upstream: ssh-agent support for U2F/FIDO keys

feedback & ok markus@

OpenBSD-Commit-ID: bb544a44bc32e45d2ec8bf652db2046f38360acb

6 files changed, 372 insertions, 32 deletions

diff --git a/Makefile.in b/Makefile.in
index bac522ad2..c4b4b9357 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -24,6 +24,7 @@ ASKPASS_PROGRAM=$(libexecdir)/ssh-askpass
 SFTP_SERVER=$(libexecdir)/sftp-server
 SSH_KEYSIGN=$(libexecdir)/ssh-keysign
 SSH_PKCS11_HELPER=$(libexecdir)/ssh-pkcs11-helper
+SSH_SK_HELPER=$(libexecdir)/ssh-sk-helper
 PRIVSEP_PATH=@PRIVSEP_PATH@
 SSH_PRIVSEP_USER=@SSH_PRIVSEP_USER@
 STRIP_OPT=@STRIP_OPT@
@@ -35,6 +36,7 @@ PATHS= -DSSHDIR=\"$(sysconfdir)\" \
         -D_PATH_SFTP_SERVER=\"$(SFTP_SERVER)\" \
         -D_PATH_SSH_KEY_SIGN=\"$(SSH_KEYSIGN)\" \
         -D_PATH_SSH_PKCS11_HELPER=\"$(SSH_PKCS11_HELPER)\" \
+        -D_PATH_SSH_SK_HELPER=\"$(SSH_SK_HELPER)\" \
         -D_PATH_SSH_PIDDIR=\"$(piddir)\" \
         -D_PATH_PRIVSEP_CHROOT_DIR=\"$(PRIVSEP_PATH)\"
 
@@ -60,7 +62,7 @@ EXEEXT=@EXEEXT@
 MANFMT=@MANFMT@
 MKDIR_P=@MKDIR_P@
 
-TARGETS=ssh$(EXEEXT) sshd$(EXEEXT) ssh-add$(EXEEXT) ssh-keygen$(EXEEXT) ssh-keyscan${EXEEXT} ssh-keysign${EXEEXT} ssh-pkcs11-helper$(EXEEXT) ssh-agent$(EXEEXT) scp$(EXEEXT) sftp-server$(EXEEXT) sftp$(EXEEXT)
+TARGETS=ssh$(EXEEXT) sshd$(EXEEXT) ssh-add$(EXEEXT) ssh-keygen$(EXEEXT) ssh-keyscan${EXEEXT} ssh-keysign${EXEEXT} ssh-pkcs11-helper$(EXEEXT) ssh-agent$(EXEEXT) scp$(EXEEXT) sftp-server$(EXEEXT) sftp$(EXEEXT) ssh-sk-helper$(EXEEXT)
 
 XMSS_OBJS=\
         ssh-xmss.o \
@@ -199,6 +201,9 @@ ssh-keysign$(EXEEXT): $(LIBCOMPAT) libssh.a ssh-keysign.o readconf.o uidswap.o c
 ssh-pkcs11-helper$(EXEEXT): $(LIBCOMPAT) libssh.a ssh-pkcs11-helper.o ssh-pkcs11.o
         $(LD) -o $@ ssh-pkcs11-helper.o ssh-pkcs11.o $(LDFLAGS) -lssh -lopenbsd-compat -lssh -lopenbsd-compat $(LIBS)
 
+ssh-sk-helper$(EXEEXT): $(LIBCOMPAT) libssh.a ssh-sk-helper.o
+        $(LD) -o $@ ssh-sk-helper.o $(LDFLAGS) -lssh -lopenbsd-compat -lssh -lopenbsd-compat $(LIBS)
+
 ssh-keyscan$(EXEEXT): $(LIBCOMPAT) libssh.a ssh-keyscan.o
         $(LD) -o $@ ssh-keyscan.o $(LDFLAGS) -lssh -lopenbsd-compat -lssh $(LIBS)
 
@@ -350,6 +355,7 @@ install-files:
         $(INSTALL) -m 0755 $(STRIP_OPT) sshd$(EXEEXT) $(DESTDIR)$(sbindir)/sshd$(EXEEXT)
         $(INSTALL) -m 4711 $(STRIP_OPT) ssh-keysign$(EXEEXT) $(DESTDIR)$(SSH_KEYSIGN)$(EXEEXT)
         $(INSTALL) -m 0755 $(STRIP_OPT) ssh-pkcs11-helper$(EXEEXT) $(DESTDIR)$(SSH_PKCS11_HELPER)$(EXEEXT)
+        $(INSTALL) -m 0755 $(STRIP_OPT) ssh-sk-helper$(EXEEXT) $(DESTDIR)$(SSH_SK_HELPER)$(EXEEXT)
         $(INSTALL) -m 0755 $(STRIP_OPT) sftp$(EXEEXT) $(DESTDIR)$(bindir)/sftp$(EXEEXT)
         $(INSTALL) -m 0755 $(STRIP_OPT) sftp-server$(EXEEXT) $(DESTDIR)$(SFTP_SERVER)$(EXEEXT)
         $(INSTALL) -m 644 ssh.1.out $(DESTDIR)$(mandir)/$(mansubdir)1/ssh.1
@@ -426,6 +432,7 @@ uninstall:
         -rm -r $(DESTDIR)$(SFTP_SERVER)$(EXEEXT)
         -rm -f $(DESTDIR)$(SSH_KEYSIGN)$(EXEEXT)
         -rm -f $(DESTDIR)$(SSH_PKCS11_HELPER)$(EXEEXT)
+        -rm -f $(DESTDIR)$(SSH_SK_HELPER)$(EXEEXT)
         -rm -f $(DESTDIR)$(mandir)/$(mansubdir)1/ssh.1
         -rm -f $(DESTDIR)$(mandir)/$(mansubdir)1/scp.1
         -rm -f $(DESTDIR)$(mandir)/$(mansubdir)1/ssh-add.1
@@ -616,10 +623,10 @@ interop-tests t-exec file-tests: regress-prep regress-binaries $(TARGETS)
         TEST_SSH_SSHADD="$${BUILDDIR}/ssh-add"; \
         TEST_SSH_SSHKEYGEN="$${BUILDDIR}/ssh-keygen"; \
         TEST_SSH_SSHPKCS11HELPER="$${BUILDDIR}/ssh-pkcs11-helper"; \
+        TEST_SSH_SSHSKHELPER="$${BUILDDIR}/ssh-sk-helper"; \
         TEST_SSH_SSHKEYSCAN="$${BUILDDIR}/ssh-keyscan"; \
         TEST_SSH_SFTP="$${BUILDDIR}/sftp"; \
         TEST_SSH_SFTPSERVER="$${BUILDDIR}/sftp-server"; \
-        TEST_SSH_SSHPKCS11HELPER="$${BUILDDIR}/ssh-pkcs11-helper"; \
         TEST_SSH_PLINK="plink"; \
         TEST_SSH_PUTTYGEN="puttygen"; \
         TEST_SSH_CONCH="conch"; \
diff --git a/pathnames.h b/pathnames.h
index 2e0c7b15b..3a1bd1977 100644
--- a/pathnames.h
+++ b/pathnames.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: pathnames.h,v 1.29 2019/10/31 21:15:14 djm Exp $ */
+/* $OpenBSD: pathnames.h,v 1.30 2019/10/31 21:22:01 djm Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -133,6 +133,11 @@
 #define _PATH_SSH_PKCS11_HELPER         "/usr/libexec/ssh-pkcs11-helper"
 #endif
 
+/* Location of ssh-sk-helper to support keys in security keys */
+#ifndef _PATH_SSH_SK_HELPER
+#define _PATH_SSH_SK_HELPER             "/usr/libexec/ssh-sk-helper"
+#endif
+
 /* xauth for X11 forwarding */
 #ifndef _PATH_XAUTH
 #define _PATH_XAUTH                     "/usr/X11R6/bin/xauth"
diff --git a/ssh-agent.1 b/ssh-agent.1
index 83b2b41c8..7719384fe 100644
--- a/ssh-agent.1
+++ b/ssh-agent.1
@@ -1,4 +1,4 @@
-.\" $OpenBSD: ssh-agent.1,v 1.64 2016/11/30 06:54:26 jmc Exp $
+.\" $OpenBSD: ssh-agent.1,v 1.65 2019/10/31 21:22:01 djm Exp $
 .\"
 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -34,7 +34,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: November 30 2016 $
+.Dd $Mdocdate: October 31 2019 $
 .Dt SSH-AGENT 1
 .Os
 .Sh NAME
@@ -46,7 +46,7 @@
 .Op Fl \&Dd
 .Op Fl a Ar bind_address
 .Op Fl E Ar fingerprint_hash
-.Op Fl P Ar pkcs11_whitelist
+.Op Fl P Ar provider_whitelist
 .Op Fl t Ar life
 .Op Ar command Op Ar arg ...
 .Nm ssh-agent
@@ -122,15 +122,17 @@ The default is
 Kill the current agent (given by the
 .Ev SSH_AGENT_PID
 environment variable).
-.It Fl P Ar pkcs11_whitelist
-Specify a pattern-list of acceptable paths for PKCS#11 shared libraries
-that may be added using the
+.It Fl P Ar provider_whitelist
+Specify a pattern-list of acceptable paths for PKCS#11 and security key shared
+libraries that may be used with the
 .Fl s
-option to
+or
+.Fl S
+options to
 .Xr ssh-add 1 .
-The default is to allow loading PKCS#11 libraries from
+The default is to allow loading libraries from
 .Dq /usr/lib/*,/usr/local/lib/* .
-PKCS#11 libraries that do not match the whitelist will be refused.
+Libraries that do not match the whitelist will be refused.
 See PATTERNS in
 .Xr ssh_config 5
 for a description of pattern-list syntax.
diff --git a/ssh-agent.c b/ssh-agent.c
index e500591a9..6bf9536fb 100644
--- a/ssh-agent.c
+++ b/ssh-agent.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-agent.c,v 1.237 2019/06/28 13:35:04 deraadt Exp $ */
+/* $OpenBSD: ssh-agent.c,v 1.238 2019/10/31 21:22:01 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -41,6 +41,7 @@
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <sys/socket.h>
+#include <sys/wait.h>
 #ifdef HAVE_SYS_TIME_H
 # include <sys/time.h>
 #endif
@@ -85,13 +86,13 @@
 #include "digest.h"
 #include "ssherr.h"
 #include "match.h"
-
-#ifdef ENABLE_PKCS11
+#include "msg.h"
+#include "pathnames.h"
 #include "ssh-pkcs11.h"
-#endif
+#include "ssh-sk.h"
 
-#ifndef DEFAULT_PKCS11_WHITELIST
-# define DEFAULT_PKCS11_WHITELIST "/usr/lib*/*,/usr/local/lib*/*"
+#ifndef DEFAULT_PROVIDER_WHITELIST
+# define DEFAULT_PROVIDER_WHITELIST "/usr/lib*/*,/usr/local/lib*/*"
 #endif
 
 /* Maximum accepted message length */
@@ -123,6 +124,7 @@ typedef struct identity {
         char *provider;
         time_t death;
         u_int confirm;
+        char *sk_provider;
 } Identity;
 
 struct idtable {
@@ -146,8 +148,8 @@ pid_t cleanup_pid = 0;
 char socket_name[PATH_MAX];
 char socket_dir[PATH_MAX];
 
-/* PKCS#11 path whitelist */
-static char *pkcs11_whitelist;
+/* PKCS#11/Security key path whitelist */
+static char *provider_whitelist;
 
 /* locking */
 #define LOCK_SIZE       32
@@ -189,6 +191,7 @@ free_identity(Identity *id)
         sshkey_free(id->key);
         free(id->provider);
         free(id->comment);
+        free(id->sk_provider);
         free(id);
 }
 
@@ -278,6 +281,121 @@ agent_decode_alg(struct sshkey *key, u_int flags)
         return NULL;
 }
 
+static int
+provider_sign(const char *provider, struct sshkey *key,
+    u_char **sigp, size_t *lenp,
+    const u_char *data, size_t datalen,
+    const char *alg, u_int compat)
+{
+        int status, pair[2], r = SSH_ERR_INTERNAL_ERROR;
+        pid_t pid;
+        char *helper, *verbosity = NULL;
+        struct sshbuf *kbuf, *req, *resp;
+        u_char version;
+
+        debug3("%s: start for provider %s", __func__, provider);
+
+        *sigp = NULL;
+        *lenp = 0;
+
+        helper = getenv("SSH_SK_HELPER");
+        if (helper == NULL || strlen(helper) == 0)
+                helper = _PATH_SSH_SK_HELPER;
+        if (log_level_get() >= SYSLOG_LEVEL_DEBUG1)
+                verbosity = "-vvv";
+
+        /* Start helper */
+        if (socketpair(AF_UNIX, SOCK_STREAM, 0, pair) == -1) {
+                error("socketpair: %s", strerror(errno));
+                return SSH_ERR_SYSTEM_ERROR;
+        }
+        if ((pid = fork()) == -1) {
+                error("fork: %s", strerror(errno));
+                close(pair[0]);
+                close(pair[1]);
+                return SSH_ERR_SYSTEM_ERROR;
+        }
+        if (pid == 0) {
+                if ((dup2(pair[1], STDIN_FILENO) == -1) ||
+                    (dup2(pair[1], STDOUT_FILENO) == -1))
+                        fatal("%s: dup2: %s", __func__, ssh_err(r));
+                close(pair[0]);
+                close(pair[1]);
+                closefrom(STDERR_FILENO + 1);
+                debug("%s: starting %s %s", __func__, helper,
+                    verbosity == NULL ? "" : verbosity);
+                execlp(helper, helper, verbosity, (char *)NULL);
+                fatal("%s: execlp: %s", __func__, strerror(errno));
+        }
+        close(pair[1]);
+
+        if ((kbuf = sshbuf_new()) == NULL ||
+            (req = sshbuf_new()) == NULL ||
+            (resp = sshbuf_new()) == NULL)
+                fatal("%s: sshbuf_new failed", __func__);
+
+        if ((r = sshkey_private_serialize(key, kbuf)) != 0 ||
+            (r = sshbuf_put_stringb(req, kbuf)) != 0 ||
+            (r = sshbuf_put_cstring(req, provider)) != 0 ||
+            (r = sshbuf_put_string(req, data, datalen)) != 0 ||
+            (r = sshbuf_put_u32(req, compat)) != 0)
+                fatal("%s: compose: %s", __func__, ssh_err(r));
+        if ((r = ssh_msg_send(pair[0], SSH_SK_HELPER_VERSION, req)) != 0) {
+                error("%s: send: %s", __func__, ssh_err(r));
+                goto out;
+        }
+        if ((r = ssh_msg_recv(pair[0], resp)) != 0) {
+                error("%s: receive: %s", __func__, ssh_err(r));
+                goto out;
+        }
+        if ((r = sshbuf_get_u8(resp, &version)) != 0) {
+                error("%s: parse version: %s", __func__, ssh_err(r));
+                goto out;
+        }
+        if (version != SSH_SK_HELPER_VERSION) {
+                error("%s: unsupported version: got %u, expected %u",
+                    __func__, version, SSH_SK_HELPER_VERSION);
+                r = SSH_ERR_INVALID_FORMAT;
+                goto out;
+        }
+        if ((r = sshbuf_get_string(resp, sigp, lenp)) != 0) {
+                error("%s: parse signature: %s", __func__, ssh_err(r));
+                r = SSH_ERR_INVALID_FORMAT;
+                goto out;
+        }
+        if (sshbuf_len(resp) != 0) {
+                error("%s: trailing data in response", __func__);
+                r = SSH_ERR_INVALID_FORMAT;
+                goto out;
+        }
+        /* success */
+        r = 0;
+ out:
+        while (waitpid(pid, &status, 0) == -1) {
+                if (errno != EINTR)
+                        fatal("%s: waitpid: %s", __func__, ssh_err(r));
+        }
+        if (!WIFEXITED(status)) {
+                error("%s: helper %s exited abnormally", __func__, helper);
+                if (r == 0)
+                        r = SSH_ERR_SYSTEM_ERROR;
+        } else if (WEXITSTATUS(status) != 0) {
+                error("%s: helper %s exited with non-zero exit status",
+                    __func__, helper);
+                if (r == 0)
+                        r = SSH_ERR_SYSTEM_ERROR;
+        }
+        if (r != 0) {
+                freezero(*sigp, *lenp);
+                *sigp = NULL;
+                *lenp = 0;
+        }
+        sshbuf_free(kbuf);
+        sshbuf_free(req);
+        sshbuf_free(resp);
+        return r;
+}
+
 /* ssh2 only */
 static void
 process_sign_request2(SocketEntry *e)
@@ -308,10 +426,19 @@ process_sign_request2(SocketEntry *e)
                 verbose("%s: user refused key", __func__);
                 goto send;
         }
-        if ((r = sshkey_sign(id->key, &signature, &slen,
-            data, dlen, agent_decode_alg(key, flags), compat)) != 0) {
-                error("%s: sshkey_sign: %s", __func__, ssh_err(r));
-                goto send;
+        if (id->sk_provider != NULL) {
+                if ((r = provider_sign(id->sk_provider, id->key, &signature,
+                    &slen, data, dlen, agent_decode_alg(key, flags),
+                    compat)) != 0) {
+                        error("%s: sshkey_sign: %s", __func__, ssh_err(r));
+                        goto send;
+                }
+        } else {
+                if ((r = sshkey_sign(id->key, &signature, &slen,
+                    data, dlen, agent_decode_alg(key, flags), compat)) != 0) {
+                        error("%s: sshkey_sign: %s", __func__, ssh_err(r));
+                        goto send;
+                }
         }
         /* Success */
         ok = 0;
@@ -411,7 +538,7 @@ process_add_identity(SocketEntry *e)
         Identity *id;
         int success = 0, confirm = 0;
         u_int seconds, maxsign;
-        char *comment = NULL;
+        char *fp, *comment = NULL, *ext_name = NULL, *sk_provider = NULL;
         time_t death = 0;
         struct sshkey *k = NULL;
         u_char ctype;
@@ -456,15 +583,58 @@ process_add_identity(SocketEntry *e)
                                 goto err;
                         }
                         break;
+                case SSH_AGENT_CONSTRAIN_EXTENSION:
+                        if ((r = sshbuf_get_cstring(e->request,
+                            &ext_name, NULL)) != 0) {
+                                error("%s: cannot parse extension: %s",
+                                    __func__, ssh_err(r));
+                                goto err;
+                        }
+                        debug("%s: constraint ext %s", __func__, ext_name);
+                        if (strcmp(ext_name, "sk-provider@openssh.com") == 0) {
+                                if (sk_provider != NULL) {
+                                        error("%s already set", ext_name);
+                                        goto err;
+                                }
+                                if ((r = sshbuf_get_cstring(e->request,
+                                    &sk_provider, NULL)) != 0) {
+                                        error("%s: cannot parse %s: %s",
+                                            __func__, ext_name, ssh_err(r));
+                                        goto err;
+                                }
+                        } else {
+                                error("%s: unsupported constraint \"%s\"",
+                                    __func__, ext_name);
+                                goto err;
+                        }
+                        free(ext_name);
+                        break;
                 default:
                         error("%s: Unknown constraint %d", __func__, ctype);
  err:
+                        free(sk_provider);
+                        free(ext_name);
                         sshbuf_reset(e->request);
                         free(comment);
                         sshkey_free(k);
                         goto send;
                 }
         }
+        if (sk_provider != NULL) {
+                if (sshkey_type_plain(k->type) != KEY_ECDSA_SK) {
+                        error("Cannot add provider: %s is not a security key",
+                            sshkey_type(k));
+                        free(sk_provider);
+                        goto send;
+                }
+                if (match_pattern_list(sk_provider,
+                    provider_whitelist, 0) != 1) {
+                        error("Refusing add key: provider %s not whitelisted",
+                            sk_provider);
+                        free(sk_provider);
+                        goto send;
+                }
+        }
 
         success = 1;
         if (lifetime && !death)
@@ -478,11 +648,21 @@ process_add_identity(SocketEntry *e)
                 /* key state might have been updated */
                 sshkey_free(id->key);
                 free(id->comment);
+                free(id->sk_provider);
         }
         id->key = k;
         id->comment = comment;
         id->death = death;
         id->confirm = confirm;
+        id->sk_provider = sk_provider;
+
+        if ((fp = sshkey_fingerprint(k, SSH_FP_HASH_DEFAULT,
+            SSH_FP_DEFAULT)) == NULL)
+                fatal("%s: sshkey_fingerprint failed", __func__);
+        debug("%s: add %s %s \"%.100s\" (life: %u) (confirm: %u) "
+            "(provider: %s)", __func__, sshkey_ssh_name(k), fp, comment,
+            seconds, confirm, sk_provider == NULL ? "none" : sk_provider);
+        free(fp);
 send:
         send_status(e, success);
 }
@@ -600,7 +780,7 @@ process_add_smartcard_key(SocketEntry *e)
                     provider, strerror(errno));
                 goto send;
         }
-        if (match_pattern_list(canonical_provider, pkcs11_whitelist, 0) != 1) {
+        if (match_pattern_list(canonical_provider, provider_whitelist, 0) != 1) {
                 verbose("refusing PKCS#11 add of \"%.100s\": "
                     "provider not whitelisted", canonical_provider);
                 goto send;
@@ -1079,7 +1259,7 @@ usage(void)
 {
         fprintf(stderr,
             "usage: ssh-agent [-c | -s] [-Dd] [-a bind_address] [-E fingerprint_hash]\n"
-            "                 [-P pkcs11_whitelist] [-t life] [command [arg ...]]\n"
+            "                 [-P provider_whitelist] [-t life] [command [arg ...]]\n"
             "       ssh-agent [-c | -s] -k\n");
         exit(1);
 }
@@ -1137,9 +1317,9 @@ main(int ac, char **av)
                         k_flag++;
                         break;
                 case 'P':
-                        if (pkcs11_whitelist != NULL)
+                        if (provider_whitelist != NULL)
                                 fatal("-P option already specified");
-                        pkcs11_whitelist = xstrdup(optarg);
+                        provider_whitelist = xstrdup(optarg);
                         break;
                 case 's':
                         if (c_flag)
@@ -1175,8 +1355,8 @@ main(int ac, char **av)
         if (ac > 0 && (c_flag || k_flag || s_flag || d_flag || D_flag))
                 usage();
 
-        if (pkcs11_whitelist == NULL)
-                pkcs11_whitelist = xstrdup(DEFAULT_PKCS11_WHITELIST);
+        if (provider_whitelist == NULL)
+                provider_whitelist = xstrdup(DEFAULT_PROVIDER_WHITELIST);
 
         if (ac == 0 && !c_flag && !s_flag) {
                 shell = getenv("SHELL");
diff --git a/ssh-sk-helper.c b/ssh-sk-helper.c
new file mode 100644
index 000000000..0a0c92a44
--- /dev/null
+++ b/ssh-sk-helper.c
@@ -0,0 +1,143 @@
+/* $OpenBSD: ssh-sk-helper.c,v 1.1 2019/10/31 21:22:01 djm Exp $ */
+/*
+ * Copyright (c) 2019 Google LLC
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * This is a tiny program used to isolate the address space used for
+ * security key middleware signing operations from ssh-agent. It is similar
+ * to ssh-pkcs11-helper.c but considerably simpler as the signing operation
+ * for this case are stateless.
+ *
+ * It receives a signing request (key, provider, message, flags) from
+ * stdin, attempts to perform a signature using the security key provider
+ * and returns the resultant signature via stdout.
+ *
+ * In the future, this program might gain additional functions to support
+ * FIDO2 tokens such as enumerating resident keys. When this happens it will
+ * be necessary to crank SSH_SK_HELPER_VERSION below.
+ */
+
+#include "includes.h"
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+
+#include "xmalloc.h"
+#include "log.h"
+#include "sshkey.h"
+#include "authfd.h"
+#include "misc.h"
+#include "sshbuf.h"
+#include "msg.h"
+#include "uidswap.h"
+#include "sshkey.h"
+#include "ssherr.h"
+#include "ssh-sk.h"
+
+extern char *__progname;
+
+int
+main(int argc, char **argv)
+{
+        SyslogFacility log_facility = SYSLOG_FACILITY_AUTH;
+        LogLevel log_level = SYSLOG_LEVEL_ERROR;
+        struct sshbuf *req, *resp, *kbuf;
+        struct sshkey *key;
+        uint32_t compat;
+        const u_char *message;
+        u_char version, *sig;
+        size_t msglen, siglen;
+        char *provider;
+        int in, out, ch, r, log_stderr = 0;
+
+        sanitise_stdfd();
+        log_init(__progname, log_level, log_facility, log_stderr);
+
+        while ((ch = getopt(argc, argv, "v")) != -1) {
+                switch (ch) {
+                case 'v':
+                        log_stderr = 1;
+                        if (log_level == SYSLOG_LEVEL_ERROR)
+                                log_level = SYSLOG_LEVEL_DEBUG1;
+                        else if (log_level < SYSLOG_LEVEL_DEBUG3)
+                                log_level++;
+                        break;
+                default:
+                        fprintf(stderr, "usage: %s [-v]\n", __progname);
+                        exit(1);
+                }
+        }
+        log_init(__progname, log_level, log_facility, log_stderr);
+
+        /*
+         * Rearrange our file descriptors a little; we don't trust the
+         * providers not to fiddle with stdin/out.
+         */
+        closefrom(STDERR_FILENO + 1);
+        if ((in = dup(STDIN_FILENO)) == -1 || (out = dup(STDOUT_FILENO)) == -1)
+                fatal("%s: dup: %s", __progname, strerror(errno));
+        close(STDIN_FILENO);
+        close(STDOUT_FILENO);
+        sanitise_stdfd(); /* resets to /dev/null */
+
+        if ((req = sshbuf_new()) == NULL || (resp = sshbuf_new()) == NULL)
+                fatal("%s: sshbuf_new failed", __progname);
+        if (ssh_msg_recv(in, req) < 0)
+                fatal("ssh_msg_recv failed");
+        close(in);
+        debug("%s: received message len %zu", __progname, sshbuf_len(req));
+
+        if ((r = sshbuf_get_u8(req, &version)) != 0)
+                fatal("%s: buffer error: %s", __progname, ssh_err(r));
+        if (version != SSH_SK_HELPER_VERSION) {
+                fatal("unsupported version: received %d, expected %d",
+                    version, SSH_SK_HELPER_VERSION);
+        }
+        if ((r = sshbuf_froms(req, &kbuf)) != 0 ||
+            (r = sshkey_private_deserialize(kbuf, &key)) != 0)
+                fatal("Unable to parse key: %s", ssh_err(r));
+        if (sshkey_type_plain(key->type) != KEY_ECDSA_SK)
+                fatal("Unsupported key type %s", sshkey_ssh_name(key));
+
+        if ((r = sshbuf_get_cstring(req, &provider, NULL)) != 0 ||
+            (r = sshbuf_get_string_direct(req, &message, &msglen)) != 0 ||
+            (r = sshbuf_get_u32(req, &compat)) != 0)
+                fatal("%s: buffer error: %s", __progname, ssh_err(r));
+        if (sshbuf_len(req) != 0)
+                fatal("%s: trailing data in request", __progname);
+
+        debug("%s: ready to sign with key %s, provider %s: "
+            "msg len %zu, compat 0x%lx", __progname, sshkey_type(key),
+            provider, msglen, (u_long)compat);
+
+        if ((r = sshsk_ecdsa_sign(provider, key, &sig, &siglen,
+            message, msglen, compat)) != 0)
+                fatal("Signing failed: %s", ssh_err(r));
+
+        /* send reply */
+        if ((r = sshbuf_put_string(resp, sig, siglen)) != 0)
+                fatal("%s: buffer error: %s", __progname, ssh_err(r));
+        debug("%s: reply len %zu", __progname, sshbuf_len(resp));
+        if (ssh_msg_send(out, SSH_SK_HELPER_VERSION, resp) == -1)
+                fatal("ssh_msg_send failed");
+        close(out);
+
+        return (0);
+}
diff --git a/ssh-sk.h b/ssh-sk.h
index 7c1d2b927..5033e6f68 100644
--- a/ssh-sk.h
+++ b/ssh-sk.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-sk.h,v 1.1 2019/10/31 21:16:20 djm Exp $ */
+/* $OpenBSD: ssh-sk.h,v 1.2 2019/10/31 21:22:01 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -21,6 +21,9 @@
 struct sshbuf;
 struct sshkey;
 
+/* Version of protocol between ssh-agent and ssh-sk-helper */
+#define SSH_SK_HELPER_VERSION   1
+
 /*
  * Enroll (generate) a new security-key hosted private key via the specified
  * provider middleware.