New sample/test configuration: small DTLS 1.2

1. Copy config-ccm-psk-tls1_2.h
2. Add DTLS support
3. Add some TLS and DTLS features that are useful in low-bandwidth,
   low-reliability networks
4. Reduce the SSL buffer to a very small size

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

configs/config-ccm-psk-dtls1_2.h

@@ -0,0 +1,103 @@
+/**
+ * \file config-ccm-psk-dtls1_2.h
+ *
+ * \brief Small configuration for DTLS 1.2 with PSK and AES-CCM ciphersuites
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  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.
+ */
+/*
+ * Minimal configuration for DTLS 1.2 with PSK and AES-CCM ciphersuites
+ *
+ * Distinguishing features:
+ * - Optimized for small code size, low bandwidth (on an unreliable network),
+ *   and low RAM usage.
+ * - No asymmetric cryptography (no certificates, no Diffie-Hellman key
+ *   exchange).
+ * - Fully modern and secure (provided the pre-shared keys are generated and
+ *   stored securely).
+ * - Very low record overhead with CCM-8.
+ * - Includes several optional DTLS features typically used in IoT.
+ *
+ * See README.txt for usage instructions.
+ */
+
+/* System support */
+//#define MBEDTLS_HAVE_TIME /* Optionally used in Hello messages */
+/* Other MBEDTLS_HAVE_XXX flags irrelevant for this configuration */
+
+/* Mbed TLS modules */
+#define MBEDTLS_AES_C
+#define MBEDTLS_CCM_C
+#define MBEDTLS_CIPHER_C
+#define MBEDTLS_CTR_DRBG_C
+#define MBEDTLS_ENTROPY_C
+#define MBEDTLS_MD_C
+#define MBEDTLS_NET_C
+/* The library does not currently support enabling SHA-224 without SHA-256.
+ * A future version of the library will have this option disabled
+ * by default. */
+#define MBEDTLS_SHA224_C
+#define MBEDTLS_SHA256_C
+#define MBEDTLS_SSL_CLI_C
+#define MBEDTLS_SSL_COOKIE_C
+#define MBEDTLS_SSL_SRV_C
+#define MBEDTLS_SSL_TLS_C
+#define MBEDTLS_TIMING_C
+
+/* TLS protocol feature support */
+#define MBEDTLS_KEY_EXCHANGE_PSK_ENABLED
+#define MBEDTLS_SSL_PROTO_TLS1_2
+#define MBEDTLS_SSL_PROTO_DTLS
+#define MBEDTLS_SSL_DTLS_ANTI_REPLAY
+#define MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE
+#define MBEDTLS_SSL_DTLS_CONNECTION_ID
+#define MBEDTLS_SSL_DTLS_HELLO_VERIFY
+#define MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
+
+/*
+ * Use only CCM_8 ciphersuites, and
+ * save ROM and a few bytes of RAM by specifying our own ciphersuite list
+ */
+#define MBEDTLS_SSL_CIPHERSUITES                        \
+        MBEDTLS_TLS_PSK_WITH_AES_256_CCM_8,             \
+        MBEDTLS_TLS_PSK_WITH_AES_128_CCM_8
+
+/*
+ * Save RAM at the expense of interoperability: do this only if you control
+ * both ends of the connection!  (See comments in "mbedtls/ssl.h".)
+ * The optimal size here depends on the typical size of records.
+ */
+#define MBEDTLS_SSL_IN_CONTENT_LEN              256
+#define MBEDTLS_SSL_OUT_CONTENT_LEN             256
+
+/* Save RAM at the expense of ROM */
+#define MBEDTLS_AES_ROM_TABLES
+
+/* Save some RAM by adjusting to your exact needs */
+#define MBEDTLS_PSK_MAX_LEN    16 /* 128-bits keys are generally enough */
+
+/*
+ * You should adjust this to the exact number of sources you're using: default
+ * is the "platform_entropy_poll" source, but you may want to add other ones
+ * Minimum is 2 for the entropy test suite.
+ */
+#define MBEDTLS_ENTROPY_MAX_SOURCES 2
+
+/* These defines are present so that the config modifying scripts can enable
+ * them during tests/scripts/test-ref-configs.pl */
+//#define MBEDTLS_USE_PSA_CRYPTO
+//#define MBEDTLS_PSA_CRYPTO_C

tests/scripts/test-ref-configs.pl

@@ -32,6 +32,10 @@ my %configs = (
         'compat' => '-m tls12 -f \'^TLS-PSK-WITH-AES-...-CCM-8\'',
         'test_again_with_use_psa' => 1
     },
+    'config-ccm-psk-dtls1_2.h' => {
+        'compat' => '-m dtls12 -f \'^TLS-PSK-WITH-AES-...-CCM-8\'',
+        'test_again_with_use_psa' => 1
+    },
     'config-no-entropy.h' => {
     },
     'config-suite-b.h' => {