agent: Make digest algorithms for ssh fingerprints configurable.

* agent/agent.h (opt): New field 'ssh_fingerprint_digest'.
* agent/command-ssh.c (data_sign, ssh_identity_register): Honor the
option for strings used to communicate with the user.
* agent/findkey.c (agent_modify_description): Likewise.
* agent/gpg-agent.c (cmd_and_opt_values): New value.
(opts): New option '--ssh-fingerprint-digest'.
(parse_rereadable_options): Set the default to MD5 for now.
(main): Handle the new option.
* doc/gpg-agent.texi: Document the new option.
--

OpenSSH has transitioned from using MD5 to compute key fingerprints to
SHA256.  This patch makes the digest used when communicating key
fingerprints to the user (e.g. in pinentry dialogs) configurable.
For now this patch conservatively defaults to MD5.

GnuPG-bug-id: 2106
Signed-off-by: Justus Winter <justus@g10code.com>

agent/agent.h

@@ -167,6 +167,10 @@ struct
      gpg-agent.c: If the value is less than 2 the name has not yet
      been malloced. */
   int browser_socket;
+
+  /* The digest algorithm to use for ssh fingerprints when
+   * communicating with the user.  */
+  int ssh_fingerprint_digest;
 } opt;
 
 

agent/command-ssh.c

@@ -2774,7 +2774,7 @@ data_sign (ctrl_t ctrl, ssh_key_type_spec_t *spec,
       err = agent_raw_key_from_file (ctrl, ctrl->keygrip, &key);
       if (err)
         goto out;
-      err = ssh_get_fingerprint_string (key, GCRY_MD_MD5, &fpr);
+      err = ssh_get_fingerprint_string (key, opt.ssh_fingerprint_digest, &fpr);
       if (!err)
         {
           gcry_sexp_t tmpsxp = gcry_sexp_find_token (key, "comment", 0);
@@ -3052,7 +3052,7 @@ ssh_identity_register (ctrl_t ctrl, ssh_key_type_spec_t *spec,
 
   bin2hex (key_grip_raw, 20, key_grip);
 
-  err = ssh_get_fingerprint_string (key, GCRY_MD_MD5, &key_fpr);
+  err = ssh_get_fingerprint_string (key, opt.ssh_fingerprint_digest, &key_fpr);
   if (err)
     goto out;
 

agent/findkey.c

@@ -412,7 +412,8 @@ agent_modify_description (const char *in, const char *comment,
 
                 case 'F': /* SSH style fingerprint.  */
                   if (!ssh_fpr && key)
-                    ssh_get_fingerprint_string (key, GCRY_MD_MD5, &ssh_fpr);
+                    ssh_get_fingerprint_string (key, opt.ssh_fingerprint_digest,
+                                                &ssh_fpr);
                   if (ssh_fpr)
                     {
                       if (out)

agent/gpg-agent.c

@@ -129,6 +129,7 @@ enum cmd_and_opt_values
   oKeepTTY,
   oKeepDISPLAY,
   oSSHSupport,
+  oSSHFingerprintDigest,
   oPuttySupport,
   oDisableScdaemon,
   oDisableCheckOwnSocket,
@@ -232,6 +233,8 @@ static ARGPARSE_OPTS opts[] = {
                 /* */    N_("allow passphrase to be prompted through Emacs")),
 
   ARGPARSE_s_n (oSSHSupport,   "enable-ssh-support", N_("enable ssh support")),
+  ARGPARSE_s_s (oSSHFingerprintDigest, "ssh-fingerprint-digest",
+                N_("digest to use when communicating ssh fingerprints")),
   ARGPARSE_s_n (oPuttySupport, "enable-putty-support",
 #ifdef HAVE_W32_SYSTEM
                 /* */           N_("enable putty support")
@@ -800,6 +803,7 @@ parse_rereadable_options (ARGPARSE_ARGS *pargs, int reread)
       opt.allow_emacs_pinentry = 0;
       opt.disable_scdaemon = 0;
       disable_check_own_socket = 0;
+      opt.ssh_fingerprint_digest = GCRY_MD_MD5;
       return 1;
     }
 
@@ -1176,6 +1180,11 @@ main (int argc, char **argv )
 	case oSSHSupport:
           ssh_support = 1;
           break;
+	case oSSHFingerprintDigest:
+          opt.ssh_fingerprint_digest = gcry_md_map_name (pargs.r.ret_str);
+          if (opt.ssh_fingerprint_digest == 0)
+            log_error ("Unknown digest algorithm: %s\n", pargs.r.ret_str);
+          break;
         case oPuttySupport:
 #        ifdef HAVE_W32_SYSTEM
           putty_support = 1;

doc/gpg-agent.texi

@@ -636,6 +636,13 @@ and allows the use of gpg-agent with the ssh implementation
 @command{putty}.  This is similar to the regular ssh-agent support but
 makes use of Windows message queue as required by @command{putty}.
 
+@anchor{option --ssh-fingerprint-digest}
+@item --ssh-fingerprint-digest
+@opindex ssh-fingerprint-digest
+
+Select the digest algorithm used to compute ssh fingerprints that are
+communicated to the user, e.g. in pinentry dialogs.  OpenSSH has
+transitioned from using MD5 to the more secure SHA256.
 
 @end table