gpg: Try hard to use MDC also for sign+symenc.

* g10/encrypt.c (use_mdc): Make it a global func.
* g10/sign.c (sign_symencrypt_file): Use that function to decide
whether to use an MDC.
* tests/openpgp/conventional-mdc.test: Add a simple test case.
--

We used --force-mdc in sign+symenc mode (-cs) only with --force-mdc.
That broke our assumption from commit 625e292 (GnuPG 2.1.9) that all
uses of modern ciphers are using MDC.

Reported-by: Ben Kibbey <bjk@luxsci.net>
Signed-off-by: Werner Koch <wk@gnupg.org>

g10/encrypt.c

@@ -101,8 +101,8 @@ encrypt_seskey (DEK *dek, DEK **seskey, byte *enckey)
 
 
 /* We try very hard to use a MDC */
-static int
-use_mdc(PK_LIST pk_list,int algo)
+int
+use_mdc (pk_list_t pk_list,int algo)
 {
   /* RFC-2440 don't has MDC */
   if (RFC2440)

g10/main.h

@@ -211,6 +211,7 @@ void display_online_help( const char *keyword );
 
 /*-- encode.c --*/
 int setup_symkey (STRING2KEY **symkey_s2k,DEK **symkey_dek);
+int use_mdc (pk_list_t pk_list,int algo);
 int encrypt_symmetric (const char *filename );
 int encrypt_store (const char *filename );
 int encrypt_crypt (ctrl_t ctrl, int filefd, const char *filename,

g10/sign.c

@@ -1261,12 +1261,7 @@ sign_symencrypt_file (const char *fname, strlist_t locusr)
         goto leave;
     }
 
-    /* We have no way to tell if the recipient can handle messages
-       with an MDC, so this defaults to no.  Perhaps in a few years,
-       this can be defaulted to yes.  Note that like regular
-       encrypting, --force-mdc overrides --disable-mdc. */
-    if(opt.force_mdc)
-      cfx.dek->use_mdc=1;
+    cfx.dek->use_mdc = use_mdc (NULL, cfx.dek->algo);
 
     /* now create the outfile */
     rc = open_outfile (-1, fname, opt.armor? 1:0, 0, &out);
@@ -1309,7 +1304,11 @@ sign_symencrypt_file (const char *fname, strlist_t locusr)
 
     /* Push the compress filter */
     if (default_compress_algo())
-      push_compress_filter(out,&zfx,default_compress_algo());
+      {
+        if (cfx.dek && cfx.dek->use_mdc)
+          zfx.new_ctb = 1;
+        push_compress_filter (out, &zfx,default_compress_algo() );
+      }
 
     /* Write the one-pass signature packets */
     /*(current filters: zip - encrypt - armor)*/

tests/openpgp/conventional-mdc.test

@@ -31,5 +31,14 @@ for ciph in `all_cipher_algos`; do
     cmp z y || error "$ciph/$i: mismatch"
   done
 done
-
 progress_end
+
+#info Checking sign+symencrypt
+for i in $plain_files $data_files; do
+    echo "Hier spricht HAL" | $GPG --passphrase-fd 0 $s2k -cs -o x --yes $i
+    echo "Hier spricht HAL" | $GPG --passphrase-fd 0 $s2k     -o y --yes x
+    cmp $i y || error "$i: mismatch in sign+symenc"
+done
+
+
+# eof