gpg: Reject signatures made with MD5.

* g10/gpg.c: Add option --allow-weak-digest-algos.
(main): Set option also in PGP2 mode.
* g10/options.h (struct opt): Add flags.allow_weak_digest_algos.
* g10/sig-check.c (do_check): Reject MD5 signatures.
* tests/openpgp/defs.inc: Add allow_weak_digest_algos to gpg.conf.

doc/gpg.texi

@@ -2276,9 +2276,10 @@ a message that PGP 2.x will not be able to handle. Note that `PGP
 available, but the MIT release is a good common baseline.
 
 This option implies @option{--rfc1991 --disable-mdc
---no-force-v4-certs --escape-from-lines --force-v3-sigs --cipher-algo
-IDEA --digest-algo MD5 --compress-algo ZIP}. It also disables
-@option{--textmode} when encrypting.
+--no-force-v4-certs --escape-from-lines --force-v3-sigs
+--allow-weak-digest-algos --cipher-algo IDEA --digest-algo
+MD5--compress-algo ZIP}. It also disables @option{--textmode} when
+encrypting.
 
 @item --pgp6
 @opindex pgp6
@@ -2734,6 +2735,13 @@ necessary to get as much data as possible out of the corrupt message.
 However, be aware that a MDC protection failure may also mean that the
 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.
+
+
 @item --no-default-keyring
 @opindex no-default-keyring
 Do not add the default keyrings to the list of keyrings. Note that

g10/gpg.c

@@ -374,6 +374,7 @@ enum cmd_and_opt_values
     oDisableDSA2,
     oAllowMultipleMessages,
     oNoAllowMultipleMessages,
+    oAllowWeakDigestAlgos,
     oFakedSystemTime,
 
     oNoop
@@ -768,6 +769,7 @@ static ARGPARSE_OPTS opts[] = {
   ARGPARSE_s_n (oDisableDSA2, "disable-dsa2", "@"),
   ARGPARSE_s_n (oAllowMultipleMessages,      "allow-multiple-messages", "@"),
   ARGPARSE_s_n (oNoAllowMultipleMessages, "no-allow-multiple-messages", "@"),
+  ARGPARSE_s_n (oAllowWeakDigestAlgos, "allow-weak-digest-algos", "@"),
 
   /* These two are aliases to help users of the PGP command line
      product use gpg with minimal pain.  Many commands are common
@@ -3066,6 +3068,10 @@ main (int argc, char **argv)
 	    opt.flags.allow_multiple_messages=0;
 	    break;
 
+          case oAllowWeakDigestAlgos:
+            opt.flags.allow_weak_digest_algos = 1;
+            break;
+
           case oFakedSystemTime:
             {
               time_t faked_time = isotime2epoch (pargs.r.ret_str);
@@ -3280,6 +3286,7 @@ main (int argc, char **argv)
 	    opt.pgp2_workarounds = 1;
 	    opt.ask_sig_expire = 0;
 	    opt.ask_cert_expire = 0;
+            opt.flags.allow_weak_digest_algos = 1;
 	    xfree(def_digest_string);
 	    def_digest_string = xstrdup("md5");
 	    xfree(s2k_digest_string);

g10/options.h

@@ -228,6 +228,7 @@ struct
     unsigned int utf8_filename:1;
     unsigned int dsa2:1;
     unsigned int allow_multiple_messages:1;
+    unsigned int allow_weak_digest_algos:1;
   } flags;
 
   /* Linked list of ways to find a key if the key isn't on the local

g10/sig-check.c

@@ -266,6 +266,22 @@ do_check( PKT_public_key *pk, PKT_signature *sig, gcry_md_hd_t digest,
     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)
+      {
+        static int shown;
+
+        if (!shown)
+          {
+            log_info
+              (_("Note: signatures using the %s algorithm are rejected\n"),
+               "MD5");
+            shown = 1;
+          }
+
+        return GPG_ERR_DIGEST_ALGO;
+      }
+
     /* Make sure the digest algo is enabled (in case of a detached
        signature).  */
     gcry_md_enable (digest, sig->digest_algo);

tests/openpgp/defs.inc

@@ -245,6 +245,7 @@ for f in gpg.conf gpg-agent.conf ; do
       gpg.conf)
         [ -n "${opt_always}" ] && echo "no-auto-check-trustdb" >>"$f"
         echo "agent-program $GPG_AGENT" >>"$f"
+        echo "allow-weak-digest-algos" >>"$f"
 
       ;;
       gpg-agent.conf)