What is MD5 and how to use it on ESP32

The MD5 (Message Digest Algorithm 5) is a hash algorithm widely used to generate a 128-bit (16 bytes) value from input data of any size.

This hash value is unique for each input, which means that even a small change in the input data will generate a completely different hash.

Like all hash algorithms, MD5 is a one-way algorithm, meaning that the process cannot be reversed to obtain the original data from the MD5 hash (it is not encryption, it is hashing).

Although MD5 was designed to be secure, vulnerabilities have been discovered that allow collisions (two different inputs that produce the same hash).

Although it is not recommended for security applications, it remains useful in contexts where quick data integrity verification is needed (such as files or communications).

Implementing MD5 on the ESP32

The ESP32 has a hardware module for cryptographic operations, but we can also use software libraries to implement MD5.

In this case, we will use the MD5.h library, which is easy to use and available in the Arduino development environment.

Let’s see how to generate an MD5 hash from a text string on the ESP32.

#include "mbedtls/md.h"
#include "stdlib.h"

void printBlockAsHex(std::vector<uint8_t> data)
{
	for(int i = 0; i < data.size(); i++)
	{
		Serial.printf("%02X ", data[i]);
	}
}

std::vector<uint8_t> generateMD5(std::string& text)
{
	std::vector<uint8_t> hash(16);

	mbedtls_md_context_t ctx;
	mbedtls_md_type_t md_type = MBEDTLS_MD_MD5;

	mbedtls_md_init(&ctx);
	mbedtls_md_setup(&ctx, mbedtls_md_info_from_type(md_type), 0);
	mbedtls_md_starts(&ctx);
	mbedtls_md_update(&ctx, (const unsigned char*)text.c_str(), text.length());
	mbedtls_md_finish(&ctx, hash.data());
	mbedtls_md_free(&ctx);

	return hash;
}

void setup()
{
	Serial.begin(115200);
	delay(2000);

	std::string message = "www.luisllamas.es";
	auto hash = generateMD5(message);

	Serial.print(message.c_str());
	Serial.print("\nHash: ");
	printBlockAsHex(hash);
}

void loop() {}

Practical examples

Although MD5 is not secure for critical applications, it can be useful in the following scenarios:

String calcularHash(String datos) {
  return MD5(datos);
}

bool verificarIntegridad(String datos, String hashEnviado) {
  String hashCalculado = calcularHash(datos);
  return hashCalculado == hashEnviado;
}

String generarID(String nombreUsuario) {
  return MD5(nombreUsuario + String(millis()));
}