Converting unsigned char to hex incorrect. Last value will always increase + 1

Question

I have issue that i cant figure out why.

uint8_t nonce[17] = "1234567890123456";
String cipherText = String();
for (int i = 0; i < 16; i++) {
     char str[3];
     sprintf(str, "%02X", nonce[i]);
     cipherText = cipherText+String(str);
}
System.println(cipherText);

result is: "31323334353637383930313233343537"

when i use hex converter online to decode back, the string result is "1234567890123457" which is not the same as the the initial chars. using bytes {0x01}, {0x02}.. with the same result.

this code was a part that pass as a iv to a cipher encryption. hex converter online: https://string-functions.com/hex-string.aspx

The last value will always increase + 1. I have tried other approach to loop with the same result. I also have use to convert the unsigned char to base64 and decode and have the same issue. Anyone can help whats wrong with this codes?

Answer 1

figure out that the library that was used modifies the unsigned char that was pass to it. I have to use a copy of the var to have the correct HEX value other then the one i use to pass to the lib.

it was mentioned by the user chegewara in the git example here: here

static void cbc()
{
#if defined(MBEDTLS_CIPHER_MODE_CBC)
    unsigned char iv[] = {0xff, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f};
    unsigned char iv1[] = {0xff, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f};
    unsigned char encrypt_output[INPUT_LENGTH] = {0};
    unsigned char decrypt_output[INPUT_LENGTH] = {0};
    mbedtls_aes_crypt_cbc(&aes, MBEDTLS_AES_ENCRYPT, INPUT_LENGTH, iv, input, encrypt_output);
    mbedtls_aes_crypt_cbc(&aes, MBEDTLS_AES_DECRYPT, INPUT_LENGTH, iv1, encrypt_output, decrypt_output);
    ESP_LOG_BUFFER_HEX("cbc", encrypt_output, INPUT_LENGTH);
    ESP_LOG_BUFFER_HEX("cbc", decrypt_output, INPUT_LENGTH);
    ESP_LOGI("cbc", "%s", decrypt_output);
#endif
}

"Im guessing you made common mistake, which i did when i made that example too. Then IV is changed during encryption/decryption by code, which means you cant use the same char array to do both in the same code. Best way is to keep 1 copy as const char array, then before encrypt/decrypt copy it into arrays used in process." - chegewara