info;
    ATTESTATION_STATUS status = SUCCESS;
    sgx_dh_session_t sgx_dh_session;
    sgx_dh_session_enclave_identity_t initiator_identity;

    if(!dh_msg2 || !dh_msg3)
    {
        return INVALID_PARAMETER_ERROR;
    }

    memset(&dh_aek,0, sizeof(sgx_key_128bit_t));
// Why is there a do-while loop anyway? It seems like there is no successful exit ... 
//    do
//    {
      // TODO: Make sure that this works - pointers
//        session_info = global_session_info;

        if(global_session_info.status != IN_PROGRESS)
        {
            status = INVALID_SESSION;
            end_session(); 
        }

        memcpy(&sgx_dh_session, &global_session_info.in_progress.dh_session, sizeof(sgx_dh_session_t));

        dh_msg3->msg3_body.additional_prop_length = 0;
        //Process message 2 from source enclave and obtain message 3
        sgx_status_t se_ret = sgx_dh_responder_proc_msg2(dh_msg2,
                                                       dh_msg3,
                                                       &sgx_dh_session,
                                                       &dh_aek,
                                                       &initiator_identity);
        if(SGX_SUCCESS != se_ret)
        {
            status = se_ret;
            end_session();
        }

        // THIS IS WHERE THE DECRYPTOR VERIFIES THE APACHE'S MRSIGNER IS THE PUBLIC KEY GIVEN AFTER THE LOCAL ATTESTATION WITH THE VERIFIER.
        //Verify source enclave's trust
        uint32_t ret = verify_peer_enclave_trust(&initiator_identity);
        if(ret != SUCCESS)
        {
            return ret; //INVALID_SESSION;
        }
	
	// TODO: Verify that these changes will be lost on update. 
        //save the session ID, status and initialize the session nonce
        global_session_info.session_id = *session_id;
        global_session_info.status = ACTIVE;
        global_session_info.active.counter = 0;
        memcpy(&global_session_info.active.AEK, &dh_aek, sizeof(sgx_key_128bit_t));
        memset(&dh_aek,0, sizeof(sgx_key_128bit_t));
        //g_session_count++;*/
//    }while(0);


    return status;
}

uint32_t create_ecdsa_key_pair(sgx_ec256_public_t* pub_key, sgx_ec256_private_t* priv_key)
{
  sgx_status_t se_ret; sgx_status_t se_ret2;
  //create ECC context
  ecc_state = NULL;
  se_ret = sgx_ecc256_open_context(&ecc_state);
  if(SGX_SUCCESS != se_ret)
    return se_ret;

  // generate private key and public key
  se_ret = sgx_ecc256_create_key_pair(priv_key, pub_key, ecc_state);
  se_ret2 = sgx_ecc256_close_context(ecc_state);

  if(SGX_SUCCESS != se_ret && se_ret2!= SGX_SUCCESS) // something weird has happened - couldn't shut it down.
    return 0xFFFFFFFF;
  return SGX_SUCCESS;
}

// todo: set to private
// todo: assumes that the length of the keystring is at least 3*SGX_ECP256_KEY_SIZE
void serialize_signing_key_pair_to_string(sgx_ec256_public_t* pub_key, sgx_ec256_private_t* signing_priv_key, uint8_t* private_public_key_string)
{
  if(private_public_key_string != NULL)  // nowhere to serialize to
  {
     uint32_t counter;
     if(pub_key != NULL)  // public key to serialize
     {
        for(counter=0;counter<SGX_ECP256_KEY_SIZE; counter++)
          *(private_public_key_string+counter)=pub_key->gx[counter];

        for(counter=SGX_ECP256_KEY_SIZE;counter<2*SGX_ECP256_KEY_SIZE; counter++)
           *(private_public_key_string+counter)=pub_key->gy[counter-SGX_ECP256_KEY_SIZE];
     }

     if(signing_priv_key != NULL) // private key to serialize
     {
       for(counter=2*SGX_ECP256_KEY_SIZE;counter<3*SGX_ECP256_KEY_SIZE; counter++)
          *(private_public_key_string+counter)=signing_priv_key->r[counter - 2*SGX_ECP256_KEY_SIZE];
     }
/*
     if(pub_key != NULL)  // public key to serialize
     {
        for(counter=SGX_ECP256_KEY_SIZE;counter<2*SGX_ECP256_KEY_SIZE; counter++)
          *(private_public_key_string+counter)=pub_key->gx[counter-SGX_ECP256_KEY_SIZE];

        for(counter=2*SGX_ECP256_KEY_SIZE;counter<3*SGX_ECP256_KEY_SIZE; counter++)
           *(private_public_key_string+counter)=pub_key->gy[counter-2*SGX_ECP256_KEY_SIZE];
     }*/
  }
}


// todo: set to private
void deserialize_string_to_public_private_key_pair(uint8_t* private_public_key_string, sgx_ec256_public_t* pub_key, sgx_ec256_private_t* signing_priv_key)
{
  if(private_public_key_string != NULL) // nowhere to deserialize from 
  {
    uint32_t counter;
    if(signing_priv_key != NULL) 
    {

     for(counter=2*SGX_ECP256_KEY_SIZE;counter<3*SGX_ECP256_KEY_SIZE; counter++)
       signing_priv_key->r[counter-2*SGX_ECP256_KEY_SIZE]=*(private_public_key_string+counter);
    }

    if(pub_key != NULL)
    {
      for(counter=0;counter<SGX_ECP256_KEY_SIZE; counter++)
        	pub_key->gx[counter]=*(private_public_key_string+counter);

      for(counter=SGX_ECP256_KEY_SIZE;counter<2*SGX_ECP256_KEY_SIZE; counter++)
  	pub_key->gy[counter-SGX_ECP256_KEY_SIZE]=*(private_public_key_string+counter);
    }
  }
}


uint32_t create_and_seal_ecdsa_signing_key_pair(__attribute__((unused))   sgx_ec256_public_t* pub_key, __attribute__((unused))  uint32_t* sealed_data_length, __attribute__((unused)) uint8_t* sealed_data)
{
    uint32_t ret_status; sgx_ec256_private_t private_key;  uint32_t counter;
    ret_status=create_ecdsa_key_pair(pub_key, &private_key);
    if(ret_status!=SGX_SUCCESS)
       return ret_status;
    for(counter=0;counter<SGX_ECP256_KEY_SIZE; counter++)
	signing_priv_key.r[counter]=private_key.r[counter]; 
    // generating the entire string as there is no SGX function to generate the public key from the private one.
    uint8_t* private_public_key_string = (uint8_t*) malloc(3*SGX_ECP256_KEY_SIZE);
    uint8_t* sealed_data2 = (uint8_t*) malloc(*sealed_data_length);
    // serializing keypair to string
    serialize_signing_key_pair_to_string(pub_key, &private_key, private_public_key_string);
    uint8_t* private_key_string = (uint8_t*) malloc(SGX_ECP256_KEY_SIZE); 
    for(counter=0;counter<SGX_ECP256_KEY_SIZE;counter++)
	*(private_key_string+counter)=private_key.r[counter];
//    return *sealed_data_length;
    ret_status = sgx_seal_data(0, NULL, 3*SGX_ECP256_KEY_SIZE, private_public_key_string, *sealed_data_length, (sgx_sealed_data_t*) sealed_data2); 
    for(counter=0;counter<*sealed_data_length;counter++)
	*(sealed_data+counter)=*(sealed_data2+counter); 
    free(sealed_data2);
    free(private_key_string);  //free(private_key);  
    free(private_public_key_string);

    return ret_status; // SGX_SUCCESS; 
}

/*
uint32_t unseal_and_restore_sealed_signing_key_pair(__attribute__((unused)) sgx_ec256_public_t* pub_key, __attribute__((unused))  uint8_t* sealed_data, __attribute__((unused))   uint32_t* sgx_sealed_data_length)
{
	return SGX_SUCCESS; 

}*/

uint32_t unseal_and_restore_sealed_signing_key_pair(__attribute__((unused)) sgx_ec256_public_t* pub_key, uint8_t* sealed_data, size_t* sgx_sealed_data_length)
{
  uint32_t expected_plaintext_msg_length; uint8_t* temp_plaintext; uint32_t counter; uint32_t ret_status;
  expected_plaintext_msg_length = sgx_get_encrypt_txt_len((sgx_sealed_data_t*)sealed_data);
  if(expected_plaintext_msg_length == 0xffffffff)
    return 0xFFFFFFFF;

//  uint32_t return_status; 
  uint8_t* sealed_data2 = (uint8_t*) malloc(*sgx_sealed_data_length); 
  for(counter=0;counter<*sgx_sealed_data_length;counter++)
  {
	*(sealed_data2+counter)=*(sealed_data+counter); 
  }

  temp_plaintext = (uint8_t*)malloc( expected_plaintext_msg_length );
  ret_status = sgx_unseal_data((sgx_sealed_data_t*)sealed_data2, NULL, 0, temp_plaintext, &expected_plaintext_msg_length);
  if(ret_status != SGX_SUCCESS)
  {
    free(temp_plaintext);free(sealed_data2);
    switch(ret_status)
    {
      case SGX_ERROR_MAC_MISMATCH:
          // MAC of the sealed data is incorrect.          The sealed data has been tampered.
          break;
      case SGX_ERROR_INVALID_ATTRIBUTE:
          // Indicates attribute field of the sealed data is incorrect.
          break;
      case SGX_ERROR_INVALID_ISVSVN:
          // Indicates isv_svn field of the sealed data is greater than            the enclave�s ISVSVN. This is a downgraded enclave.
          break;
      case SGX_ERROR_INVALID_CPUSVN:
          // Indicates cpu_svn field of the sealed data is greater than            the platform�s cpu_svn. enclave is  on a downgraded platform.
          break;
      case SGX_ERROR_INVALID_KEYNAME:
          // Indicates key_name field of the sealed data is incorrect.
          break;
      default:
          // other errors
          break;
    }
    return ret_status;
  }
   
  deserialize_string_to_public_private_key_pair(temp_plaintext, pub_key, &signing_priv_key);
  free(temp_plaintext); free(sealed_data2);  
  return SGX_SUCCESS;
}



uint32_t calculate_sealed_data_size( uint32_t input_size) 
{
//	*op_size=sgx_calc_sealed_data_size(0, input_size); 
	return sgx_calc_sealed_data_size(0, input_size);

}
























/*
uint32_t create_ecdsa_key_pair( sgx_ec256_public_t* pub_key ) 
{
//sgx_ec256_public_t pub_key;
sgx_status_t se_ret; 
//create ECC context
ecc_state = NULL;

se_ret = sgx_ecc256_open_context(&ecc_state);
if(SGX_SUCCESS != se_ret)
{
   return se_ret;
}
// generate private key and public key
se_ret = sgx_ecc256_create_key_pair(&signing_priv_key, pub_key, ecc_state);
if(SGX_SUCCESS != se_ret)
  return se_ret; 

se_ret = sgx_ecc256_close_context(ecc_state); 
// if(SGX_SUCCESS != se_ret)
//  return se_ret; 

return se_ret; 

}
*/
/*
uint32_t generate_and_seal_signing_private_key(uint8_t* pub_key, )
{
    uint32_t ret_status; 
    ret_status=create_ecdsa_key_pair(pub_key); 
    if(ret_status!=SGX_SUCCESS) 
       return ret_status; 
    uint8_t* public_key_string = (uint8_t*) malloc(2*SGX_ECP256_KEY_SIZE); 
    uint32_t counter;
    for(counter=0;counter<SGX_ECP256_KEY_SIZE; counter++)
    {
	*(public_key_string+counter)=pub_key->gx[counter];
    }
    for(counter=SGX_ECP256_KEY_SIZE;counter<2*SGX_ECP256_KEY_SIZE; counter++)
    {
        *(public_key_string+counter)=pub_key->gy[counter];
    }

    // sgx_seal_data() call 
    uint32_t expected_sealed_data_length=sgx_calc_sealed_data_size(0,2*ECP256_KEY_SIZE); 
    if(expected_sealed_data_length == 0xFFFFFFFF)
	return 0xFFFFFFFF; 
    uint8_t* sealed_data=(uint8_t*) malloc(expected_sealed_data_length);
    sgx_sealed_data_t sealed_data; 
    sgx_seal_data(0, NULL, public_key_string, 2*ECP256_KEY_SIZE, );
    free(public_key_string); 

}
*/
/*
uint32_t sign_with_signing_private_key(uint8_t* data, uint8_t* length) 
{



}
*/
// uint32_t create_


/*
//Request for the response size, send the request message to the destination enclave and receive the response message back
ATTESTATION_STATUS send_request_receive_response(
                                  sgx_enclave_id_t dest_enclave_id,
                                  dh_session_t *session_info,
                                  char *inp_buff,
                                  size_t inp_buff_len,
                                  size_t max_out_buff_size,
                                  char **out_buff,
                                  size_t* out_buff_len)
{
    const uint8_t* plaintext;
    uint32_t plaintext_length;
    sgx_status_t status;
    uint32_t retstatus;
    secure_message_t* req_message;
    secure_message_t* resp_message;
    uint8_t *decrypted_data;
    uint32_t decrypted_data_length;
    uint32_t plain_text_offset;
    uint8_t l_tag[TAG_SIZE];
    size_t max_resp_message_length;
    plaintext = (const uint8_t*)(" ");
    plaintext_length = 0;

    if(!session_info || !inp_buff)
    {
        return INVALID_PARAMETER_ERROR;
    }
    // TODO: Figure out what this was supposed to be for.
    //Check if the nonce for the session has not exceeded 2^32-2 if so end session and start a new session
    if(session_info->active.counter == ((uint32_t) - 2))
    {
        close_session(src_enclave_id, dest_enclave_id);
        create_session(src_enclave_id, dest_enclave_id, session_info);
    }

    //Allocate memory for the AES-GCM request message
    req_message = (secure_message_t*)malloc(sizeof(secure_message_t)+ inp_buff_len);
    if(!req_message)
    {
        return MALLOC_ERROR;
    }

    memset(req_message,0,sizeof(secure_message_t)+ inp_buff_len);
    const uint32_t data2encrypt_length = (uint32_t)inp_buff_len;
    //Set the payload size to data to encrypt length
    req_message->message_aes_gcm_data.payload_size = data2encrypt_length;

    //Use the session nonce as the payload IV
    memcpy(req_message->message_aes_gcm_data.reserved,&session_info->active.counter,sizeof(session_info->active.counter));

    //Set the session ID of the message to the current session id
    req_message->session_id = session_info->session_id;

    //Prepare the request message with the encrypted payload
    status = sgx_rijndael128GCM_encrypt(&session_info->active.AEK, (uint8_t*)inp_buff, data2encrypt_length,
                reinterpret_cast<uint8_t *>(&(req_message->message_aes_gcm_data.payload)),
                reinterpret_cast<uint8_t *>(&(req_message->message_aes_gcm_data.reserved)),
                sizeof(req_message->message_aes_gcm_data.reserved), plaintext, plaintext_length,
                &(req_message->message_aes_gcm_data.payload_tag));

    if(SGX_SUCCESS != status)
    {
        SAFE_FREE(req_message);
        return status;
    }

    //Allocate memory for the response payload to be copied
    *out_buff = (char*)malloc(max_out_buff_size);
    if(!*out_buff)
    {
        SAFE_FREE(req_message);
        return MALLOC_ERROR;
    }

    memset(*out_buff, 0, max_out_buff_size);

    //Allocate memory for the response message
    resp_message = (secure_message_t*)malloc(sizeof(secure_message_t)+ max_out_buff_size);
    if(!resp_message)
    {
        SAFE_FREE(req_message);
        return MALLOC_ERROR;
    }

    memset(resp_message, 0, sizeof(secure_message_t)+ max_out_buff_size);

    // TODO: This should not exist.
    //Ocall to send the request to the Destination Enclave and get the response message back
    status = send_request_ocall(&retstatus, src_enclave_id, dest_enclave_id, req_message,
                                (sizeof(secure_message_t)+ inp_buff_len), max_out_buff_size,
                                resp_message, (sizeof(secure_message_t)+ max_out_buff_size));
    if (status == SGX_SUCCESS)
    {
        if ((ATTESTATION_STATUS)retstatus != SUCCESS)
        {
            SAFE_FREE(req_message);
            SAFE_FREE(resp_message);
            return ((ATTESTATION_STATUS)retstatus);
        }
    }
    else
    {
        SAFE_FREE(req_message);
        SAFE_FREE(resp_message);
        return ATTESTATION_SE_ERROR;
    }

    max_resp_message_length = sizeof(secure_message_t)+ max_out_buff_size;

    if(sizeof(resp_message) > max_resp_message_length)
    {
        SAFE_FREE(req_message);
        SAFE_FREE(resp_message);
        return INVALID_PARAMETER_ERROR;
    }

    //Code to process the response message from the Destination Enclave

    decrypted_data_length = resp_message->message_aes_gcm_data.payload_size;
    plain_text_offset = decrypted_data_length;
    decrypted_data = (uint8_t*)malloc(decrypted_data_length);
    if(!decrypted_data)
    {
        SAFE_FREE(req_message);
        SAFE_FREE(resp_message);
        return MALLOC_ERROR;
    }
    memset(&l_tag, 0, 16);

    memset(decrypted_data, 0, decrypted_data_length);

    //Decrypt the response message payload
    status = sgx_rijndael128GCM_decrypt(&session_info->active.AEK, resp_message->message_aes_gcm_data.payload,
                decrypted_data_length, decrypted_data,
                reinterpret_cast<uint8_t *>(&(resp_message->message_aes_gcm_data.reserved)),
                sizeof(resp_message->message_aes_gcm_data.reserved), &(resp_message->message_aes_gcm_data.payload[plain_text_offset]), plaintext_length,
                &resp_message->message_aes_gcm_data.payload_tag);

    if(SGX_SUCCESS != status)
    {
        SAFE_FREE(req_message);
        SAFE_FREE(decrypted_data);
        SAFE_FREE(resp_message);
        return status;
    }

    // Verify if the nonce obtained in the response is equal to the session nonce + 1 (Prevents replay attacks)
    if(*(resp_message->message_aes_gcm_data.reserved) != (session_info->active.counter + 1 ))
    {
        SAFE_FREE(req_message);
        SAFE_FREE(resp_message);
        SAFE_FREE(decrypted_data);
        return INVALID_PARAMETER_ERROR;
    }

        //Update the value of the session nonce in the source enclave
    session_info->active.counter = session_info->active.counter + 1;

    memcpy(out_buff_len, &decrypted_data_length, sizeof(decrypted_data_length));
    memcpy(*out_buff, decrypted_data, decrypted_data_length);

    SAFE_FREE(decrypted_data);
    SAFE_FREE(req_message);
    SAFE_FREE(resp_message);
    return SUCCESS;


}
*/

/*
//Process the request from the Source enclave and send the response message back to the Source enclave
ATTESTATION_STATUS generate_response(sgx_enclave_id_t src_enclave_id,
                                     secure_message_t* req_message,
                                     size_t req_message_size,
                                     size_t max_payload_size,
                                     secure_message_t* resp_message,
                                     size_t resp_message_size)
{
    const uint8_t* plaintext;
    uint32_t plaintext_length;
    uint8_t *decrypted_data;
    uint32_t decrypted_data_length;
    uint32_t plain_text_offset;
    ms_in_msg_exchange_t * ms;
    size_t resp_data_length;
    size_t resp_message_calc_size;
    char* resp_data;
    uint8_t l_tag[TAG_SIZE];
    size_t header_size, expected_payload_size;
    dh_session_t *session_info;
    secure_message_t* temp_resp_message;
    uint32_t ret;
    sgx_status_t status;

    plaintext = (const uint8_t*)(" ");
    plaintext_length = 0;

    if(!req_message || !resp_message)
    {
        return INVALID_PARAMETER_ERROR;
    }
// TODO: Set session_info from somewhere.

    //Get the session information from the map corresponding to the source enclave id
    std::map<sgx_enclave_id_t, dh_session_t>::iterator it = g_dest_session_info_map.find(src_enclave_id);
    if(it != g_dest_session_info_map.end())
    {
        session_info = &it->second;
    }
    else
    {
        return INVALID_SESSION;
    }

    if(session_info->status != ACTIVE)
    {
        return INVALID_SESSION;
    }

    //Set the decrypted data length to the payload size obtained from the message
    decrypted_data_length = req_message->message_aes_gcm_data.payload_size;

    header_size = sizeof(secure_message_t);
    expected_payload_size = req_message_size - header_size;

    //Verify the size of the payload
    if(expected_payload_size != decrypted_data_length)
        return INVALID_PARAMETER_ERROR;

    memset(&l_tag, 0, 16);
    plain_text_offset = decrypted_data_length;
    decrypted_data = (uint8_t*)malloc(decrypted_data_length);
    if(!decrypted_data)
    {
            return MALLOC_ERROR;
    }

    memset(decrypted_data, 0, decrypted_data_length);

    //Decrypt the request message payload from source enclave
    status = sgx_rijndael128GCM_decrypt(&session_info->active.AEK, req_message->message_aes_gcm_data.payload,
                decrypted_data_length, decrypted_data,
                reinterpret_cast<uint8_t *>(&(req_message->message_aes_gcm_data.reserved)),
                sizeof(req_message->message_aes_gcm_data.reserved), &(req_message->message_aes_gcm_data.payload[plain_text_offset]), plaintext_length,
                &req_message->message_aes_gcm_data.payload_tag);

    if(SGX_SUCCESS != status)
    {
        SAFE_FREE(decrypted_data);
        return status;
    }

    //Casting the decrypted data to the marshaling structure type to obtain type of request (generic message exchange/enclave to enclave call)
    ms = (ms_in_msg_exchange_t *)decrypted_data;


    // Verify if the nonce obtained in the request is equal to the session nonce
    if((uint32_t)*(req_message->message_aes_gcm_data.reserved) != session_info->active.counter || *(req_message->message_aes_gcm_data.reserved) > ((2^32)-2))
    {
        SAFE_FREE(decrypted_data);
        return INVALID_PARAMETER_ERROR;
    }

    if(ms->msg_type == MESSAGE_EXCHANGE)
    {
        //Call the generic secret response generator for message exchange
        ret = message_exchange_response_generator((char*)decrypted_data, &resp_data, &resp_data_length);
        if(ret !=0)
        {
            SAFE_FREE(decrypted_data);
            SAFE_FREE(resp_data);
            return INVALID_SESSION;
        }
    }
    else if(ms->msg_type == ENCLAVE_TO_ENCLAVE_CALL)
    {
        //Call the destination enclave's dispatcher to call the appropriate function in the destination enclave
        ret = enclave_to_enclave_call_dispatcher((char*)decrypted_data, decrypted_data_length, &resp_data, &resp_data_length);
        if(ret !=0)
        {
            SAFE_FREE(decrypted_data);
            SAFE_FREE(resp_data);
            return INVALID_SESSION;
        }
    }
    else
    {
        SAFE_FREE(decrypted_data);
        return INVALID_REQUEST_TYPE_ERROR;
    }


    if(resp_data_length > max_payload_size)
    {
        SAFE_FREE(resp_data);
        SAFE_FREE(decrypted_data);
        return OUT_BUFFER_LENGTH_ERROR;
    }

    resp_message_calc_size = sizeof(secure_message_t)+ resp_data_length;

    if(resp_message_calc_size > resp_message_size)
    {
        SAFE_FREE(resp_data);
        SAFE_FREE(decrypted_data);
        return OUT_BUFFER_LENGTH_ERROR;
    }

    //Code to build the response back to the Source Enclave
    temp_resp_message = (secure_message_t*)malloc(resp_message_calc_size);
    if(!temp_resp_message)
    {
            SAFE_FREE(resp_data);
            SAFE_FREE(decrypted_data);
            return MALLOC_ERROR;
    }

    memset(temp_resp_message,0,sizeof(secure_message_t)+ resp_data_length);
    const uint32_t data2encrypt_length = (uint32_t)resp_data_length;
    temp_resp_message->session_id = session_info->session_id;
    temp_resp_message->message_aes_gcm_data.payload_size = data2encrypt_length;

    //Increment the Session Nonce (Replay Protection)
    session_info->active.counter = session_info->active.counter + 1;

    //Set the response nonce as the session nonce
    memcpy(&temp_resp_message->message_aes_gcm_data.reserved,&session_info->active.counter,sizeof(session_info->active.counter));

    //Prepare the response message with the encrypted payload
    status = sgx_rijndael128GCM_encrypt(&session_info->active.AEK, (uint8_t*)resp_data, data2encrypt_length,
                reinterpret_cast<uint8_t *>(&(temp_resp_message->message_aes_gcm_data.payload)),
                reinterpret_cast<uint8_t *>(&(temp_resp_message->message_aes_gcm_data.reserved)),
                sizeof(temp_resp_message->message_aes_gcm_data.reserved), plaintext, plaintext_length,
                &(temp_resp_message->message_aes_gcm_data.payload_tag));

    if(SGX_SUCCESS != status)
    {
        SAFE_FREE(resp_data);
        SAFE_FREE(decrypted_data);
        SAFE_FREE(temp_resp_message);
        return status;
    }

    memset(resp_message, 0, sizeof(secure_message_t)+ resp_data_length);
    memcpy(resp_message, temp_resp_message, sizeof(secure_message_t)+ resp_data_length);

    SAFE_FREE(decrypted_data);
    SAFE_FREE(resp_data);
    SAFE_FREE(temp_resp_message);

    return SUCCESS;
}
*/
/*
//Close a current session
ATTESTATION_STATUS close_session(sgx_enclave_id_t src_enclave_id,
                        sgx_enclave_id_t dest_enclave_id)
{
    sgx_status_t status;

    uint32_t retstatus;

    //Ocall to ask the destination enclave to end the session
    status = end_session_ocall(&retstatus, src_enclave_id, dest_enclave_id);
    if (status == SGX_SUCCESS)
    {
        if ((ATTESTATION_STATUS)retstatus != SUCCESS)
            return ((ATTESTATION_STATUS)retstatus);
    }
    else
    {
        return ATTESTATION_SE_ERROR;
    }
    return SUCCESS;
}
*/
// TODO: Fix this.
//Respond to the request from the Source Enclave to close the session
ATTESTATION_STATUS end_session(/**/)
{
  return SUCCESS;
}
/*
// Session_id is set to the first index of the pointer array that is non-null.(Not sure how it is ensured that all of them point to NULL at the start)
// Why can't it just keep a counter that is incremented? What are the values of g_session_id_tracker array?
//Returns a new sessionID for the source destination session
ATTESTATION_STATUS generate_session_id(uint32_t *session_id)
{
    ATTESTATION_STATUS status = SUCCESS;

    if(!session_id)
    {
        return INVALID_PARAMETER_ERROR;
    }
    //if the session structure is untintialized, set that as the next session ID
    for (int i = 0; i < MAX_SESSION_COUNT; i++)
    {
        if (g_session_id_tracker[i] == NULL)
        {
            *session_id = i;
            return status;
        }
    }

    status = NO_AVAILABLE_SESSION_ERROR;

    return status;
*/
  // *session_id=++global_session_id;
//}