gpg: Add option --weak-digest to gpg and gpgv.

* g10/options.h: Add additional_weak_digests linked list to opts.
* g10/main.h: Declare weakhash linked list struct and
additional_weak_digest() function to insert newly-declared weak
digests into opts.
* g10/misc.c: (additional_weak_digest): New function.
(print_digest_algo_note): Check for deprecated digests; use proper
gcry_md_algos type.
* g10/sig-check.c: (do_check): Reject weak digests in addition to MD5.
* g10/gpg.c: Add --weak-digest option to gpg.
* doc/gpg.texi: Document gpg --weak-digest option.
* g10/gpgv.c: Add --weak-digest option to gpgv.
* doc/gpgv.texi: Document gpgv --weak-digest option.

--
gpg and gpgv treat signatures made over MD5 as unreliable, unless the
user supplies --allow-weak-digests to gpg.  Signatures over any other
digest are considered acceptable.

Despite SHA-1 being a mandatory-to-implement digest algorithm in RFC
4880, the collision-resistance of SHA-1 is weaker than anyone would
like it to be.

Some operators of high-value targets that depend on OpenPGP signatures
may wish to require their signers to use a stronger digest algorithm
than SHA1, even if the OpenPGP ecosystem at large cannot deprecate
SHA1 entirely today.

This changeset adds a new "--weak-digest DIGEST" option for both gpg
and gpgv, which makes it straightforward for anyone to treat any
signature or certification made over the specified digest as
unreliable.

This option can be supplied multiple times if the operator wishes to
deprecate multiple digest algorithms, and will be ignored completely
if the operator supplies --allow-weak-digests (as before).

MD5 is still always considered weak, regardless of any further
--weak-digest options supplied.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

Capitialized some comments, shorted a line in do_check, and changed
subject to name the option.  -wk

doc/gpg.texi

@@ -2789,9 +2789,20 @@ message was tampered with intentionally by an attacker.
 
 @item --allow-weak-digest-algos
 @opindex allow-weak-digest-algos
-Signatures made with the broken MD5 algorithm are normally rejected
-with an ``invalid digest algorithm'' message.  This option allows the
-verification of signatures made with such weak algorithms.
+Signatures made with known-weak digest algorithms are normally
+rejected with an ``invalid digest algorithm'' message.  This option
+allows the verification of signatures made with such weak algorithms.
+MD5 is the only digest algorithm considered weak by default.  See also
+@option{--weak-digest} to reject other digest algorithms.
+
+@item --weak-digest @code{name}
+@opindex weak-digest
+Treat the specified digest algorithm as weak.  Signatures made over
+weak digests algorithms are normally rejected. This option can be
+supplied multiple times if multiple algorithms should be considered
+weak.  See also @option{--allow-weak-digest-algos} to disable
+rejection of weak digests.  MD5 is always considered weak, and does
+not need to be listed explicitly.
 
 @item --no-default-keyring
 @opindex no-default-keyring

doc/gpgv.texi

@@ -118,6 +118,14 @@ checks into warnings.
 
 @include opt-homedir.texi
 
+@item --weak-digest @code{name}
+@opindex weak-digest
+Treat the specified digest algorithm as weak.  Signatures made over
+weak digests algorithms are normally rejected. This option can be
+supplied multiple times if multiple algorithms should be considered
+weak.  MD5 is always considered weak, and does not need to be listed
+explicitly.
+
 @end table
 
 @mansect return value

g10/gpg.c

@@ -389,6 +389,7 @@ enum cmd_and_opt_values
     oPrintDANERecords,
     oTOFUDefaultPolicy,
     oTOFUDBFormat,
+    oWeakDigest,
 
     oNoop
   };
@@ -749,6 +750,7 @@ static ARGPARSE_OPTS opts[] = {
   ARGPARSE_s_s (oPersonalCompressPreferences,
                                          "personal-compress-preferences", "@"),
   ARGPARSE_s_s (oFakedSystemTime, "faked-system-time", "@"),
+  ARGPARSE_s_s (oWeakDigest, "weak-digest","@"),
 
   /* Aliases.  I constantly mistype these, and assume other people do
      as well. */
@@ -2208,6 +2210,7 @@ main (int argc, char **argv)
     set_homedir (default_homedir ());
     opt.passphrase_repeat = 1;
     opt.emit_version = 1; /* Limit to the major number.  */
+    opt.additional_weak_digests = NULL;
 
     /* Check whether we have a config file on the command line.  */
     orig_argc = argc;
@@ -3124,6 +3127,9 @@ main (int argc, char **argv)
 	    break;
           case oAgentProgram: opt.agent_program = pargs.r.ret_str;  break;
           case oDirmngrProgram: opt.dirmngr_program = pargs.r.ret_str; break;
+          case oWeakDigest:
+	    additional_weak_digest(pargs.r.ret_str);
+	    break;
 
           case oDisplay:
             set_opt_session_env ("DISPLAY", pargs.r.ret_str);

g10/gpgv.c

@@ -61,6 +61,7 @@ enum cmd_and_opt_values {
   oStatusFD,
   oLoggerFD,
   oHomedir,
+  oWeakDigest,
   aTest
 };
 
@@ -78,6 +79,7 @@ static ARGPARSE_OPTS opts[] = {
                 N_("|FD|write status info to this FD")),
   ARGPARSE_s_i (oLoggerFD, "logger-fd", "@"),
   ARGPARSE_s_s (oHomedir, "homedir", "@"),
+  ARGPARSE_s_s (oWeakDigest, "weak-digest", "@"),
 
   ARGPARSE_end ()
 };
@@ -192,6 +194,9 @@ main( int argc, char **argv )
           log_set_fd (translate_sys2libc_fd_int (pargs.r.ret_int, 1));
           break;
         case oHomedir: opt.homedir = pargs.r.ret_str; break;
+        case oWeakDigest:
+          additional_weak_digest(pargs.r.ret_str);
+          break;
         case oIgnoreTimeConflict: opt.ignore_time_conflict = 1; break;
         default : pargs.err = ARGPARSE_PRINT_ERROR; break;
 	}

g10/main.h

@@ -69,6 +69,12 @@ struct groupitem
   struct groupitem *next;
 };
 
+struct weakhash
+{
+  enum gcry_md_algos algo;
+  struct weakhash *next;
+};
+
 
 /*-- gpg.c --*/
 extern int g10_errors_seen;
@@ -82,6 +88,7 @@ void print_pubkey_algo_note (pubkey_algo_t algo);
 void print_cipher_algo_note (cipher_algo_t algo);
 void print_digest_algo_note (digest_algo_t algo);
 void print_md5_rejected_note (void);
+void additional_weak_digest (const char* digestname);
 
 /*-- armor.c --*/
 char *make_radix64_string( const byte *data, size_t len );

g10/misc.c

@@ -307,6 +307,10 @@ print_cipher_algo_note (cipher_algo_t algo)
 void
 print_digest_algo_note (digest_algo_t algo)
 {
+  int deprecated = 0;
+  const enum gcry_md_algos galgo = map_md_openpgp_to_gcry (algo);
+  const struct weakhash *weak;
+
   if(algo >= 100 && algo <= 110)
     {
       static int warn=0;
@@ -315,14 +319,21 @@ print_digest_algo_note (digest_algo_t algo)
 	  warn=1;
           es_fflush (es_stdout);
 	  log_info (_("WARNING: using experimental digest algorithm %s\n"),
-                    gcry_md_algo_name (algo));
+                    gcry_md_algo_name (galgo));
 	}
     }
-  else if(algo==DIGEST_ALGO_MD5)
+  else if(algo == DIGEST_ALGO_MD5)
+    deprecated = 1;
+  else
+      for (weak = opt.additional_weak_digests; weak != NULL; weak = weak->next)
+        if (weak->algo == galgo)
+          deprecated = 1;
+
+  if (deprecated)
     {
       es_fflush (es_stdout);
       log_info (_("WARNING: digest algorithm %s is deprecated\n"),
-                gcry_md_algo_name (algo));
+                gcry_md_algo_name (galgo));
     }
 }
 
@@ -1676,3 +1687,37 @@ ecdsa_qbits_from_Q (unsigned int qbits)
   qbits /= 2;
   return qbits;
 }
+
+
+/* Ignore signatures and certifications made over certain digest
+ * algorithms by default, MD5 is considered weak.  This allows users
+ * to deprecate support for other algorithms as well.
+ */
+void
+additional_weak_digest (const char* digestname)
+{
+  struct weakhash *weak = NULL;
+  const enum gcry_md_algos algo = string_to_digest_algo(digestname);
+
+  if (algo == GCRY_MD_MD5)
+    return; /* MD5 is always considered weak, no need to add it.  */
+
+  if (algo == GCRY_MD_NONE)
+    {
+      log_error(_("Unknown weak digest '%s'\n"), digestname);
+      return;
+    }
+
+  /* Check to ensure it's not already present.  */
+  for (weak = opt.additional_weak_digests; weak != NULL; weak = weak->next)
+    {
+      if (algo == weak->algo)
+        return;
+    }
+
+  /* Add it to the head of the list.  */
+  weak = xmalloc(sizeof(*weak));
+  weak->algo = algo;
+  weak->next = opt.additional_weak_digests;
+  opt.additional_weak_digests = weak;
+}

g10/options.h

@@ -170,6 +170,7 @@ struct
   prefitem_t *personal_cipher_prefs;
   prefitem_t *personal_digest_prefs;
   prefitem_t *personal_compress_prefs;
+  struct weakhash *additional_weak_digests;
   int no_perm_warn;
   int no_mdc_warn;
   char *temp_dir;

g10/sig-check.c

@@ -274,15 +274,18 @@ do_check( PKT_public_key *pk, PKT_signature *sig, gcry_md_hd_t digest,
 {
     gcry_mpi_t result = NULL;
     int rc = 0;
+    const struct weakhash *weak;
 
     if( (rc=do_check_messages(pk,sig,r_expired,r_revoked)) )
         return rc;
 
-    if (sig->digest_algo == GCRY_MD_MD5
-        && !opt.flags.allow_weak_digest_algos)
+    if (!opt.flags.allow_weak_digest_algos)
       {
-        print_md5_rejected_note ();
-        return GPG_ERR_DIGEST_ALGO;
+        if (sig->digest_algo == GCRY_MD_MD5)
+          return GPG_ERR_DIGEST_ALGO;
+        for (weak = opt.additional_weak_digests; weak; weak = weak->next)
+          if (sig->digest_algo == weak->algo)
+            return GPG_ERR_DIGEST_ALGO;
       }
 
     /* Make sure the digest algo is enabled (in case of a detached