How to convert an unsigned char to string

Question

I am working with the Mbed TLS Library on an ESP32 board and a function i am using takes only unsigned char and also gives unsigned char as the output

  mbedtls_aes_setkey_enc( &aes, key, 256 );
  mbedtls_aes_crypt_cbc( &aes, MBEDTLS_AES_ENCRYPT, 48, iv, input, output );

I now need to display output using this method

Heltec.display->drawString(0, 0, String(output));

but here it only takes strings type... how do i go about converting the unsigned char to string here? or is there a better way to display the output (I am using a Heltec LoRa32 board with a 0.96-inch display)

here is the full code

#include "mbedtls/aes.h"
#include "heltec.h"

mbedtls_aes_context aes;

unsigned char key[32];
unsigned char iv[16];

unsigned char input [128];
unsigned char output[128];

size_t input_len = 40;
size_t output_len = 0;

void setup() {
  Heltec.display->init();
  Heltec.display->flipScreenVertically();
  Heltec.display->setFont(ArialMT_Plain_10);
  delay(1500);
  Heltec.display->clear();

  Heltec.display->drawString(0, 0, "Heltec.LoRa Initial success!");
  Heltec.display->display();
  delay(1000);

  mbedtls_aes_setkey_enc( &aes, key, 256 );
  mbedtls_aes_crypt_cbc( &aes, MBEDTLS_AES_ENCRYPT, 48, iv, input, output );
  }

void loop() {
  Heltec.display->clear();
  Heltec.display->setTextAlignment(TEXT_ALIGN_LEFT);
  Heltec.display->setFont(ArialMT_Plain_10);
  Heltec.display->drawString(0, 0, String(output);
  Heltec.display->display();
}

String((char*)output)? (assuming output is null terminated) — Alan Birtles
– Alan Birtles, Commented Apr 20, 2022 at 17:25
It looks like output is binary (encrypted) data, in which case displaying it as-is is not a good idea. You likely want to encode the raw bytes as a hex-encoded string first, then display that instead. — Remy Lebeau
– Remy Lebeau, Commented Apr 20, 2022 at 17:26

maark · Accepted Answer · 2022-04-20 18:35:12Z

0

mbedtls_aes_crypt_cbc has 'strings' (unsigned char arrays) as input and output, not a single character:

mbedtls_aes_crypt_cbc (mbedtls_aes_context *ctx, int mode, size_t length, unsigned char iv[16], const unsigned char *input, unsigned char *output)

drawString takes 'string' (signed char array) as input:

void U8X8::drawString(uint8_t x, uint8_t y, const char *s)

and your output variable is char array, so all is correct (or some conversion to signed char pointer might be necessary).

You forgot closing parenthesis:

Heltec.display->drawString(0, 0, String(output));

of course if 'String' is a conversion :)

should be enough to:

Heltec.display->drawString(0, 0, (char*)(output));

Good comment is that from Remy Lebeau - output is a scrambled stream of bytes, there is no sense to print them (better display as HEX instead).

edited Apr 20, 2022 at 18:35

answered Apr 20, 2022 at 17:33

maark

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1 Comment

reconn Over a year ago

A minor correction, char is neither defined to be signed nor unsigned. It depends on the platform. Char is actually a third distinct type.

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