Clarify descriptions of constant-flow helpers

Signed-off-by: Manuel Pégourié-Gonnard <[email protected]>

library/ssl_msg.c

@@ -1048,6 +1048,12 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
  * Turn a bit into a mask:
  * - if bit == 1, return the all-bits 1 mask, aka (size_t) -1
  * - if bit == 0, return the all-bits 0 mask, aka 0
+ *
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations using masks.
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
  */
 static size_t mbedtls_ssl_cf_mask_from_bit( size_t bit )
 {
@@ -1068,15 +1074,18 @@ static size_t mbedtls_ssl_cf_mask_from_bit( size_t bit )
  * - if x < y,  return all bits 1, that is (size_t) -1
  * - otherwise, return all bits 0, that is 0
  *
- * Use only bit operations to avoid branches that could be used by some
- * compilers on some platforms to translate comparison operators.
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations using masks.
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
  */
 static size_t mbedtls_ssl_cf_mask_lt( size_t x, size_t y )
 {
-    /* This has the msb set if and only if x < y */
+    /* This has the most significant bit set if and only if x < y */
     const size_t sub = x - y;
 
-    /* sub1 = (x < y) in {0, 1} */
+    /* sub1 = (x < y) ? 1 : 0 */
     const size_t sub1 = sub >> ( sizeof( sub ) * 8 - 1 );
 
     /* mask = (x < y) ? 0xff... : 0x00... */
@@ -1090,8 +1099,11 @@ static size_t mbedtls_ssl_cf_mask_lt( size_t x, size_t y )
  * - if x >= y, return all bits 1, that is (size_t) -1
  * - otherwise, return all bits 0, that is 0
  *
- * Use only bit operations to avoid branches that could be used by some
- * compilers on some platforms to translate comparison operators.
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations using masks.
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
  */
 static size_t mbedtls_ssl_cf_mask_ge( size_t x, size_t y )
 {
@@ -1102,8 +1114,12 @@ static size_t mbedtls_ssl_cf_mask_ge( size_t x, size_t y )
  * Constant-flow boolean "equal" comparison:
  * return x == y
  *
- * Use only bit operations to avoid branches that could be used by some
- * compilers on some platforms to translate comparison operators.
+ * This function can be used to write constant-time code by replacing branches
+ * with bit operations - it can be used in conjunction with
+ * mbedtls_ssl_cf_mask_from_bit().
+ *
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
  */
 static size_t mbedtls_ssl_cf_bool_eq( size_t x, size_t y )
 {
@@ -1124,7 +1140,7 @@ static size_t mbedtls_ssl_cf_bool_eq( size_t x, size_t y )
 #pragma warning( pop )
 #endif
 
-    /* diff1 = (x != y) in {0, 1} */
+    /* diff1 = (x != y) ? 1 : 0 */
     const size_t diff1 = diff_msb >> ( sizeof( diff_msb ) * 8 - 1 );
 
     return( 1 ^ diff1 );
@@ -1136,8 +1152,8 @@ static size_t mbedtls_ssl_cf_bool_eq( size_t x, size_t y )
  *  - otherwise, a no-op,
  * but with execution flow independent of the values of c1 and c2.
  *
- * Use only bit operations to avoid branches that could be used by some
- * compilers on some platforms to translate comparison operators.
+ * This function is implemented without using comparison operators, as those
+ * might be translated to branches by some compilers on some platforms.
  */
 static void mbedtls_ssl_cf_memcpy_if_eq( unsigned char *dst,
                                          const unsigned char *src,
@@ -1148,9 +1164,9 @@ static void mbedtls_ssl_cf_memcpy_if_eq( unsigned char *dst,
     const size_t equal = mbedtls_ssl_cf_bool_eq( c1, c2 );
     const unsigned char mask = (unsigned char) mbedtls_ssl_cf_mask_from_bit( equal );
 
-    /* dst[i] = c1 != c2 ? dst[i] : src[i] */
+    /* dst[i] = c1 == c2 ? src[i] : dst[i] */
     for( size_t i = 0; i < len; i++ )
-        dst[i] = ( dst[i] & ~mask ) | ( src[i] & mask );
+        dst[i] = ( src[i] & mask ) | ( dst[i] & ~mask );
 }
 
 /*
@@ -1630,7 +1646,8 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
         {
             /* This is the SSL 3.0 path, we don't have to worry about Lucky
              * 13, because there's a strictly worse padding attack built in
-             * the protocol (known as part of POODLE), so branches are OK. */
+             * the protocol (known as part of POODLE), so we don't care if the
+             * code is not constant-time, in particular branches are OK. */
             if( padlen > transform->ivlen )
             {
 #if defined(MBEDTLS_SSL_DEBUG_ALL)
@@ -1666,7 +1683,7 @@ int mbedtls_ssl_decrypt_buf( mbedtls_ssl_context const *ssl,
             for( idx = start_idx; idx < rec->data_len; idx++ )
             {
                 /* pad_count += (idx >= padding_idx) &&
-                 *              (chech[idx] == padlen - 1);
+                 *              (check[idx] == padlen - 1);
                  */
                 const size_t mask = mbedtls_ssl_cf_mask_ge( idx, padding_idx );
                 const size_t equal = mbedtls_ssl_cf_bool_eq( check[idx],