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/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;