gpg: Do not allow old cipher algorithms for encryption.

* g10/gpg.c: New option --allow-old-cipher-algos.
(set_compliance_option): Set --rfc4880bis explictly to SHA256 and
AES256.  Allow old cipher algos for OpenPGP, rfc4880, and rfc2440.
* g10/options.h (opt): Add flags.allow_old_cipher_algos.
* g10/misc.c (print_sha1_keysig_rejected_note): Always print the note
unless in --quiet mode.
* g10/encrypt.c (setup_symkey): Disallow by default algos with a
blocklengt < 128.
(encrypt_crypt): Ditto.  Fallback by default to AES instead of 3DES.
* g10/pkclist.c (algo_available): Take care of old cipher also.
(select_algo_from_prefs): Use AES as implicit algorithm by default.

* tests/openpgp/defs.scm (create-gpghome): Set allow-old-cipher-algos.
--

GnuPG-bug-id: 3415

doc/gpg.texi

@@ -2848,16 +2848,17 @@ different compliance option in the gpg.conf file.
 @item --openpgp
 @opindex openpgp
 Reset all packet, cipher and digest options to strict OpenPGP
-behavior. Use this option to reset all previous options like
-@option{--s2k-*}, @option{--cipher-algo}, @option{--digest-algo} and
+behavior.  This option implies @option{--allow-old-cipher-algos}.  Use
+this option to reset all previous options like @option{--s2k-*},
+@option{--cipher-algo}, @option{--digest-algo} and
 @option{--compress-algo} to OpenPGP compliant values. All PGP
 workarounds are disabled.
 
 @item --rfc4880
 @opindex rfc4880
 Reset all packet, cipher and digest options to strict RFC-4880
-behavior. Note that this is currently the same thing as
-@option{--openpgp}.
+behavior.  This option implies @option{--allow-old-cipher-algos}.
+Note that this is currently the same thing as @option{--openpgp}.
 
 @item --rfc4880bis
 @opindex rfc4880bis
@@ -2869,7 +2870,8 @@ proposed updates of RFC-4880.
 Reset all packet, cipher and digest options to strict RFC-2440
 behavior.  Note that by using this option encryption packets are
 created in a legacy mode without MDC protection.  This is dangerous
-and should thus only be used for experiments.  See also option
+and should thus only be used for experiments.  This option implies
+@option{--allow-old-cipher-algos}.  See also option
 @option{--ignore-mdc-error}.
 
 @item --pgp6
@@ -3391,6 +3393,15 @@ necessary to get as much data as possible out of that garbled message.
 Be aware that a missing or failed MDC can be an indication of an
 attack.  Use with great caution; see also option @option{--rfc2440}.
 
+@item --allow-old-cipher-algos
+@opindex allow-old-cipher-algos
+Old cipher algorithms like 3DES, IDEA, or CAST5 encrypt data using
+blocks of 64 bits; modern algorithms use blocks of 128 bit instead.
+To avoid certain attack on these old algorithms it is suggested not to
+encrypt more than 150 MiByte using the same key.  For this reason gpg
+does not allow the use of 64 bit block size algorithms for encryption
+unless this option is specified.
+
 @item --allow-weak-digest-algos
 @opindex allow-weak-digest-algos
 Signatures made with known-weak digest algorithms are normally

g10/encrypt.c

@@ -538,6 +538,17 @@ setup_symkey (STRING2KEY **symkey_s2k, DEK **symkey_dek)
   int s2kdigest;
 
   defcipher = default_cipher_algo ();
+  if (openpgp_cipher_blocklen (defcipher) < 16
+      && !opt.flags.allow_old_cipher_algos)
+    {
+      log_error (_("cipher algorithm '%s' may not be used for encryption\n"),
+		 openpgp_cipher_algo_name (defcipher));
+      if (!opt.quiet)
+        log_info (_("(use option \"%s\" to override)\n"),
+                  "--allow-old-cipher-algos");
+      return gpg_error (GPG_ERR_CIPHER_ALGO);
+    }
+
   if (!gnupg_cipher_is_allowed (opt.compliance, 1, defcipher,
                                 GCRY_CIPHER_MODE_CFB))
     {
@@ -741,10 +752,18 @@ encrypt_crypt (ctrl_t ctrl, int filefd, const char *filename,
          entry for 3DES, and the pk_list cannot be empty.  In this
          case, use 3DES anyway as it's the safest choice - perhaps the
          v3 key is being used in an OpenPGP implementation and we know
-         that the implementation behind any v4 key can handle 3DES. */
+         that the implementation behind any v4 key can handle 3DES.
+         Note that we do not support v3 keys since version 2.2 so the
+         above description gives only historical background. */
       if (cfx.dek->algo == -1)
         {
-          cfx.dek->algo = CIPHER_ALGO_3DES;
+          /* If does not make sense to fallback to the rfc4880
+           * required 3DES if we will reject that algo later.  Thus we
+           * fallback to AES anticipating RFC4880bis rules.  */
+          if (opt.flags.allow_old_cipher_algos)
+            cfx.dek->algo = CIPHER_ALGO_3DES;
+          else
+            cfx.dek->algo = CIPHER_ALGO_AES;
         }
 
       /* In case 3DES has been selected, print a warning if any key
@@ -770,6 +789,18 @@ encrypt_crypt (ctrl_t ctrl, int filefd, const char *filename,
       cfx.dek->algo = opt.def_cipher_algo;
     }
 
+  if (openpgp_cipher_blocklen (cfx.dek->algo) < 16
+      && !opt.flags.allow_old_cipher_algos)
+    {
+      log_error (_("cipher algorithm '%s' may not be used for encryption\n"),
+		 openpgp_cipher_algo_name (cfx.dek->algo));
+      if (!opt.quiet)
+        log_info (_("(use option \"%s\" to override)\n"),
+                  "--allow-old-cipher-algos");
+      rc = gpg_error (GPG_ERR_CIPHER_ALGO);
+      goto leave;
+    }
+
   /* Check compliance.  */
   if (! gnupg_cipher_is_allowed (opt.compliance, 1, cfx.dek->algo,
                                  GCRY_CIPHER_MODE_CFB))

g10/gpg.c

@@ -345,6 +345,7 @@ enum cmd_and_opt_values
     oAllowFreeformUID,
     oNoAllowFreeformUID,
     oAllowSecretKeyImport,
+    oAllowOldCipherAlgos,
     oEnableSpecialFilenames,
     oNoLiteral,
     oSetFilesize,
@@ -854,6 +855,7 @@ static gpgrt_opt_t opts[] = {
   /* Options to override new security defaults.  */
   ARGPARSE_s_n (oAllowWeakKeySignatures, "allow-weak-key-signatures", "@"),
   ARGPARSE_s_n (oAllowWeakDigestAlgos, "allow-weak-digest-algos", "@"),
+  ARGPARSE_s_n (oAllowOldCipherAlgos, "allow-old-cipher-algos", "@"),
   ARGPARSE_s_s (oWeakDigest, "weak-digest","@"),
   ARGPARSE_s_s (oVerifyOptions, "verify-options", "@"),
   ARGPARSE_s_n (oEnableSpecialFilenames, "enable-special-filenames", "@"),
@@ -2186,7 +2188,23 @@ set_compliance_option (enum cmd_and_opt_values option)
     {
     case oRFC4880bis:
       opt.flags.rfc4880bis = 1;
-      /* fall through.  */
+      opt.compliance = CO_RFC4880;
+      opt.flags.dsa2 = 1;
+      opt.flags.require_cross_cert = 1;
+      opt.rfc2440_text = 0;
+      opt.allow_non_selfsigned_uid = 1;
+      opt.allow_freeform_uid = 1;
+      opt.escape_from = 1;
+      opt.not_dash_escaped = 0;
+      opt.def_cipher_algo = 0;
+      opt.def_aead_algo = 0;
+      opt.def_digest_algo = 0;
+      opt.cert_digest_algo = 0;
+      opt.compress_algo = -1;
+      opt.s2k_mode = 3; /* iterated+salted */
+      opt.s2k_digest_algo = DIGEST_ALGO_SHA256;
+      opt.s2k_cipher_algo = CIPHER_ALGO_AES256;
+      break;
     case oOpenPGP:
     case oRFC4880:
       /* This is effectively the same as RFC2440, but with
@@ -2208,6 +2226,7 @@ set_compliance_option (enum cmd_and_opt_values option)
       opt.s2k_mode = 3; /* iterated+salted */
       opt.s2k_digest_algo = DIGEST_ALGO_SHA1;
       opt.s2k_cipher_algo = CIPHER_ALGO_3DES;
+      opt.flags.allow_old_cipher_algos = 1;
       break;
     case oRFC2440:
       opt.compliance = CO_RFC2440;
@@ -2225,6 +2244,7 @@ set_compliance_option (enum cmd_and_opt_values option)
       opt.s2k_mode = 3; /* iterated+salted */
       opt.s2k_digest_algo = DIGEST_ALGO_SHA1;
       opt.s2k_cipher_algo = CIPHER_ALGO_3DES;
+      opt.flags.allow_old_cipher_algos = 1;
       break;
     case oPGP7:  opt.compliance = CO_PGP7;  break;
     case oPGP8:  opt.compliance = CO_PGP8;  break;
@@ -3604,6 +3624,10 @@ main (int argc, char **argv)
             opt.flags.allow_weak_key_signatures = 1;
             break;
 
+          case oAllowOldCipherAlgos:
+            opt.flags.allow_old_cipher_algos = 1;
+            break;
+
           case oFakedSystemTime:
             {
               size_t len = strlen (pargs.r.ret_str);

g10/misc.c

@@ -396,8 +396,9 @@ print_sha1_keysig_rejected_note (void)
   log_info (_("Note: third-party key signatures using"
               " the %s algorithm are rejected\n"),
             gcry_md_algo_name (GCRY_MD_SHA1));
-  print_further_info ("use option \"%s\" to override",
-                      "--allow-weak-key-signatures");
+  if (!opt.quiet)
+    log_info (_("(use option \"%s\" to override)\n"),
+              "--allow-weak-key-signatures");
 }
 
 

g10/options.h

@@ -237,6 +237,7 @@ struct
     unsigned int use_embedded_filename:1;
     unsigned int utf8_filename:1;
     unsigned int dsa2:1;
+    unsigned int allow_old_cipher_algos:1;
     unsigned int allow_weak_digest_algos:1;
     unsigned int allow_weak_key_signatures:1;
     unsigned int large_rsa:1;

g10/pkclist.c

@@ -1397,6 +1397,10 @@ algo_available( preftype_t preftype, int algo, const struct pref_hint *hint)
 {
   if( preftype == PREFTYPE_SYM )
     {
+      if (!opt.flags.allow_old_cipher_algos
+          && openpgp_cipher_blocklen (algo) < 16)
+        return 0;  /* We don't want this one.  */
+
       if(PGP7 && (algo != CIPHER_ALGO_IDEA
 		  && algo != CIPHER_ALGO_3DES
 		  && algo != CIPHER_ALGO_CAST5
@@ -1494,12 +1498,15 @@ select_algo_from_prefs(PK_LIST pk_list, int preftype,
       switch(preftype)
 	{
 	case PREFTYPE_SYM:
-	  /* IDEA is implicitly there for v3 keys with v3 selfsigs if
-	     --pgp2 mode is on.  This was a 2440 thing that was
-	     dropped from 4880 but is still relevant to GPG's 1991
-	     support.  All this doesn't mean IDEA is actually
-	     available, of course. */
-          implicit=CIPHER_ALGO_3DES;
+	  /* Historical note: IDEA is implicitly there for v3 keys
+	     with v3 selfsigs if --pgp2 mode is on.  This was a 2440
+	     thing that was dropped from 4880 but is still relevant to
+	     GPG's 1991 support.  All this doesn't mean IDEA is
+	     actually available, of course. */
+          if (opt.flags.allow_old_cipher_algos)
+            implicit = CIPHER_ALGO_3DES;
+          else
+            implicit = CIPHER_ALGO_AES;
 	  break;
 
 	case PREFTYPE_AEAD:

tests/openpgp/defs.scm

@@ -345,6 +345,7 @@
                "no-auto-key-retrieve"
                "no-auto-key-locate"
 	       "allow-weak-digest-algos"
+	       "allow-old-cipher-algos"
                "ignore-mdc-error"
 	       (if have-opt-always-trust
 		   "no-auto-check-trustdb" "#no-auto-check-trustdb")