Combine 3 separate byte arrays into one byte array

Question

I would like to combine 3 byte arrays into one byte array. I tried the following code and I'm not sure why not all of the arrays are being copied into one byte array.

What seems to be happening is, all of the hmacDigest array and cipher array are being copied, but then a part of the cipher array is being copied again. What I would like to accomplish is combine the cipher array, iv array and the hmacDigest array and store the combined arrays into the combined byte array. Also, I would like to combine those arrays in this order: cipher, iv, hmacdigest

#include "mbed.h"

#include "cyassl/ctaocrypt/hmac.h"
#include "cyassl/ctaocrypt/aes.h"

#include "MbedJSONValue.h"
#include "LinearTempSensor.h"

#include <string>

using namespace std;

Serial pc(USBTX, USBRX);

LinearTempSensor sensor(p20, 300, LinearTempSensor::MCP9701);
MbedJSONValue sensorResults;

Aes enc;
Aes dec;

Hmac hmac;

float Vout, Tav, To, TempValue;

std::string s;

int main() {

    pc.baud(115200);

    const byte key[16] = { 0x6d, 0x6e, 0x62, 0x76, 0x63, 0x78, 0x7a, 0x6c, 0x6b, 0x6a, 0x68, 0x67, 0x66, 0x64, 0x73, 0x61 };
    const byte iv[16]  = { 0x61, 0x73, 0x64, 0x66, 0x67, 0x68, 0x6a, 0x6b, 0x6c, 0x70, 0x6f, 0x69, 0x75, 0x79, 0x74, 0x72 };

    byte plain[128] = { 'a', 'b', 'c', 'd', 'e', 'f' };   // an increment of 16, fill with data
    byte cipher[128];
    byte deciphered[128];

    byte hkey[24] = { 0x8b, 0x21, 0x31, 0x21, 0xb7, 0xbe, 0x33, 0x1a, 0xcf, 0x1f, 0x71, 0x70, 0x45, 0xaf, 0x5c, 0x02, 0xa7, 0xa1, 0x4c, 0x34, 0xd4, 0xbc, 0x4b, 0x4a };    // fill key with keying material
    byte buffer[2048];   // fill buffer with data to digest
    byte hmacDigest[SHA256_DIGEST_SIZE];

    Vout = sensor.Sense();          // Sample data (read sensor)
    Tav  = sensor.GetAverageTemp(); // Calculate average temperature from N samples
    To   = sensor.GetLatestTemp();  // Calculate temperature from the latest sample

    TempValue = sensor.GetAverageTemp();

    //Create JSON
    sensorResults["DATA1"][0] = "Result";
    sensorResults["DATA1"][1] = 5.5;
    sensorResults["DATA2"][0] = "Result";
    sensorResults["DATA2"][1] = 700;
    sensorResults["DATA3"][0] = "Result";
    sensorResults["DATA3"][1] = TempValue;

    //Serialize JSON
    s = sensorResults.serialize();
    //sl = s.size();

    //Print JSON string
    pc.printf("json: %s\r\n", s.c_str());

    //Convert JSON string to a char array to encrypt
    //char *a=new char[s.size()+1];
    plain[s.size()]=0;
    memcpy(plain,s.c_str(),s.size());//<-- Fills plain array with the JSON values

    // encrypt
    AesSetKey(&enc, key, sizeof(key), iv, AES_ENCRYPTION);
    AesCbcEncrypt(&enc, cipher, plain, sizeof(plain));

    HmacSetKey(&hmac, SHA256, hkey, sizeof(key));
    HmacUpdate(&hmac, buffer, sizeof(buffer));
    HmacFinal(&hmac, hmacDigest);

    //cipher now contains the cipher text from the plain text.

    pc.printf("\r\nAES Key: ");
    for(int i=0; i<sizeof(key); i++) 
    {
        //if(i%16==0) pc.printf("\r\n");
        pc.printf("%.2X",key[i]);
    }

    pc.printf("\r\nAES IV: ");
    for(int i=0; i<sizeof(iv); i++) 
    {
        //if(i%16==0) pc.printf("\r\n");
        pc.printf("%.2X",iv[i]);
    }

    pc.printf("\r\nPlain HEX: ");
    for(int i=0; i<sizeof(plain); i++) 
    {
        //if(i%16==0) pc.printf("\r\n");
        pc.printf("%.2X",plain[i]);
    }

    pc.printf("\r\nEncrypted: ");
    for(int i=0; i<sizeof(cipher); i++) 
    {
        //if(i%16==0) pc.printf("\r\n");
        pc.printf("%.2X",cipher[i]);
    }

    pc.printf("\r\nhmacDigest: ");
    for(int i=0; i<sizeof(hmacDigest); i++) 
    {
        //if(i%16==0) pc.printf("\r\n");
        pc.printf("%.2X",hmacDigest[i]);
    }

    // decrypt
    AesSetKey(&dec, key, sizeof(key), iv, AES_DECRYPTION);
    AesCbcDecrypt(&dec, deciphered, cipher, sizeof(cipher));

    pc.printf("\r\nDecrypted: ");
    for(int  i=0; i<sizeof(deciphered); i++) 
    {
        //if(i%16==0) pc.printf("\r\n");
        //pc.printf("%.2X",deciphered[i]);
        pc.printf("%c",deciphered[i]);
    }

    //Combine the EncryptedData + IV + HMAC
    const int S_CIPHER = sizeof(cipher);
    const int S_IV = sizeof(iv);
    const int S_HMACDIGEST = sizeof(hmacDigest);

    const int S_TOTAL = S_CIPHER + S_IV + S_HMACDIGEST;

    byte combined[S_TOTAL];
    //Copy arrays in individually.
    memcpy(combined, cipher, S_CIPHER);
    memcpy(combined, iv, S_IV);
    memcpy(combined, hmacDigest, S_HMACDIGEST);

    pc.printf("\r\nOutput: ");
    for(int i=0; i<sizeof(combined); i++) 
    {
        //if(i%16==0) pc.printf("\r\n");
        //pc.printf("%.2X",deciphered[i]);
        pc.printf("%.2X",combined[i]);
    }   
}

You memcpy arrays always at the same point of combined array. — LPs
– LPs, Commented Jun 8, 2015 at 7:26
Btw. be careful with sizeof()ing, here it is OK as all sizeofed things are fully-typed arrays, but when you change to pointers etc, it will cease to be correct. — quetzalcoatl
– quetzalcoatl, Commented Jun 8, 2015 at 7:32

LPs · Accepted Answer · 2015-06-08 07:30:33Z

6

You memcpy arrays always at the same point of combined array

Change it to:

memcpy(combined, cipher, S_CIPHER);
memcpy(combined+S_CIPHER, iv, S_IV);
memcpy(combined+S_CIPHER+S_IV, hmacDigest, S_HMACDIGEST);

memcpy prototype is

void *memcpy(void *dest, const void *src, size_t n);

You have to move dest pointer to the correct position each time you copy a single array.

edited Jun 8, 2015 at 7:30

answered Jun 8, 2015 at 7:27

LPs

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