Split authcrypt code in .cpp and .h

Author: Andres Amaya Garcia

authcrypt/authcrypt.cpp

@@ -0,0 +1,185 @@
+/*
+ *  Hello world example of using the authenticated encryption with mbed TLS
+ *
+ *  Copyright (C) 2017, ARM Limited, All Rights Reserved
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#include "authcrypt.h"
+
+#include "mbed.h"
+
+#include "mbedtls/cipher.h"
+#include "mbedtls/entropy.h"
+#include "mbedtls/ctr_drbg.h"
+#if DEBUG_LEVEL > 0
+#include "mbedtls/debug.h"
+#endif
+
+#include "mbedtls/platform.h"
+
+#include <string.h>
+
+const unsigned char Authcrypt::secret_key[16] = {
+    0xf4, 0x82, 0xc6, 0x70, 0x3c, 0xc7, 0x61, 0x0a,
+    0xb9, 0xa0, 0xb8, 0xe9, 0x87, 0xb8, 0xc1, 0x72,
+};
+
+const char Authcrypt::message[] = "Some things are better left unread";
+
+const char Authcrypt::metadata[] = "eg sequence number, routing info";
+
+Authcrypt::Authcrypt()
+{
+    memset(ciphertext, 0, sizeof(ciphertext));
+    memset(decrypted, 0, sizeof(decrypted));
+
+    mbedtls_entropy_init(&entropy);
+    mbedtls_ctr_drbg_init(&drbg);
+    mbedtls_cipher_init(&cipher);
+}
+
+Authcrypt::~Authcrypt()
+{
+    memset(ciphertext, 0, sizeof(ciphertext));
+    memset(decrypted, 0, sizeof(decrypted));
+
+    mbedtls_cipher_free(&cipher);
+    mbedtls_ctr_drbg_free(&drbg);
+    mbedtls_entropy_free(&entropy);
+}
+
+int Authcrypt::run()
+{
+    mbedtls_printf("\r\n\r\n");
+    print_hex("plaintext message",
+              reinterpret_cast<const 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 ret;
+    }
+
+    /* 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 ret;
+    }
+
+    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 ret;
+    }
+
+    /*
+     * 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 ret;
+    }
+    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 ret;
+    }
+
+    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 ret;
+    } else if (ret != 0) {
+        mbedtls_printf("mbedtls_cipher_authdecrypt() returned -0x%04X\r\n",
+                       -ret);
+        return ret;
+    }
+
+    print_hex("decrypted", decrypted, decrypted_len);
+
+    mbedtls_printf("\r\nDONE\r\n");
+
+    return 0;
+}
+
+void Authcrypt::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");
+}

authcrypt/authcrypt.h

@@ -0,0 +1,108 @@
+/*
+ *  Hello world example of using the authenticated encryption with mbed TLS
+ *
+ *  Copyright (C) 2017, ARM Limited, All Rights Reserved
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#ifndef _AUTHCRYPT_H_
+#define _AUTHCRYPT_H_
+
+#include "mbedtls/cipher.h"
+#include "mbedtls/entropy.h"
+#include "mbedtls/ctr_drbg.h"
+
+/**
+ * This class implements the logic to demonstrate authenticated encryption using
+ * mbed TLS.
+ */
+class Authcrypt
+{
+public:
+    /**
+     * Construct an Authcrypt instance
+     */
+    Authcrypt();
+
+    /**
+     * Free any allocated resources
+     */
+    ~Authcrypt();
+
+    /**
+     * Run the authenticated encryption example
+     *
+     * \return  0 if successful
+     */
+    int run();
+
+    /**
+     * The pre-shared key
+     *
+     * \note This should be generated randomly and be unique to the
+     *       device/channel/etc. Just used a fixed on here for simplicity.
+     */
+    static const unsigned char secret_key[16];
+
+    /**
+     * Message that should be protected
+     */
+    static const char message[];
+
+    /**
+     * Metadata transmitted in the clear but authenticated
+     */
+    static const char metadata[];
+
+private:
+    /**
+     * Print a buffer's contents in hexadecimal
+     *
+     * \param[in]   title
+     *              The string to print before the hex string
+     * \param[in]   buf
+     *              The buffer to print in hex
+     * \param[in]   len
+     *              The length of the buffer
+     */
+    void print_hex(const char *title, const unsigned char buf[], size_t len);
+
+    /**
+     * Ciphertext buffer large enough to hold message + nonce + tag
+     */
+    unsigned char ciphertext[128];
+
+    /**
+     * Plaintext buffer large enough to hold the decrypted message
+     */
+    unsigned char decrypted[128];
+
+    /**
+     * Entropy pool for seeding PRNG
+     */
+    mbedtls_entropy_context entropy;
+
+    /**
+     * Pseudo-random generator
+     */
+    mbedtls_ctr_drbg_context drbg;
+
+    /**
+     * The block cipher configuration
+     */
+    mbedtls_cipher_context_t cipher;
+};
+
+#endif /* _AUTHCRYPT_H_ */