How to compare a byte array with a String or char array?

Projects Programming
1

Hello! This is part of the code, regarding this issue:

#include <Crypto.h>             // AES 128 CBC with pkcs7, RNG, SHA256 and SHA256HMAC  
#include <base64.hpp> 
#define HMAC_KEY_LENGTH 16

byte hmacKey[HMAC_KEY_LENGTH] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};
byte message[HMAC_KEY_LENGTH] = {0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30};

void setup() {
  Serial.begin(115200);
 SHA256HMAC hmac(hmacKey, HMAC_KEY_LENGTH);
    hmac.doUpdate(message, HMAC_KEY_LENGTH);
    byte authCode2[SHA256HMAC_SIZE];
    hmac.doFinal(authCode2);
    
Serial.print("AUTHCODE2 IS : ");
     for (byte i=0; i < SHA256HMAC_SIZE; i++)
  {
      if (authCode2[i]<0x10) {  }
      Serial.print(authCode2[i], HEX); //This prints 9CF3809536BBBC0B3D5536E6247C182BB1DC29C6DDC21D604CFAB92FDF9
  }

  String received = "9CF3809536BBBC0B3D5536E6247C182BB1DC29C6DDC21D604CFAB92FDF9";

  //How to compare?


}

void loop() {
  // put your main code here, to run repeatedly:

}

Basically i want to compare the byte array authCode2 with the string i have called received. I have tried converting the byte array to a String multiple times but it is always wrongly converted. I have tried once to convert the String into a byte array but it also fails when comparing the two of them... Does anyone know of a solution for this?

Thanks

2

memcmp?

3

Serial.print(authCode2[i], HEX);
One problem with this approach is that leading zeros are omitted from hex digits when printed.

Better to compare bytes in memory, as suggested above.

4

You can't directly compare binary bytes to the string of hexadecimal digits.

You could store the constant as binary:

byte received[] = {0x9C, 0xF3, 0x80, 0x95, 0x36, 0xBB, 0xBC, 0x0B, 0x3D, 0x55, 0x36, 0xE6, 0x24, 0x7C, 0x18, 0x2B, 0xB1, 0xDC, 0x29, 0xC6, 0xDD, 0xC2, 0x1D, 0x60, 0x4C, 0xFA, 0xB9, 0x2F, 0xDF, 0x9};

You have an odd number of digits so I suspect a leading 0 has been lost. Try printing out the value as:

  Serial.print("AUTHCODE2 IS : ");
  for (byte i = 0; i < SHA256HMAC_SIZE; i++)
  {
    Serial.print(, "0x");
    Serial.print(authCode2[i], HEX);
  }
5

Thanks a lot, it makes sense now i wasn't thinking straight about the byte array. It's fixed now :slight_smile:

6

What EXACTLY are you looking for when you say "compare"? Equal length? All bytes equal? One byte being less than the other?