Arduino ESP32 RSA OAEP encryption with Base64 encoding

rsa-oaep-enc-b64.cpp

#include "mbedtls/pk.h"
#include "mbedtls/rsa.h"
#include "mbedtls/error.h"
#include "mbedtls/base64.h"
#include "mbedtls/entropy.h"
#include "mbedtls/ctr_drbg.h"

#define PERS "rsa_encrypt"

mbedtls_pk_context public_key;
mbedtls_entropy_context entropy;
mbedtls_ctr_drbg_context ctr_drbg;

const char* esp_pub_key_pem_test = "-----BEGIN PUBLIC KEY-----\n\
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCvEn4AzPLEjQuD2bBKa5SQAEnT\n\
e2IuuURqYiALOuGPXW+XppcywC38FHuWnHlVtDS2NIk6F6V287wwkBX/a6r5XMH7\n\
pE9Qzlv6hWgQUYeoX7dKRLn5BlsqaxWsgvYpGoUfUhutL6I6q0g3LQDlvWKI5Kvc\n\
9dWb27rSTMPsO27MmwIDAQAB\n\
-----END PUBLIC KEY-----";

const char* esp_priv_key_pem_test = "-----BEGIN PRIVATE KEY-----\n\
MIICdwIBADANBgkqhkiG9w0BAQEFAASCAmEwggJdAgEAAoGBAK8SfgDM8sSNC4PZ\n\
sEprlJAASdN7Yi65RGpiIAs64Y9db5emlzLALfwUe5aceVW0NLY0iToXpXbzvDCQ\n\
Ff9rqvlcwfukT1DOW/qFaBBRh6hft0pEufkGWyprFayC9ikahR9SG60vojqrSDct\n\
AOW9Yojkq9z11ZvbutJMw+w7bsybAgMBAAECgYArgh38VvKyNBc7gjsUk53NF7VB\n\
MHWbBQHiqlYqVEzAE0lrV8RVoXsMHY8FRAfHa8x5x56e6Ed/w1zueLBu1LQdcPJc\n\
COf9noH1onkAuW52ofwuuuCGq+L8gyw5tTxmHhuyOTBy4syD9M4AMUDbb4wJPi1L\n\
WYe12pm6P3v+j6MDQQJBAOdsNhwciwxPGEle+HosSYrZJAK5dM8tL+w7doWdDCJ5\n\
oP1+f1sOa3n2xXfN7Kdr9t1jsz1UIQ4cKrX2OpbNgeECQQDBqkGnQb5Y0xYQ3Rxk\n\
hJEDIQC436g4GLy0kKsA2f7kboMURPfVv/senUSj4IVc9EdlUIhMjZ+sQcNoHfcf\n\
aRX7AkEAwNZo2E/ohd86q5jZkGRq+6oo2aRT83cB4eVCw9+zht63CcRotyVy5XR9\n\
43DSbEgIsz4q00XRLYLHDE0bUSH5wQJBAJ68g4WpUHCTzBFUetI1CnsrEOF3iPU5\n\
6WqC7XcMN21a25vAj/cmnZlRRJKiFA3Ft8NmR+Gfb5IMcTMk5IpdYwMCQEgpuLqq\n\
AxLc8u2j6I/FvbKvbFx8ZHKq0jPRXjzJz/0a39F8JPLT7M5LTSJ1zDIv4FQKY376\n\
D+3/XxFpEl40k3I=\n\
-----END PRIVATE KEY-----";

size_t get_encoded_size(size_t plain_data_size) {
  return (4 * ceil((double)plain_data_size / 3)) + 1;
}

void setup() {
  printf("\n");
  
  mbedtls_pk_init(&public_key);
  mbedtls_ctr_drbg_init(&ctr_drbg);
  mbedtls_entropy_init(&entropy);
  int res = mbedtls_ctr_drbg_seed(&ctr_drbg, mbedtls_entropy_func, &entropy, (const uint8_t*)PERS, strlen(PERS));
  if (res != 0) {
    char buff[512];
    mbedtls_strerror(res, buff, sizeof(buff));
    printf("Error mbedtls_ctr_drbg_seed = %s", buff);
    return;
  }
  
  size_t key_len = strlen(esp_pub_key_pem_test) + 1;
  res = mbedtls_pk_parse_public_key(&public_key, (const uint8_t*)esp_pub_key_pem_test, key_len);
  if (res != 0) {
    char buff[512];
    mbedtls_strerror(res, buff, sizeof(buff));
    printf("Parse key error: %s\n", buff);
    return;
  }
  printf("Parse public key ok\n");

  size_t ciphertext_buff_len = mbedtls_pk_rsa(public_key)->len;
  uint8_t *ciphertext_buff = (uint8_t*)malloc(ciphertext_buff_len);
  if (ciphertext_buff == NULL) {
    printf("Failed to allocate %zu bytes...\n", ciphertext_buff_len);
    return;
  }

  char *plain_text_p = "Hello"; 
  mbedtls_rsa_set_padding(mbedtls_pk_rsa(public_key), MBEDTLS_RSA_PKCS_V21, MBEDTLS_MD_SHA256);
  res = mbedtls_rsa_rsaes_oaep_encrypt(
    mbedtls_pk_rsa(public_key),
    mbedtls_ctr_drbg_random,
    &ctr_drbg,
    MBEDTLS_RSA_PUBLIC,
    NULL,
    0,
    strlen(plain_text_p),
    (const uint8_t*)plain_text_p,
    ciphertext_buff
  );
  
  if (res != 0) {
    char buff[512];
    mbedtls_strerror(res, buff, sizeof(buff));
    printf("Encryption error: %s\n", buff);
    free(ciphertext_buff);
    return;
  }

  printf("Encryption finished\n");

  size_t outlen = 0;
  size_t encoded_buff_size = get_encoded_size(ciphertext_buff_len);
  
  char *ciphertext_base64_buff = (char*)malloc(encoded_buff_size);
  if (ciphertext_base64_buff == NULL) {
    printf("Failed to allocate %zu bytes...\n", encoded_buff_size);
    free(ciphertext_buff);
    return;
  }

  res = mbedtls_base64_encode(
    (uint8_t*)ciphertext_base64_buff, 
    encoded_buff_size, 
    &outlen, 
    ciphertext_buff, 
    ciphertext_buff_len
  );
  if (res != 0) {
    char buff[512];
    mbedtls_strerror(res, buff, sizeof(buff));
    printf("Encode error: %s\n", buff);
    free(ciphertext_base64_buff);
    free(ciphertext_buff);
    return;
  }

  printf("Ciphertext (Base64) [%zu]: %s\n", strlen(ciphertext_base64_buff), ciphertext_base64_buff);
  free(ciphertext_base64_buff);
  free(ciphertext_buff);
}

void loop() {
  delay(10);
}