upstream: Extends the SK API to accept a set of key/value options

for all operations. These are intended to future-proof the API a little by
making it easier to specify additional fields for without having to change
the API version for each.

At present, only two options are defined: one to explicitly specify
the device for an operation (rather than accepting the middleware's
autoselection) and another to specify the FIDO2 username that may
be used when generating a resident key. These new options may be
invoked at key generation time via ssh-keygen -O

This also implements a suggestion from Markus to avoid "int" in favour
of uint32_t for the algorithm argument in the API, to make implementation
of ssh-sk-client/helper a little easier.

feedback, fixes and ok markus@

OpenBSD-Commit-ID: 973ce11704609022ab36abbdeb6bc23c8001eabc

10 files changed, 404 insertions, 121 deletions

diff --git a/PROTOCOL.u2f b/PROTOCOL.u2f
index 5f44c3acc..fd0cd0de0 100644
--- a/PROTOCOL.u2f
+++ b/PROTOCOL.u2f
@@ -233,7 +233,7 @@ support for the common case of USB HID security keys internally.
 
 The middleware library need only expose a handful of functions:
 
-        #define SSH_SK_VERSION_MAJOR            0x00030000 /* API version */
+        #define SSH_SK_VERSION_MAJOR            0x00040000 /* API version */
         #define SSH_SK_VERSION_MAJOR_MASK       0xffff0000
 
         /* Flags */
@@ -245,6 +245,11 @@ The middleware library need only expose a handful of functions:
         #define SSH_SK_ECDSA                   0x00
         #define SSH_SK_ED25519                 0x01
 
+        /* Error codes */
+        #define SSH_SK_ERR_GENERAL              -1
+        #define SSH_SK_ERR_UNSUPPORTED          -2
+        #define SSH_SK_ERR_PIN_REQUIRED         -3
+
         struct sk_enroll_response {
                 uint8_t *public_key;
                 size_t public_key_len;
@@ -266,35 +271,63 @@ The middleware library need only expose a handful of functions:
         };
 
         struct sk_resident_key {
-                uint8_t alg;
+                uint32_t alg;
                 size_t slot;
                 char *application;
                 struct sk_enroll_response key;
         };
 
+        struct sk_option {
+                char *name;
+                char *value;
+                uint8_t important;
+        };
+
         /* Return the version of the middleware API */
         uint32_t sk_api_version(void);
 
         /* Enroll a U2F key (private key generation) */
-        int sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
+        int sk_enroll(uint32_t alg,
+            const uint8_t *challenge, size_t challenge_len,
             const char *application, uint8_t flags, const char *pin,
+            struct sk_option **options,
             struct sk_enroll_response **enroll_response);
 
         /* Sign a challenge */
-        int sk_sign(int alg, const uint8_t *message, size_t message_len,
+        int sk_sign(uint32_t alg, const uint8_t *message, size_t message_len,
             const char *application,
             const uint8_t *key_handle, size_t key_handle_len,
-            uint8_t flags, const char *pin,
+            uint8_t flags, const char *pin, struct sk_option **options,
             struct sk_sign_response **sign_response);
 
         /* Enumerate all resident keys */
-        int sk_load_resident_keys(const char *pin,
+        int sk_load_resident_keys(const char *pin, struct sk_option **options,
             struct sk_resident_key ***rks, size_t *nrks);
 
 The SSH_SK_VERSION_MAJOR should be incremented for each incompatible
 API change.
 
-In OpenSSH, these will be invoked by using a similar mechanism to
+The options may be used to pass miscellaneous options to the middleware
+as a NULL-terminated array of pointers to struct sk_option. The middleware
+may ignore unsupported or unknown options unless the "important" flag is
+set, in which case it should return failure if an unsupported option is
+requested.
+
+At present the following options names are supported:
+
+        "device"
+
+        Specifies a specific FIDO device on which to perform the
+        operation. The value in this field is interpreted by the
+        middleware but it would be typical to specify a path to
+        a /dev node for the device in question.
+
+        "user"
+
+        Specifies the FIDO2 username used when enrolling a key,
+        overriding OpenSSH's default of using an all-zero username.
+
+In OpenSSH, the middleware will be invoked by using a similar mechanism to
 ssh-pkcs11-helper to provide address-space containment of the
 middleware from ssh-agent.
 
diff --git a/sk-api.h b/sk-api.h
index dc786d556..93d6a1229 100644
--- a/sk-api.h
+++ b/sk-api.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: sk-api.h,v 1.6 2019/12/30 09:24:45 djm Exp $ */
+/* $OpenBSD: sk-api.h,v 1.7 2020/01/06 02:00:46 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -58,30 +58,37 @@ struct sk_sign_response {
 };
 
 struct sk_resident_key {
-        uint8_t alg;
+        uint32_t alg;
         size_t slot;
         char *application;
         struct sk_enroll_response key;
 };
 
-#define SSH_SK_VERSION_MAJOR            0x00030000 /* current API version */
+struct sk_option {
+        char *name;
+        char *value;
+        uint8_t required;
+};
+
+#define SSH_SK_VERSION_MAJOR            0x00040000 /* current API version */
 #define SSH_SK_VERSION_MAJOR_MASK       0xffff0000
 
 /* Return the version of the middleware API */
 uint32_t sk_api_version(void);
 
 /* Enroll a U2F key (private key generation) */
-int sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
+int sk_enroll(uint32_t alg, const uint8_t *challenge, size_t challenge_len,
     const char *application, uint8_t flags, const char *pin,
-    struct sk_enroll_response **enroll_response);
+    struct sk_option **options, struct sk_enroll_response **enroll_response);
 
 /* Sign a challenge */
-int sk_sign(int alg, const uint8_t *message, size_t message_len,
+int sk_sign(uint32_t alg, const uint8_t *message, size_t message_len,
     const char *application, const uint8_t *key_handle, size_t key_handle_len,
-    uint8_t flags, const char *pin, struct sk_sign_response **sign_response);
+    uint8_t flags, const char *pin, struct sk_option **options,
+    struct sk_sign_response **sign_response);
 
 /* Enumerate all resident keys */
-int sk_load_resident_keys(const char *pin,
+int sk_load_resident_keys(const char *pin, struct sk_option **options,
     struct sk_resident_key ***rks, size_t *nrks);
 
 #endif /* _SK_API_H */
diff --git a/sk-usbhid.c b/sk-usbhid.c
index 22a4c5df5..2e1573c48 100644
--- a/sk-usbhid.c
+++ b/sk-usbhid.c
@@ -54,7 +54,7 @@
         } while (0)
 #endif
 
-#define SK_VERSION_MAJOR        0x00030000 /* current API version */
+#define SK_VERSION_MAJOR        0x00040000 /* current API version */
 
 /* Flags */
 #define SK_USER_PRESENCE_REQD           0x01
@@ -91,12 +91,18 @@ struct sk_sign_response {
 };
 
 struct sk_resident_key {
-        uint8_t alg;
+        uint32_t alg;
         size_t slot;
         char *application;
         struct sk_enroll_response key;
 };
 
+struct sk_option {
+        char *name;
+        char *value;
+        uint8_t required;
+};
+
 /* If building as part of OpenSSH, then rename exported functions */
 #if !defined(SK_STANDALONE)
 #define sk_api_version          ssh_sk_api_version
@@ -109,17 +115,18 @@ struct sk_resident_key {
 uint32_t sk_api_version(void);
 
 /* Enroll a U2F key (private key generation) */
-int sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
+int sk_enroll(uint32_t alg, const uint8_t *challenge, size_t challenge_len,
     const char *application, uint8_t flags, const char *pin,
-    struct sk_enroll_response **enroll_response);
+    struct sk_option **options, struct sk_enroll_response **enroll_response);
 
 /* Sign a challenge */
-int sk_sign(int alg, const uint8_t *message, size_t message_len,
+int sk_sign(uint32_t alg, const uint8_t *message, size_t message_len,
     const char *application, const uint8_t *key_handle, size_t key_handle_len,
-    uint8_t flags, const char *pin, struct sk_sign_response **sign_response);
+    uint8_t flags, const char *pin, struct sk_option **options,
+    struct sk_sign_response **sign_response);
 
 /* Load resident keys */
-int sk_load_resident_keys(const char *pin,
+int sk_load_resident_keys(const char *pin, struct sk_option **options,
     struct sk_resident_key ***rks, size_t *nrks);
 
 static void skdebug(const char *func, const char *fmt, ...)
@@ -235,15 +242,27 @@ try_device(fido_dev_t *dev, const uint8_t *message, size_t message_len,
 
 /* Iterate over configured devices looking for a specific key handle */
 static fido_dev_t *
-find_device(const uint8_t *message, size_t message_len, const char *application,
-    const uint8_t *key_handle, size_t key_handle_len)
+find_device(const char *path, const uint8_t *message, size_t message_len,
+    const char *application, const uint8_t *key_handle, size_t key_handle_len)
 {
         fido_dev_info_t *devlist = NULL;
         fido_dev_t *dev = NULL;
         size_t devlist_len = 0, i;
-        const char *path;
         int r;
 
+        if (path != NULL) {
+                if ((dev = fido_dev_new()) == NULL) {
+                        skdebug(__func__, "fido_dev_new failed");
+                        return NULL;
+                }
+                if ((r = fido_dev_open(dev, path)) != FIDO_OK) {
+                        skdebug(__func__, "fido_dev_open failed");
+                        fido_dev_free(&dev);
+                        return NULL;
+                }
+                return dev;
+        }
+
         if ((devlist = fido_dev_info_new(MAX_FIDO_DEVICES)) == NULL) {
                 skdebug(__func__, "fido_dev_info_new failed");
                 goto out;
@@ -402,7 +421,7 @@ pack_public_key_ed25519(const fido_cred_t *cred,
 }
 
 static int
-pack_public_key(int alg, const fido_cred_t *cred,
+pack_public_key(uint32_t alg, const fido_cred_t *cred,
     struct sk_enroll_response *response)
 {
         switch(alg) {
@@ -431,10 +450,45 @@ fidoerr_to_skerr(int fidoerr)
         }
 }
 
+static int
+check_enroll_options(struct sk_option **options, char **devicep,
+    uint8_t *user_id, size_t user_id_len)
+{
+        size_t i;
+
+        if (options == NULL)
+                return 0;
+        for (i = 0; options[i] != NULL; i++) {
+                if (strcmp(options[i]->name, "device") == 0) {
+                        if ((*devicep = strdup(options[i]->value)) == NULL) {
+                                skdebug(__func__, "strdup device failed");
+                                return -1;
+                        }
+                        skdebug(__func__, "requested device %s", *devicep);
+                } if (strcmp(options[i]->name, "user") == 0) {
+                        if (strlcpy(user_id, options[i]->value, user_id_len) >=
+                            user_id_len) {
+                                skdebug(__func__, "user too long");
+                                return -1;
+                        }
+                        skdebug(__func__, "requested user %s",
+                            (char *)user_id);
+                } else {
+                        skdebug(__func__, "requested unsupported option %s",
+                            options[i]->name);
+                        if (options[i]->required) {
+                                skdebug(__func__, "unknown required option");
+                                return -1;
+                        }
+                }
+        }
+        return 0;
+}
+
 int
-sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
+sk_enroll(uint32_t alg, const uint8_t *challenge, size_t challenge_len,
     const char *application, uint8_t flags, const char *pin,
-    struct sk_enroll_response **enroll_response)
+    struct sk_option **options, struct sk_enroll_response **enroll_response)
 {
         fido_cred_t *cred = NULL;
         fido_dev_t *dev = NULL;
@@ -454,6 +508,11 @@ sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
                 skdebug(__func__, "enroll_response == NULL");
                 goto out;
         }
+        memset(user_id, 0, sizeof(user_id));
+        if (check_enroll_options(options, &device,
+            user_id, sizeof(user_id)) != 0)
+                goto out; /* error already logged */
+
         *enroll_response = NULL;
         switch(alg) {
 #ifdef WITH_OPENSSL
@@ -468,7 +527,7 @@ sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
                 skdebug(__func__, "unsupported key type %d", alg);
                 goto out;
         }
-        if ((device = pick_first_device()) == NULL) {
+        if (device == NULL && (device = pick_first_device()) == NULL) {
                 skdebug(__func__, "pick_first_device failed");
                 goto out;
         }
@@ -477,7 +536,6 @@ sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
                 skdebug(__func__, "fido_cred_new failed");
                 goto out;
         }
-        memset(user_id, 0, sizeof(user_id));
         if ((r = fido_cred_set_type(cred, cose_alg)) != FIDO_OK) {
                 skdebug(__func__, "fido_cred_set_type: %s", fido_strerr(r));
                 goto out;
@@ -654,7 +712,8 @@ pack_sig_ed25519(fido_assert_t *assert, struct sk_sign_response *response)
 }
 
 static int
-pack_sig(int alg, fido_assert_t *assert, struct sk_sign_response *response)
+pack_sig(uint32_t  alg, fido_assert_t *assert,
+    struct sk_sign_response *response)
 {
         switch(alg) {
 #ifdef WITH_OPENSSL
@@ -668,13 +727,42 @@ pack_sig(int alg, fido_assert_t *assert, struct sk_sign_response *response)
         }
 }
 
+/* Checks sk_options for sk_sign() and sk_load_resident_keys() */
+static int
+check_sign_load_resident_options(struct sk_option **options, char **devicep)
+{
+        size_t i;
+
+        if (options == NULL)
+                return 0;
+        for (i = 0; options[i] != NULL; i++) {
+                if (strcmp(options[i]->name, "device") == 0) {
+                        if ((*devicep = strdup(options[i]->value)) == NULL) {
+                                skdebug(__func__, "strdup device failed");
+                                return -1;
+                        }
+                        skdebug(__func__, "requested device %s", *devicep);
+                } else {
+                        skdebug(__func__, "requested unsupported option %s",
+                            options[i]->name);
+                        if (options[i]->required) {
+                                skdebug(__func__, "unknown required option");
+                                return -1;
+                        }
+                }
+        }
+        return 0;
+}
+
 int
-sk_sign(int alg, const uint8_t *message, size_t message_len,
+sk_sign(uint32_t alg, const uint8_t *message, size_t message_len,
     const char *application,
     const uint8_t *key_handle, size_t key_handle_len,
-    uint8_t flags, const char *pin, struct sk_sign_response **sign_response)
+    uint8_t flags, const char *pin, struct sk_option **options,
+    struct sk_sign_response **sign_response)
 {
         fido_assert_t *assert = NULL;
+        char *device = NULL;
         fido_dev_t *dev = NULL;
         struct sk_sign_response *response = NULL;
         int ret = SSH_SK_ERR_GENERAL;
@@ -689,8 +777,10 @@ sk_sign(int alg, const uint8_t *message, size_t message_len,
                 goto out;
         }
         *sign_response = NULL;
-        if ((dev = find_device(message, message_len, application, key_handle,
-            key_handle_len)) == NULL) {
+        if (check_sign_load_resident_options(options, &device) != 0)
+                goto out; /* error already logged */
+        if ((dev = find_device(device, message, message_len,
+            application, key_handle, key_handle_len)) == NULL) {
                 skdebug(__func__, "couldn't find device for key handle");
                 goto out;
         }
@@ -737,6 +827,7 @@ sk_sign(int alg, const uint8_t *message, size_t message_len,
         response = NULL;
         ret = 0;
  out:
+        free(device);
         if (response != NULL) {
                 free(response->sig_r);
                 free(response->sig_s);
@@ -789,6 +880,7 @@ read_rks(const char *devpath, const char *pin,
                 }
                 skdebug(__func__, "get metadata for %s failed: %s",
                     devpath, fido_strerr(r));
+                ret = fidoerr_to_skerr(r);
                 goto out;
         }
         skdebug(__func__, "existing %llu, remaining %llu",
@@ -904,7 +996,7 @@ read_rks(const char *devpath, const char *pin,
 }
 
 int
-sk_load_resident_keys(const char *pin,
+sk_load_resident_keys(const char *pin, struct sk_option **options,
     struct sk_resident_key ***rksp, size_t *nrksp)
 {
         int ret = SSH_SK_ERR_GENERAL, r = -1;
@@ -912,39 +1004,57 @@ sk_load_resident_keys(const char *pin,
         size_t i, ndev = 0, nrks = 0;
         const fido_dev_info_t *di;
         struct sk_resident_key **rks = NULL;
+        char *device = NULL;
         *rksp = NULL;
         *nrksp = 0;
 
-        if ((devlist = fido_dev_info_new(MAX_FIDO_DEVICES)) == NULL) {
-                skdebug(__func__, "fido_dev_info_new failed");
-                goto out;
-        }
-        if ((r = fido_dev_info_manifest(devlist,
-            MAX_FIDO_DEVICES, &ndev)) != FIDO_OK) {
-                skdebug(__func__, "fido_dev_info_manifest failed: %s",
-                    fido_strerr(r));
-                goto out;
-        }
-        for (i = 0; i < ndev; i++) {
-                if ((di = fido_dev_info_ptr(devlist, i)) == NULL) {
-                        skdebug(__func__, "no dev info at %zu", i);
-                        continue;
-                }
-                skdebug(__func__, "trying %s", fido_dev_info_path(di));
-                if ((r = read_rks(fido_dev_info_path(di), pin,
-                    &rks, &nrks)) != 0) {
+        if (check_sign_load_resident_options(options, &device) != 0)
+                goto out; /* error already logged */
+        if (device != NULL) {
+                skdebug(__func__, "trying %s", device);
+                if ((r = read_rks(device, pin, &rks, &nrks)) != 0) {
                         skdebug(__func__, "read_rks failed for %s",
                             fido_dev_info_path(di));
-                        continue;
+                        ret = r;
+                        goto out;
+                }
+        } else {
+                /* Try all devices */
+                if ((devlist = fido_dev_info_new(MAX_FIDO_DEVICES)) == NULL) {
+                        skdebug(__func__, "fido_dev_info_new failed");
+                        goto out;
+                }
+                if ((r = fido_dev_info_manifest(devlist,
+                    MAX_FIDO_DEVICES, &ndev)) != FIDO_OK) {
+                        skdebug(__func__, "fido_dev_info_manifest failed: %s",
+                            fido_strerr(r));
+                        goto out;
+                }
+                for (i = 0; i < ndev; i++) {
+                        if ((di = fido_dev_info_ptr(devlist, i)) == NULL) {
+                                skdebug(__func__, "no dev info at %zu", i);
+                                continue;
+                        }
+                        skdebug(__func__, "trying %s", fido_dev_info_path(di));
+                        if ((r = read_rks(fido_dev_info_path(di), pin,
+                            &rks, &nrks)) != 0) {
+                                skdebug(__func__, "read_rks failed for %s",
+                                    fido_dev_info_path(di));
+                                /* remember last error */
+                                ret = r;
+                                continue;
+                        }
                 }
         }
-        /* success */
-        ret = 0;
+        /* success, unless we have no keys but a specific error */
+        if (nrks > 0 || ret == SSH_SK_ERR_GENERAL)
+                ret = 0;
         *rksp = rks;
         *nrksp = nrks;
         rks = NULL;
         nrks = 0;
  out:
+        free(device);
         for (i = 0; i < nrks; i++) {
                 free(rks[i]->application);
                 freezero(rks[i]->key.public_key, rks[i]->key.public_key_len);
diff --git a/ssh-add.c b/ssh-add.c
index c25b57cc1..fbb2578dd 100644
--- a/ssh-add.c
+++ b/ssh-add.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-add.c,v 1.148 2019/12/30 09:22:49 djm Exp $ */
+/* $OpenBSD: ssh-add.c,v 1.149 2020/01/06 02:00:46 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -549,7 +549,8 @@ load_resident_keys(int agent_fd, const char *skprovider, int qflag)
         char *fp;
 
         pass = read_passphrase("Enter PIN for security key: ", RP_ALLOW_STDIN);
-        if ((r = sshsk_load_resident(skprovider, pass, &keys, &nkeys)) != 0) {
+        if ((r = sshsk_load_resident(skprovider, NULL, pass,
+            &keys, &nkeys)) != 0) {
                 error("Unable to load resident keys: %s", ssh_err(r));
                 return r;
         }
diff --git a/ssh-keygen.1 b/ssh-keygen.1
index 7b83a2240..92c516588 100644
--- a/ssh-keygen.1
+++ b/ssh-keygen.1
@@ -1,4 +1,4 @@
-.\"     $OpenBSD: ssh-keygen.1,v 1.188 2020/01/03 07:33:33 jmc Exp $
+.\"     $OpenBSD: ssh-keygen.1,v 1.189 2020/01/06 02:00:46 djm Exp $
 .\"
 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -35,7 +35,7 @@
 .\" (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: January 3 2020 $
+.Dd $Mdocdate: January 6 2020 $
 .Dt SSH-KEYGEN 1
 .Os
 .Sh NAME
@@ -462,8 +462,18 @@ section may be specified.
 .Pp
 When generating a key that will be hosted on a FIDO authenticator, this
 flag may be used to specify key-specific options.
-Two FIDO authenticator options are supported at present:
-.Pp
+The FIDO authenticator options are supported at present are:
+.Pp
+.Cm application
+overrides the default FIDO application/origin string of
+.Dq ssh: .
+This option may be useful when generating host or domain-specific resident
+keys.
+.Cm device
+explicitly specify a device to generate the key on, rather than accepting
+the authenticator middleware's automatic selection.
+.Xr fido 4
+device to use, rather than letting the token middleware select one.
 .Cm no-touch-required
 indicates that the generated private key should not require touch
 events (user presence) when making signatures.
@@ -478,6 +488,11 @@ Resident keys may be supported on FIDO2 tokens and typically require that
 a PIN be set on the token prior to generation.
 Resident keys may be loaded off the token using
 .Xr ssh-add 1 .
+.Cm user
+allows specification of a username to be associated with a resident key,
+overriding the empty default username.
+Specifying a username may be useful when generating multiple resident keys
+for the same application name.
 .Pp
 The
 .Fl O
diff --git a/ssh-keygen.c b/ssh-keygen.c
index 7731339f7..d0ffa5cd7 100644
--- a/ssh-keygen.c
+++ b/ssh-keygen.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-keygen.c,v 1.381 2020/01/02 22:40:09 djm Exp $ */
+/* $OpenBSD: ssh-keygen.c,v 1.382 2020/01/06 02:00:46 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1994 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -2915,7 +2915,7 @@ skip_ssh_url_preamble(const char *s)
 }
 
 static int
-do_download_sk(const char *skprovider)
+do_download_sk(const char *skprovider, const char *device)
 {
         struct sshkey **keys;
         size_t nkeys, i;
@@ -2927,7 +2927,8 @@ do_download_sk(const char *skprovider)
                 fatal("Cannot download keys without provider");
 
         pin = read_passphrase("Enter PIN for security key: ", RP_ALLOW_STDIN);
-        if ((r = sshsk_load_resident(skprovider, pin, &keys, &nkeys)) != 0) {
+        if ((r = sshsk_load_resident(skprovider, device, pin,
+            &keys, &nkeys)) != 0) {
                 freezero(pin, strlen(pin));
                 error("Unable to load resident keys: %s", ssh_err(r));
                 return -1;
@@ -3067,6 +3068,7 @@ main(int argc, char **argv)
         int do_gen_candidates = 0, do_screen_candidates = 0, download_sk = 0;
         unsigned long long cert_serial = 0;
         char *identity_comment = NULL, *ca_key_path = NULL, **opts = NULL;
+        char *sk_application = NULL, *sk_device = NULL, *sk_user = NULL;
         size_t i, nopts = 0;
         u_int32_t bits = 0;
         uint8_t sk_flags = SSH_SK_USER_PRESENCE_REQD;
@@ -3396,8 +3398,17 @@ main(int argc, char **argv)
         }
         if (pkcs11provider != NULL)
                 do_download(pw);
-        if (download_sk)
-                return do_download_sk(sk_provider);
+        if (download_sk) {
+                for (i = 0; i < nopts; i++) {
+                        if (strncasecmp(opts[i], "device=", 7) == 0) {
+                                sk_device = xstrdup(opts[i] + 7);
+                        } else {
+                                fatal("Option \"%s\" is unsupported for "
+                                    "FIDO authenticator download", opts[i]);
+                        }
+                }
+                return do_download_sk(sk_provider, sk_device);
+        }
         if (print_fingerprint || print_bubblebabble)
                 do_fingerprint(pw);
         if (change_passphrase)
@@ -3484,6 +3495,13 @@ main(int argc, char **argv)
                                 sk_flags &= ~SSH_SK_USER_PRESENCE_REQD;
                         } else if (strcasecmp(opts[i], "resident") == 0) {
                                 sk_flags |= SSH_SK_RESIDENT_KEY;
+                        } else if (strncasecmp(opts[i], "device=", 7) == 0) {
+                                sk_device = xstrdup(opts[i] + 7);
+                        } else if (strncasecmp(opts[i], "user=", 5) == 0) {
+                                sk_user = xstrdup(opts[i] + 5);
+                        } else if (strncasecmp(opts[i],
+                            "application=", 12) == 0) {
+                                sk_application = xstrdup(opts[i] + 12);
                         } else {
                                 fatal("Option \"%s\" is unsupported for "
                                     "FIDO authenticator enrollment", opts[i]);
@@ -3495,14 +3513,11 @@ main(int argc, char **argv)
                 }
                 passphrase = NULL;
                 for (i = 0 ; i < 3; i++) {
-                        if (!quiet) {
-                                printf("You may need to touch your security "
-                                    "key to authorize key generation.\n");
-                        }
                         fflush(stdout);
-                        r = sshsk_enroll(type, sk_provider,
-                            cert_key_id == NULL ? "ssh:" : cert_key_id,
-                            sk_flags, passphrase, NULL, &private, NULL);
+                        r = sshsk_enroll(type, sk_provider, sk_device,
+                            sk_application == NULL ? "ssh:" : sk_application,
+                            sk_user, sk_flags, passphrase, NULL,
+                            &private, NULL);
                         if (r == 0)
                                 break;
                         if (r != SSH_ERR_KEY_WRONG_PASSPHRASE)
diff --git a/ssh-sk-client.c b/ssh-sk-client.c
index 0033a6655..d3d37f792 100644
--- a/ssh-sk-client.c
+++ b/ssh-sk-client.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-sk-client.c,v 1.3 2019/12/30 09:23:28 djm Exp $ */
+/* $OpenBSD: ssh-sk-client.c,v 1.4 2020/01/06 02:00:46 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -282,8 +282,9 @@ sshsk_sign(const char *provider, struct sshkey *key,
 }
 
 int
-sshsk_enroll(int type, const char *provider_path, const char *application,
-    uint8_t flags, const char *pin, struct sshbuf *challenge_buf,
+sshsk_enroll(int type, const char *provider_path, const char *device,
+    const char *application, const char *userid, uint8_t flags,
+    const char *pin, struct sshbuf *challenge_buf,
     struct sshkey **keyp, struct sshbuf *attest)
 {
         int oerrno, r = SSH_ERR_INTERNAL_ERROR;
@@ -311,7 +312,9 @@ sshsk_enroll(int type, const char *provider_path, const char *application,
         if ((r = sshbuf_put_u32(req, SSH_SK_HELPER_ENROLL)) != 0 ||
             (r = sshbuf_put_u32(req, (u_int)type)) != 0 ||
             (r = sshbuf_put_cstring(req, provider_path)) != 0 ||
+            (r = sshbuf_put_cstring(req, device)) != 0 ||
             (r = sshbuf_put_cstring(req, application)) != 0 ||
+            (r = sshbuf_put_cstring(req, userid)) != 0 ||
             (r = sshbuf_put_u8(req, flags)) != 0 ||
             (r = sshbuf_put_cstring(req, pin)) != 0 ||
             (r = sshbuf_put_stringb(req, challenge_buf)) != 0) {
@@ -358,8 +361,8 @@ sshsk_enroll(int type, const char *provider_path, const char *application,
 }
 
 int
-sshsk_load_resident(const char *provider_path, const char *pin,
-    struct sshkey ***keysp, size_t *nkeysp)
+sshsk_load_resident(const char *provider_path, const char *device,
+    const char *pin, struct sshkey ***keysp, size_t *nkeysp)
 {
         int oerrno, r = SSH_ERR_INTERNAL_ERROR;
         struct sshbuf *kbuf = NULL, *req = NULL, *resp = NULL;
@@ -378,6 +381,7 @@ sshsk_load_resident(const char *provider_path, const char *pin,
 
         if ((r = sshbuf_put_u32(req, SSH_SK_HELPER_LOAD_RESIDENT)) != 0 ||
             (r = sshbuf_put_cstring(req, provider_path)) != 0 ||
+            (r = sshbuf_put_cstring(req, device)) != 0 ||
             (r = sshbuf_put_cstring(req, pin)) != 0) {
                 error("%s: compose: %s", __func__, ssh_err(r));
                 goto out;
diff --git a/ssh-sk-helper.c b/ssh-sk-helper.c
index 590ff8501..85a461d53 100644
--- a/ssh-sk-helper.c
+++ b/ssh-sk-helper.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-sk-helper.c,v 1.6 2019/12/30 09:23:28 djm Exp $ */
+/* $OpenBSD: ssh-sk-helper.c,v 1.7 2020/01/06 02:00:46 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -77,6 +77,17 @@ reply_error(int r, char *fmt, ...)
         return resp;
 }
 
+/* If the specified string is zero length, then free it and replace with NULL */
+static void
+null_empty(char **s)
+{
+        if (s == NULL || *s == NULL || **s != '\0')
+                return;
+
+        free(*s);
+        *s = NULL;
+}
+
 static struct sshbuf *
 process_sign(struct sshbuf *req)
 {
@@ -108,10 +119,7 @@ process_sign(struct sshbuf *req)
             "msg len %zu, compat 0x%lx", __progname, sshkey_type(key),
             provider, msglen, (u_long)compat);
 
-        if (*pin == 0) {
-                free(pin);
-                pin = NULL;
-        }
+        null_empty(&pin);
 
         if ((r = sshsk_sign(provider, key, &sig, &siglen,
             message, msglen, compat, pin)) != 0) {
@@ -138,7 +146,7 @@ process_enroll(struct sshbuf *req)
 {
         int r;
         u_int type;
-        char *provider, *application, *pin;
+        char *provider, *application, *pin, *device, *userid;
         uint8_t flags;
         struct sshbuf *challenge, *attest, *kbuf, *resp;
         struct sshkey *key;
@@ -149,7 +157,9 @@ process_enroll(struct sshbuf *req)
 
         if ((r = sshbuf_get_u32(req, &type)) != 0 ||
             (r = sshbuf_get_cstring(req, &provider, NULL)) != 0 ||
+            (r = sshbuf_get_cstring(req, &device, NULL)) != 0 ||
             (r = sshbuf_get_cstring(req, &application, NULL)) != 0 ||
+            (r = sshbuf_get_cstring(req, &userid, NULL)) != 0 ||
             (r = sshbuf_get_u8(req, &flags)) != 0 ||
             (r = sshbuf_get_cstring(req, &pin, NULL)) != 0 ||
             (r = sshbuf_froms(req, &challenge)) != 0)
@@ -163,13 +173,12 @@ process_enroll(struct sshbuf *req)
                 sshbuf_free(challenge);
                 challenge = NULL;
         }
-        if (*pin == 0) {
-                free(pin);
-                pin = NULL;
-        }
+        null_empty(&device);
+        null_empty(&userid);
+        null_empty(&pin);
 
-        if ((r = sshsk_enroll((int)type, provider, application, flags, pin,
-            challenge, &key, attest)) != 0) {
+        if ((r = sshsk_enroll((int)type, provider, device, application, userid,
+            flags, pin, challenge, &key, attest)) != 0) {
                 resp = reply_error(r, "Enrollment failed: %s", ssh_err(r));
                 goto out;
         }
@@ -200,7 +209,7 @@ static struct sshbuf *
 process_load_resident(struct sshbuf *req)
 {
         int r;
-        char *provider, *pin;
+        char *provider, *pin, *device;
         struct sshbuf *kbuf, *resp;
         struct sshkey **keys = NULL;
         size_t nkeys = 0, i;
@@ -209,17 +218,17 @@ process_load_resident(struct sshbuf *req)
                 fatal("%s: sshbuf_new failed", __progname);
 
         if ((r = sshbuf_get_cstring(req, &provider, NULL)) != 0 ||
+            (r = sshbuf_get_cstring(req, &device, NULL)) != 0 ||
             (r = sshbuf_get_cstring(req, &pin, NULL)) != 0)
                 fatal("%s: buffer error: %s", __progname, ssh_err(r));
         if (sshbuf_len(req) != 0)
                 fatal("%s: trailing data in request", __progname);
 
-        if (*pin == 0) {
-                free(pin);
-                pin = NULL;
-        }
+        null_empty(&device);
+        null_empty(&pin);
 
-        if ((r = sshsk_load_resident(provider, pin, &keys, &nkeys)) != 0) {
+        if ((r = sshsk_load_resident(provider, device, pin,
+            &keys, &nkeys)) != 0) {
                 resp = reply_error(r, " sshsk_load_resident failed: %s",
                     ssh_err(r));
                 goto out;
diff --git a/ssh-sk.c b/ssh-sk.c
index b1d0d6c58..0ef52e299 100644
--- a/ssh-sk.c
+++ b/ssh-sk.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-sk.c,v 1.23 2019/12/30 09:24:45 djm Exp $ */
+/* $OpenBSD: ssh-sk.c,v 1.24 2020/01/06 02:00:47 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -53,29 +53,32 @@ struct sshsk_provider {
         /* Enroll a U2F key (private key generation) */
         int (*sk_enroll)(int alg, const uint8_t *challenge,
             size_t challenge_len, const char *application, uint8_t flags,
-            const char *pin, struct sk_enroll_response **enroll_response);
+            const char *pin, struct sk_option **opts,
+            struct sk_enroll_response **enroll_response);
 
         /* Sign a challenge */
         int (*sk_sign)(int alg, const uint8_t *message, size_t message_len,
             const char *application,
             const uint8_t *key_handle, size_t key_handle_len,
-            uint8_t flags, const char *pin,
+            uint8_t flags, const char *pin, struct sk_option **opts,
             struct sk_sign_response **sign_response);
 
         /* Enumerate resident keys */
-        int (*sk_load_resident_keys)(const char *pin,
+        int (*sk_load_resident_keys)(const char *pin, struct sk_option **opts,
             struct sk_resident_key ***rks, size_t *nrks);
 };
 
 /* Built-in version */
 int ssh_sk_enroll(int alg, const uint8_t *challenge,
     size_t challenge_len, const char *application, uint8_t flags,
-    const char *pin, struct sk_enroll_response **enroll_response);
+    const char *pin, struct sk_option **opts,
+    struct sk_enroll_response **enroll_response);
 int ssh_sk_sign(int alg, const uint8_t *message, size_t message_len,
     const char *application,
     const uint8_t *key_handle, size_t key_handle_len,
-    uint8_t flags, const char *pin, struct sk_sign_response **sign_response);
-int ssh_sk_load_resident_keys(const char *pin,
+    uint8_t flags, const char *pin, struct sk_option **opts,
+    struct sk_sign_response **sign_response);
+int ssh_sk_load_resident_keys(const char *pin, struct sk_option **opts,
     struct sk_resident_key ***rks, size_t *nrks);
 
 static void
@@ -339,9 +342,80 @@ skerr_to_ssherr(int skerr)
         }
 }
 
+static void
+sshsk_free_options(struct sk_option **opts)
+{
+        size_t i;
+
+        if (opts == NULL)
+                return;
+        for (i = 0; opts[i] != NULL; i++) {
+                free(opts[i]->name);
+                free(opts[i]->value);
+                free(opts[i]);
+        }
+        free(opts);
+}
+
+static int
+sshsk_add_option(struct sk_option ***optsp, size_t *noptsp,
+    const char *name, const char *value, uint8_t required)
+{
+        struct sk_option **opts = *optsp;
+        size_t nopts = *noptsp;
+
+        if ((opts = recallocarray(opts, nopts, nopts + 2, /* extra for NULL */
+            sizeof(*opts))) == NULL) {
+                error("%s: array alloc failed", __func__);
+                return SSH_ERR_ALLOC_FAIL;
+        }
+        *optsp = opts;
+        *noptsp = nopts + 1;
+        if ((opts[nopts] = calloc(1, sizeof(**opts))) == NULL) {
+                error("%s: alloc failed", __func__);
+                return SSH_ERR_ALLOC_FAIL;
+        }
+        if ((opts[nopts]->name = strdup(name)) == NULL ||
+            (opts[nopts]->value = strdup(value)) == NULL) {
+                error("%s: alloc failed", __func__);
+                return SSH_ERR_ALLOC_FAIL;
+        }
+        opts[nopts]->required = required;
+        return 0;
+}
+
+static int
+make_options(const char *device, const char *user_id,
+    struct sk_option ***optsp)
+{
+        struct sk_option **opts = NULL;
+        size_t nopts = 0;
+        int r, ret = SSH_ERR_INTERNAL_ERROR;
+
+        if (device != NULL &&
+            (r = sshsk_add_option(&opts, &nopts, "device", device, 0)) != 0) {
+                ret = r;
+                goto out;
+        }
+        if (user_id != NULL &&
+            (r = sshsk_add_option(&opts, &nopts, "user", user_id, 0)) != 0) {
+                ret = r;
+                goto out;
+        }
+        /* success */
+        *optsp = opts;
+        opts = NULL;
+        nopts = 0;
+        ret = 0;
+ out:
+        sshsk_free_options(opts);
+        return ret;
+}
+
 int
-sshsk_enroll(int type, const char *provider_path, const char *application,
-    uint8_t flags, const char *pin, struct sshbuf *challenge_buf,
+sshsk_enroll(int type, const char *provider_path, const char *device,
+    const char *application, const char *userid, uint8_t flags,
+    const char *pin, struct sshbuf *challenge_buf,
     struct sshkey **keyp, struct sshbuf *attest)
 {
         struct sshsk_provider *skp = NULL;
@@ -350,17 +424,23 @@ sshsk_enroll(int type, const char *provider_path, const char *application,
         const u_char *challenge;
         size_t challenge_len;
         struct sk_enroll_response *resp = NULL;
+        struct sk_option **opts = NULL;
         int r = SSH_ERR_INTERNAL_ERROR;
         int alg;
 
-        debug("%s: provider \"%s\", application \"%s\", flags 0x%02x, "
-            "challenge len %zu%s", __func__, provider_path, application,
-            flags, challenge_buf == NULL ? 0 : sshbuf_len(challenge_buf),
+        debug("%s: provider \"%s\", device \"%s\", application \"%s\", "
+            "userid \"%s\", flags 0x%02x, challenge len %zu%s", __func__,
+            provider_path, device, application, userid, flags,
+            challenge_buf == NULL ? 0 : sshbuf_len(challenge_buf),
             (pin != NULL && *pin != '\0') ? " with-pin" : "");
 
         *keyp = NULL;
         if (attest)
                 sshbuf_reset(attest);
+
+        if ((r = make_options(device, userid, &opts)) != 0)
+                goto out;
+
         switch (type) {
 #ifdef WITH_OPENSSL
         case KEY_ECDSA_SK:
@@ -407,7 +487,7 @@ sshsk_enroll(int type, const char *provider_path, const char *application,
         /* XXX validate flags? */
         /* enroll key */
         if ((r = skp->sk_enroll(alg, challenge, challenge_len, application,
-            flags, pin, &resp)) != 0) {
+            flags, pin, opts, &resp)) != 0) {
                 error("Security key provider \"%s\" returned failure %d",
                     provider_path, r);
                 r = skerr_to_ssherr(r);
@@ -437,6 +517,7 @@ sshsk_enroll(int type, const char *provider_path, const char *application,
         key = NULL; /* transferred */
         r = 0;
  out:
+        sshsk_free_options(opts);
         sshsk_free(skp);
         sshkey_free(key);
         sshsk_free_enroll_response(resp);
@@ -528,6 +609,7 @@ sshsk_sign(const char *provider_path, struct sshkey *key,
         struct sk_sign_response *resp = NULL;
         struct sshbuf *inner_sig = NULL, *sig = NULL;
         uint8_t message[32];
+        struct sk_option **opts = NULL;
 
         debug("%s: provider \"%s\", key %s, flags 0x%02x%s", __func__,
             provider_path, sshkey_type(key), key->sk_flags,
@@ -571,7 +653,7 @@ sshsk_sign(const char *provider_path, struct sshkey *key,
         if ((r = skp->sk_sign(alg, message, sizeof(message),
             key->sk_application,
             sshbuf_ptr(key->sk_key_handle), sshbuf_len(key->sk_key_handle),
-            key->sk_flags, pin, &resp)) != 0) {
+            key->sk_flags, pin, opts, &resp)) != 0) {
                 debug("%s: sk_sign failed with code %d", __func__, r);
                 r = skerr_to_ssherr(r);
                 goto out;
@@ -617,6 +699,7 @@ sshsk_sign(const char *provider_path, struct sshkey *key,
         /* success */
         r = 0;
  out:
+        sshsk_free_options(opts);
         explicit_bzero(message, sizeof(message));
         sshsk_free(skp);
         sshsk_free_sign_response(resp);
@@ -645,8 +728,8 @@ sshsk_free_sk_resident_keys(struct sk_resident_key **rks, size_t nrks)
 }
 
 int
-sshsk_load_resident(const char *provider_path, const char *pin,
-    struct sshkey ***keysp, size_t *nkeysp)
+sshsk_load_resident(const char *provider_path, const char *device,
+    const char *pin, struct sshkey ***keysp, size_t *nkeysp)
 {
         struct sshsk_provider *skp = NULL;
         int r = SSH_ERR_INTERNAL_ERROR;
@@ -654,6 +737,7 @@ sshsk_load_resident(const char *provider_path, const char *pin,
         size_t i, nrks = 0, nkeys = 0;
         struct sshkey *key = NULL, **keys = NULL, **tmp;
         uint8_t flags;
+        struct sk_option **opts = NULL;
 
         debug("%s: provider \"%s\"%s", __func__, provider_path,
             (pin != NULL && *pin != '\0') ? ", have-pin": "");
@@ -663,11 +747,13 @@ sshsk_load_resident(const char *provider_path, const char *pin,
         *keysp = NULL;
         *nkeysp = 0;
 
+        if ((r = make_options(device, NULL, &opts)) != 0)
+                goto out;
         if ((skp = sshsk_open(provider_path)) == NULL) {
                 r = SSH_ERR_INVALID_FORMAT; /* XXX sshsk_open return code? */
                 goto out;
         }
-        if ((r = skp->sk_load_resident_keys(pin, &rks, &nrks)) != 0) {
+        if ((r = skp->sk_load_resident_keys(pin, opts, &rks, &nrks)) != 0) {
                 error("Security key provider \"%s\" returned failure %d",
                     provider_path, r);
                 r = skerr_to_ssherr(r);
@@ -710,6 +796,7 @@ sshsk_load_resident(const char *provider_path, const char *pin,
         nkeys = 0;
         r = 0;
  out:
+        sshsk_free_options(opts);
         sshsk_free(skp);
         sshsk_free_sk_resident_keys(rks, nrks);
         sshkey_free(key);
diff --git a/ssh-sk.h b/ssh-sk.h
index 348759a98..ea9ff6e1a 100644
--- a/ssh-sk.h
+++ b/ssh-sk.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-sk.h,v 1.8 2019/12/30 09:23:28 djm Exp $ */
+/* $OpenBSD: ssh-sk.h,v 1.9 2020/01/06 02:00:47 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -20,9 +20,10 @@
 
 struct sshbuf;
 struct sshkey;
+struct sk_option;
 
 /* Version of protocol expected from ssh-sk-helper */
-#define SSH_SK_HELPER_VERSION           3
+#define SSH_SK_HELPER_VERSION           4
 
 /* ssh-sk-helper messages */
 #define SSH_SK_HELPER_ERROR             0       /* Only valid H->C */
@@ -40,8 +41,9 @@ struct sshkey;
  * If successful and the attest_data buffer is not NULL then attestation
  * information is placed there.
  */
-int sshsk_enroll(int type, const char *provider_path, const char *application,
-    uint8_t flags, const char *pin, struct sshbuf *challenge_buf,
+int sshsk_enroll(int type, const char *provider_path, const char *device,
+    const char *application, const char *userid, uint8_t flags,
+    const char *pin, struct sshbuf *challenge_buf,
     struct sshkey **keyp, struct sshbuf *attest);
 
 /*
@@ -60,8 +62,8 @@ int sshsk_sign(const char *provider_path, struct sshkey *key,
  *
  * Returns 0 on success or a ssherr.h error code on failure.
  */
-int sshsk_load_resident(const char *provider_path, const char *pin,
-    struct sshkey ***keysp, size_t *nkeysp);
+int sshsk_load_resident(const char *provider_path, const char *device,
+    const char *pin, struct sshkey ***keysp, size_t *nkeysp);
 
 #endif /* _SSH_SK_H */