From b1aa1b3616dcbf6cb4a110ac2704f490f1f03b4c Mon Sep 17 00:00:00 2001
From: Hanno Becker <hanno.becker@arm.com>
Date: Wed, 8 May 2019 17:37:58 +0100
Subject: [PATCH] Allow the configuration of padding when using CID extension

---
 include/mbedtls/config.h       | 16 ++++++++++++++++
 include/mbedtls/ssl.h          |  4 ++++
 include/mbedtls/ssl_internal.h |  4 +---
 library/ssl_tls.c              |  4 +++-
 programs/ssl/query_config.c    |  8 ++++++++
 5 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 6f6d7f064..e7f42e5a3 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -3354,6 +3354,22 @@
  */
 //#define MBEDTLS_SSL_CID_OUT_LEN_MAX 32
 
+/** \def MBEDTLS_SSL_CID_PADDING_GRANULARITY
+ *
+ * This option controls the use of record plaintext padding
+ * when using the Connection ID extension in DTLS 1.2.
+ *
+ * The padding will always be chosen so that the length of the
+ * padded plaintext is a multiple of the value of this option.
+ *
+ * Note: A value of \c 1 means that no padding will be used
+ *       for outgoing records.
+ *
+ * The value MUST be a power of 2.
+ *
+ */
+//#define MBEDTLS_SSL_CID_PADDING_GRANULARITY 16
+
 /** \def MBEDTLS_SSL_OUT_CONTENT_LEN
  *
  * Maximum length (in bytes) of outgoing plaintext fragments.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index d0ecd0b11..1acd388f2 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -270,6 +270,10 @@
 #define MBEDTLS_SSL_CID_OUT_LEN_MAX         32
 #endif
 
+#if !defined(MBEDTLS_SSL_CID_PADDING_GRANULARITY)
+#define MBEDTLS_SSL_CID_PADDING_GRANULARITY 16
+#endif
+
 /* \} name SECTION: Module settings */
 
 /*
diff --git a/include/mbedtls/ssl_internal.h b/include/mbedtls/ssl_internal.h
index 41a1c7fb8..15ab7f83f 100644
--- a/include/mbedtls/ssl_internal.h
+++ b/include/mbedtls/ssl_internal.h
@@ -176,9 +176,7 @@
 #endif
 
 #if defined(MBEDTLS_SSL_CID)
-#define MBEDTLS_SSL_MAX_CID_EXPANSION       16 /* Currently, we pad records
-                                                * to lengths which are multiples
-                                                * of 16 Bytes. */
+#define MBEDTLS_SSL_MAX_CID_EXPANSION      MBEDTLS_SSL_CID_PADDING_GRANULARITY
 #else
 #define MBEDTLS_SSL_MAX_CID_EXPANSION        0
 #endif
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 7e7d5a0b4..3882622bc 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2013,7 +2013,9 @@ static int ssl_cid_build_inner_plaintext( unsigned char *content,
                                           uint8_t rec_type )
 {
     size_t len = *content_size;
-    size_t pad = ~len & 0xF; /* Pad to a multiple of 16 */
+
+    /* MBEDTLS_SSL_CID_PADDING_GRANULARITY must be a power of 2. */
+    size_t pad = ~len & ( MBEDTLS_SSL_CID_PADDING_GRANULARITY - 1 );
 
     /* Write real content type */
     if( remaining == 0 )
diff --git a/programs/ssl/query_config.c b/programs/ssl/query_config.c
index a7f2d2de5..9286766aa 100644
--- a/programs/ssl/query_config.c
+++ b/programs/ssl/query_config.c
@@ -2530,6 +2530,14 @@ int query_config( const char *config )
     }
 #endif /* MBEDTLS_SSL_CID_OUT_LEN_MAX */
 
+#if defined(MBEDTLS_SSL_CID_PADDING_GRANULARITY)
+    if( strcmp( "MBEDTLS_SSL_CID_PADDING_GRANULARITY", config ) == 0 )
+    {
+        MACRO_EXPANSION_TO_STR( MBEDTLS_SSL_CID_PADDING_GRANULARITY );
+        return( 0 );
+    }
+#endif /* MBEDTLS_SSL_CID_PADDING_GRANULARITY */
+
 #if defined(MBEDTLS_SSL_OUT_CONTENT_LEN)
     if( strcmp( "MBEDTLS_SSL_OUT_CONTENT_LEN", config ) == 0 )
     {