Add encryption/decryption tests for small records

This commit adds tests to check the behavior of the record encryption
routine `ssl_encrypt_buf` when the buffer surrounding the plaintext is
too small to hold the expansion in the beginning and end (due to IV's,
padding, and MAC).

Each test starts successively increases the space available at the
beginning, end, or both, of the record buffer, and checks that the
record encryption either fails with a BUFFER_TOO_SMALL error, or
that it succeeds. Moreover, if it succeeds, it is checked that
decryption succeeds, too, and results in the original record.

tests/suites/test_suite_ssl.function

@@ -381,3 +381,149 @@ void ssl_crypt_record( int cipher_type, int hash_id,
     free( buf );
 }
 /* END_CASE */
+
+/* BEGIN_CASE */
+void ssl_crypt_record_small( int cipher_type, int hash_id,
+                             int etm, int tag_mode, int ver )
+{
+    /*
+     * Test pairs of encryption and decryption with an increasing
+     * amount of space in the record buffer - in more detail:
+     * 1) Try to encrypt with 0, 1, 2, ... bytes available
+     *    in front of the plaintext, and expect the encryption
+     *    to succeed starting from some offset. Always keep
+     *    enough space in the end of the buffer.
+     * 2) Try to encrypt with 0, 1, 2, ... bytes available
+     *    at the end of the plaintext, and expect the encryption
+     *    to succeed starting from some offset. Always keep
+     *    enough space at the beginning of the buffer.
+     * 3) Try to encrypt with 0, 1, 2, ... bytes available
+     *    both at the front and end of the plaintext,
+     *    and expect the encryption to succeed starting from
+     *    some offset.
+     *
+     * If encryption succeeds, check that decryption succeeds
+     * and yields the original record.
+     */
+
+    mbedtls_ssl_context ssl; /* ONLY for debugging */
+
+    mbedtls_ssl_transform t0, t1;
+    unsigned char *buf;
+    size_t const buflen = 150;
+    mbedtls_record rec, rec_backup;
+
+    int ret;
+    int mode;               /* Mode 1, 2 or 3 as explained above     */
+    size_t offset;          /* Available space at beginning/end/both */
+    size_t threshold = 64;  /* Maximum offset to test against        */
+
+    size_t default_pre_padding  = 64; /* Pre-padding to use in mode 2  */
+    size_t default_post_padding = 64; /* Post-padding to use in mode 1 */
+
+    int seen_success; /* Indicates if in the current mode we've
+                       * already seen a successful test. */
+
+    mbedtls_ssl_init( &ssl );
+    mbedtls_ssl_transform_init( &t0 );
+    mbedtls_ssl_transform_init( &t1 );
+    TEST_ASSERT( build_transforms( &t0, &t1, cipher_type, hash_id,
+                                   etm, tag_mode, ver ) == 0 );
+
+    TEST_ASSERT( ( buf = malloc( buflen ) ) != NULL );
+
+    for( mode=1; mode <= 3; mode++ )
+    {
+        seen_success = 0;
+        for( offset=0; offset <= threshold; offset++ )
+        {
+            mbedtls_ssl_transform *t_dec, *t_enc;
+            /* Take turns in who's sending and who's receiving. */
+            if( offset % 3 == 0 )
+            {
+                t_dec = &t0;
+                t_enc = &t1;
+            }
+            else
+            {
+                t_dec = &t1;
+                t_enc = &t0;
+            }
+
+            memset( rec.ctr, offset, sizeof( rec.ctr ) );
+            rec.type    = 42;
+            rec.ver[0]  = offset;
+            rec.ver[1]  = offset;
+            rec.buf     = buf;
+            rec.buf_len = buflen;
+
+            switch( mode )
+            {
+                case 1: /* Space in the beginning */
+                    rec.data_offset = offset;
+                    rec.data_len = buflen - offset - default_post_padding;
+                    break;
+
+                case 2: /* Space in the end */
+                    rec.data_offset = default_pre_padding;
+                    rec.data_len = buflen - default_pre_padding - offset;
+                    break;
+
+                case 3: /* Space in the beginning and end */
+                    rec.data_offset = offset;
+                    rec.data_len = buflen - 2 * offset;
+                    break;
+
+                default:
+                    TEST_ASSERT( 0 );
+                    break;
+            }
+
+            memset( rec.buf + rec.data_offset, 42, rec.data_len );
+
+            /* Make a copy for later comparison */
+            rec_backup = rec;
+
+            /* Encrypt record */
+            ret = mbedtls_ssl_encrypt_buf( &ssl, t_enc, &rec, rnd_std_rand, NULL );
+
+            if( ( mode == 1 || mode == 2 ) && seen_success )
+            {
+                TEST_ASSERT( ret == 0 );
+            }
+            else
+            {
+                TEST_ASSERT( ret == 0 || ret == MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );
+                if( ret == 0 )
+                    seen_success = 1;
+            }
+
+            if( ret != 0 )
+                continue;
+
+            /* Decrypt record with t_dec */
+            TEST_ASSERT( mbedtls_ssl_decrypt_buf( &ssl, t_dec, &rec ) == 0 );
+
+            /* Compare results */
+            TEST_ASSERT( rec.type == rec_backup.type );
+            TEST_ASSERT( memcmp( rec.ctr, rec_backup.ctr, 8 ) == 0 );
+            TEST_ASSERT( rec.ver[0] == rec_backup.ver[0] );
+            TEST_ASSERT( rec.ver[1] == rec_backup.ver[1] );
+            TEST_ASSERT( rec.data_len == rec_backup.data_len );
+            TEST_ASSERT( rec.data_offset == rec_backup.data_offset );
+            TEST_ASSERT( memcmp( rec.buf + rec.data_offset,
+                                 rec_backup.buf + rec_backup.data_offset,
+                                 rec.data_len ) == 0 );
+        }
+
+        TEST_ASSERT( seen_success == 1 );
+    }
+
+    /* Cleanup */
+    mbedtls_ssl_free( &ssl );
+    mbedtls_ssl_transform_free( &t0 );
+    mbedtls_ssl_transform_free( &t1 );
+
+    free( buf );
+}
+/* END_CASE */