Refactor authcrypt

The example is refactored to use C++ classes and allocate the bulk of
the mbed TLS context structures in the heap instead of stack.

Author: Andres Amaya Garcia

authcrypt/main.cpp

@@ -30,20 +30,6 @@
 
 #include <string.h>
 
-#if defined(MBEDTLS_MEMORY_BUFFER_ALLOC_C)
-#include "mbedtls/memory_buffer_alloc.h"
-#endif
-
-static void print_hex(const char *title, const unsigned char buf[], size_t len)
-{
-    mbedtls_printf("%s: ", title);
-
-    for (size_t i = 0; i < len; i++)
-        mbedtls_printf("%02x", buf[i]);
-
-    mbedtls_printf("\r\n");
-}
-
 /*
  * The pre-shared key. Should be generated randomly and be unique to the
  * device/channel/etc. Just used a fixed on here for simplicity.
@@ -53,132 +39,174 @@ static const unsigned char secret_key[16] = {
     0xb9, 0xa0, 0xb8, 0xe9, 0x87, 0xb8, 0xc1, 0x72,
 };
 
-static int example(void)
-{
-    /* message that should be protected */
-    const char message[] = "Some things are better left unread";
-    /* metadata transmitted in the clear but authenticated */
-    const char metadata[] = "eg sequence number, routing info";
-    /* ciphertext buffer large enough to hold message + nonce + tag */
-    unsigned char ciphertext[128] = { 0 };
-    int ret;
-
-    mbedtls_printf("\r\n\r\n");
-    print_hex("plaintext message", (unsigned char *) message, sizeof message);
-
-    /*
-     * Setup random number generator
-     * (Note: later this might be done automatically.)
-     */
-    mbedtls_entropy_context entropy;    /* entropy pool for seeding PRNG */
-    mbedtls_ctr_drbg_context drbg;      /* pseudo-random generator */
-
-    mbedtls_entropy_init(&entropy);
-    mbedtls_ctr_drbg_init(&drbg);
-
-    /* Seed the PRNG using the entropy pool, and throw in our secret key as an
-     * additional source of randomness. */
-    ret = mbedtls_ctr_drbg_seed(&drbg, mbedtls_entropy_func, &entropy,
-                                       secret_key, sizeof secret_key);
-    if (ret != 0) {
-        return 1;
+/* Message that should be protected */
+static const char message[] = "Some things are better left unread";
+/* Metadata transmitted in the clear but authenticated */
+static const char metadata[] = "eg sequence number, routing info";
+
+class Authcrypt {
+public:
+    Authcrypt() : ciphertext(), decrypted()
+    {
+        mbedtls_entropy_init(&entropy);
+        mbedtls_ctr_drbg_init(&drbg);
+        mbedtls_cipher_init(&cipher);
     }
 
-    /*
-     * Setup AES-CCM contex
-     */
-    mbedtls_cipher_context_t ctx;
-
-    mbedtls_cipher_init(&ctx);
+    ~Authcrypt()
+    {
+        memset(ciphertext, 0, sizeof(ciphertext));
+        memset(decrypted, 0, sizeof(decrypted));
 
-    ret = mbedtls_cipher_setup(&ctx, mbedtls_cipher_info_from_type(MBEDTLS_CIPHER_AES_128_CCM));
-    if (ret != 0) {
-        mbedtls_printf("mbedtls_cipher_setup() returned -0x%04X\r\n", -ret);
-        return 1;
+        mbedtls_entropy_free(&entropy);
+        mbedtls_ctr_drbg_free(&drbg);
+        mbedtls_cipher_free(&cipher);
     }
 
-    ret = mbedtls_cipher_setkey(&ctx, secret_key, 8 * sizeof secret_key, MBEDTLS_ENCRYPT);
-    if (ret != 0) {
-        mbedtls_printf("mbedtls_cipher_setkey() returned -0x%04X\r\n", -ret);
-        return 1;
+    int run()
+    {
+        mbedtls_printf("\r\n\r\n");
+        print_hex("plaintext message", (unsigned char *)message,
+                  sizeof(message));
+
+        /*
+         * Seed the PRNG using the entropy pool, and throw in our secret key
+         * as an additional source of randomness.
+         */
+        int ret = mbedtls_ctr_drbg_seed(&drbg, mbedtls_entropy_func, &entropy,
+                                        secret_key, sizeof(secret_key));
+        if (ret != 0) {
+            mbedtls_printf("mbedtls_ctr_drbg_seed() returned -0x%04X\r\n",
+                           -ret);
+            return 1;
+        }
+
+        /* Setup AES-CCM contex */
+        ret = mbedtls_cipher_setup(&cipher,
+                    mbedtls_cipher_info_from_type(MBEDTLS_CIPHER_AES_128_CCM));
+        if (ret != 0) {
+            mbedtls_printf("mbedtls_cipher_setup() returned -0x%04X\r\n",
+                           -ret);
+            return 1;
+        }
+
+        ret = mbedtls_cipher_setkey(&cipher, secret_key,
+                                    8 * sizeof(secret_key), MBEDTLS_ENCRYPT);
+        if (ret != 0) {
+            mbedtls_printf("mbedtls_cipher_setkey() returned -0x%04X\r\n",
+                           -ret);
+            return 1;
+        }
+
+        /*
+         * Encrypt-authenticate the message and authenticate additional data
+         *
+         * First generate a random 8-byte nonce.
+         * Put it directly in the output buffer as the recipient will need it.
+         *
+         * Warning: you must never re-use the same (key, nonce) pair. One of
+         * the best ways to ensure this to use a counter for the nonce.
+         * However, this means you should save the counter accross rebots, if
+         * the key is a long-term one. The alternative we choose here is to
+         * generate the nonce randomly. However it only works if you have a
+         * good source of randomness.
+         */
+        const size_t nonce_len = 8;
+        mbedtls_ctr_drbg_random(&drbg, ciphertext, nonce_len);
+
+        size_t ciphertext_len = 0;
+        /* Go for a conservative 16-byte (128-bit) tag
+         * and append it to the ciphertext */
+        const size_t tag_len = 16;
+        ret = mbedtls_cipher_auth_encrypt(&cipher, ciphertext, nonce_len,
+                            reinterpret_cast<const unsigned char *>(metadata),
+                            sizeof(metadata),
+                            reinterpret_cast<const unsigned char *>(message),
+                            sizeof(message),
+                            ciphertext + nonce_len, &ciphertext_len,
+                            ciphertext + nonce_len + sizeof(message),
+                            tag_len);
+        if (ret != 0) {
+            mbedtls_printf("mbedtls_cipher_auth_encrypt() returned -0x%04X\r\n",
+                           -ret);
+            return 1;
+        }
+        ciphertext_len += nonce_len + tag_len;
+
+        /*
+         * The following information should now be transmitted:
+         * - First ciphertext_len bytes of ciphertext buffer
+         * - Metadata if not already transmitted elsewhere
+         */
+        print_hex("ciphertext", ciphertext, ciphertext_len);
+
+        /*
+         * Decrypt-authenticate
+         */
+        size_t decrypted_len = 0;
+
+        ret = mbedtls_cipher_setkey(&cipher, secret_key,
+                                    8 * sizeof(secret_key), MBEDTLS_DECRYPT);
+        if (ret != 0) {
+            mbedtls_printf("mbedtls_cipher_setkey() returned -0x%04X\r\n",
+                           -ret);
+            return 1;
+        }
+
+        ret = mbedtls_cipher_auth_decrypt(&cipher, ciphertext, nonce_len,
+                        reinterpret_cast<const unsigned char *>(metadata),
+                        sizeof(metadata), ciphertext + nonce_len,
+                        ciphertext_len - nonce_len - tag_len, decrypted,
+                        &decrypted_len, ciphertext + ciphertext_len - tag_len,
+                        tag_len);
+        /* Checking the return code is CRITICAL for security here */
+        if (ret == MBEDTLS_ERR_CIPHER_AUTH_FAILED) {
+            mbedtls_printf("Something bad is happening! Data is not "
+                           "authentic!\r\n");
+            return 1;
+        } else if (ret != 0) {
+            mbedtls_printf("mbedtls_cipher_authdecrypt() returned -0x%04X\r\n",
+                           -ret);
+            return 1;
+        }
+
+        print_hex("decrypted", decrypted, decrypted_len);
+
+        mbedtls_printf("\r\nDONE\r\n");
+
+        return 0;
     }
 
-    /*
-     * Encrypt-authenticate the message and authenticate additional data
-     *
-     * First generate a random 8-byte nonce.
-     * Put it directly in the output buffer as the recipient will need it.
-     *
-     * Warning: you must never re-use the same (key, nonce) pair. One of the
-     * best ways to ensure this to use a counter for the nonce. However this
-     * means you should save the counter accross rebots, if the key is a
-     * long-term one. The alternative we choose here is to generate the nonce
-     * randomly. However it only works if you have a good source of
-     * randomness.
-     */
-    const size_t nonce_len = 8;
-    mbedtls_ctr_drbg_random(&drbg, ciphertext, nonce_len);
-
-    size_t ciphertext_len = 0;
-    /* Go for a conservative 16-byte (128-bit) tag
-     * and append it to the ciphertext */
-    const size_t tag_len = 16;
-    ret = mbedtls_cipher_auth_encrypt(&ctx, ciphertext, nonce_len,
-                              (const unsigned char *) metadata, sizeof metadata,
-                              (const unsigned char *) message, sizeof message,
-                              ciphertext + nonce_len, &ciphertext_len,
-                              ciphertext + nonce_len + sizeof message, tag_len );
-    if (ret != 0) {
-        mbedtls_printf("mbedtls_cipher_auth_encrypt() returned -0x%04X\r\n", -ret);
-        return 1;
-    }
-    ciphertext_len += nonce_len + tag_len;
-
-    /*
-     * The following information should now be transmitted:
-     * - first ciphertext_len bytes of ciphertext buffer
-     * - metadata if not already transmitted elsewhere
-     */
-    print_hex("ciphertext", ciphertext, ciphertext_len);
-
-    /*
-     * Decrypt-authenticate
-     */
-    unsigned char decrypted[128] = { 0 };
-    size_t decrypted_len = 0;
-
-    ret = mbedtls_cipher_setkey(&ctx, secret_key, 8 * sizeof secret_key, MBEDTLS_DECRYPT);
-    if (ret != 0) {
-        mbedtls_printf("mbedtls_cipher_setkey() returned -0x%04X\r\n", -ret);
-        return 1;
-    }
+private:
+    void print_hex(const char *title, const unsigned char buf[], size_t len)
+    {
+        mbedtls_printf("%s: ", title);
 
-    ret = mbedtls_cipher_auth_decrypt(&ctx,
-                              ciphertext, nonce_len,
-                              (const unsigned char *) metadata, sizeof metadata,
-                              ciphertext + nonce_len, ciphertext_len - nonce_len - tag_len,
-                              decrypted, &decrypted_len,
-                              ciphertext + ciphertext_len - tag_len, tag_len );
-    /* Checking the return code is CRITICAL for security here */
-    if (ret == MBEDTLS_ERR_CIPHER_AUTH_FAILED) {
-        mbedtls_printf("Something bad is happening! Data is not authentic!\r\n");
-        return 1;
-    }
-    if (ret != 0) {
-        mbedtls_printf("mbedtls_cipher_authdecrypt() returned -0x%04X\r\n", -ret);
-        return 1;
+        for (size_t i = 0; i < len; i++)
+            mbedtls_printf("%02x", buf[i]);
+
+        mbedtls_printf("\r\n");
     }
 
-    print_hex("decrypted", decrypted, decrypted_len);
+    /* Ciphertext buffer large enough to hold message + nonce + tag */
+    unsigned char ciphertext[128];
+    unsigned char decrypted[128];
 
-    mbedtls_printf("\r\nDONE\r\n");
+    /* Entropy pool for seeding PRNG */
+    mbedtls_entropy_context entropy;
 
-    return 0;
-}
+    /* Pseudo-random generator */
+    mbedtls_ctr_drbg_context drbg;
+    mbedtls_cipher_context_t cipher;
+};
 
 int main() {
-    int ret = example();
-    if (ret != 0) {
+    int ret;
+    Authcrypt *authcrypt = new Authcrypt();
+
+    if ((ret = authcrypt->run()) != 0) {
         mbedtls_printf("Example failed with error %d\r\n", ret);
     }
+
+    delete authcrypt;
 }