ESP8266 Crypto

This is a minimal, lightweight crypto library for the ESP8266 IOT device. It provides the following functions:

SHA256
AES 128 and 256
SHA256HMAC
RNG

The SHA256 and AES implementations are based upon the implementations in axTLS except ported to the ESP8266 Arduino platform, credit to Cameron Rich for the axTLS project.

Usage

SHA256HMAC

The following snippet demonstrates how to compute the SHA256 HMAC authentication code for a message.

/* Include the crypto library into your project */
#include <Crypto.h>

/* The length of the key we will use for this HMAC */
/* The key can be of any length, 16 and 32 are common */
#define KEY_LENGTH 16

/* Define our */
byte key[KEY_LENGTH] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15};

/* Create the HMAC instance with our key */
SHA256HMAC hmac(key, KEY_LENGTH);

/* Update the HMAC with just a plain string (null terminated) */
hmac.doUpdate("Hello World");

/* And or with a string and length */
const char *goodbye = "GoodBye World";
hmac.doUpdate(goodbye, strlen(goodbye));

/* And or with a binary message */
byte message[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
hmac.doUpdate(message, sizeof(message));

/* Finish the HMAC calculation and return the authentication code */
byte authCode[SHA256HMAC_SIZE];
hmac.doFinal(authCode);

/* authCode now contains our 32 byte authentication code */
for (byte i; i < SHA256HMAC_SIZE; i++)
{
    Serial.print(authCode[i], HEX);
}

SHA256

The following snippet demonstrates how to compute the SHA256 hash of a message.

/* Create a SHA256 hash */
SHA256 hasher;

/* Update the hash with your message, as many times as you like */
const char *hello = "Hello World";
hasher.doUpdate(hello, strlen(hello));

/* Update the hash with just a plain string*/
hasher.doUpdate("Goodbye World");

/* Update the hash with a binary message */
byte message[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
hasher.doUpdate(message, sizeof(message));

/* Compute the final hash */
byte hash[SHA256_SIZE];
hasher.doFinal(hash);

/* hash now contains our 32 byte hash */
for (byte i; i < SHA256_SIZE; i++)
{
    Serial.print(hash[i], HEX);
}

AES-128

#include <Crypto.h>
#include <base64.hpp>

#define BLOCK_SIZE 16

uint8_t key[BLOCK_SIZE] = { 0x1C,0x3E,0x4B,0xAF,0x13,0x4A,0x89,0xC3,0xF3,0x87,0x4F,0xBC,0xD7,0xF3, 0x31, 0x31 };
uint8_t iv[BLOCK_SIZE] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };

void bufferSize(char* text, int &length)
{
  int i = strlen(text);
  int buf = round(i / BLOCK_SIZE) * BLOCK_SIZE;
  length = (buf <= i) ? buf + BLOCK_SIZE : length = buf;
}
    
void encrypt(char* plain_text, char* output, int length)
{
  byte enciphered[length];
  RNG::fill(iv, BLOCK_SIZE); 
  AES aesEncryptor(key, iv, AES::AES_MODE_128, AES::CIPHER_ENCRYPT);
  aesEncryptor.process((uint8_t*)plain_text, enciphered, length);
  int encrypted_size = sizeof(enciphered);
  char encoded[encrypted_size];
  encode_base64(enciphered, encrypted_size, (unsigned char*)encoded);
  strcpy(output, encoded);
}

void decrypt(char* enciphered, char* output, int length)
{
  length = length + 1; //re-adjust
  char decoded[length];
  decode_base64((unsigned char*)enciphered, (unsigned char*)decoded);
  bufferSize(enciphered, length);
  byte deciphered[length];
  AES aesDecryptor(key, iv, AES::AES_MODE_128, AES::CIPHER_DECRYPT);
  aesDecryptor.process((uint8_t*)decoded, deciphered, length);
  strcpy(output, (char*)deciphered);
}

void setup() {
  Serial.begin(115200);
  while (!Serial) {
    ; //wait
  }
}

void loop() {
  char plain_text[] = "1234567890ABCDEF1234567890ABCDEF";
  
  // encrypt
  int length = 0;
  bufferSize(plain_text, length);
  char encrypted[length];
  encrypt(plain_text, encrypted, length);

  Serial.println("");
  Serial.print("Encrypted: ");
  Serial.println(encrypted); 

  // decrypt
  length = strlen(encrypted);
  char decrypted[length];
  decrypt(encrypted, decrypted, length);

  Serial.print("Decrypted: ");
  Serial.println(decrypted);

  delay(5000);
}

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Author

Chris Ellis

Twitter: @intrbiz