Introduce LTC_NO_STRUCT_PADDING.

Directly use it in `ltc_rsa_op_parameters`.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

.ci/build_options.sh

@@ -18,6 +18,7 @@ options=(
 -DLTC_MECC_FP
 -DLTC_NO_TABLES
 -DLTC_NO_FAST
+-DLTC_NO_STRUCT_PADDING
 -DLTC_NO_ASM
 -DLTC_NO_DEPRECATED_APIS
 -DLTC_NO_ECC_TIMING_RESISTANT

doc/crypt.tex

@@ -9454,6 +9454,10 @@ file I/O on embedded platforms.
 When this functions is defined the functions that store key material on the stack will clean up afterwards.
 Assumes that you have no memory paging with the stack.
 
+\subsection{LTC\_NO\_STRUCT\_PADDING}
+Per default the library adds padding to some structs, which are likely to be extended sometime in the future, in order
+to provide ABI compatibility to previous versions. To trim down struct sizes where possible, define this macro.
+
 \subsection{LTC\_TEST}
 When this has been defined the various self--test functions (for ciphers, hashes, prngs, etc) are included in the build.  This is the default configuration.
 If LTC\_NO\_TEST has been defined, the testing routines will be compacted and only return CRYPT\_NOP.

src/headers/tomcrypt_custom.h

@@ -153,6 +153,9 @@
 /* clean the stack of functions which put private information on stack */
 /* #define LTC_CLEAN_STACK */
 
+/* enable this in case you want to disable padding elements in structs */
+/* #define LTC_NO_STRUCT_PADDING */
+
 /* disable all file related functions */
 /* #define LTC_NO_FILE */
 

src/headers/tomcrypt_pk.h

@@ -125,8 +125,10 @@ typedef struct ltc_rsa_op_parameters {
          const unsigned char *lparam;
                unsigned long  lparamlen;
       } crypt;
+#ifndef LTC_NO_STRUCT_PADDING
       /* let's make space for potential future extensions */
-      ulong64 dummy[8];
+      void* padding[8];
+#endif
    } u;
 } ltc_rsa_op_parameters;