diff --git a/Makefile.in b/Makefile.in
index f22c836..bac522a 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -96,7 +96,7 @@
 	msg.o progressmeter.o dns.o entropy.o gss-genr.o umac.o umac128.o \
 	ssh-pkcs11.o smult_curve25519_ref.o \
 	poly1305.o chacha.o cipher-chachapoly.o \
-	ssh-ed25519.o digest-openssl.o digest-libc.o hmac.o \
+	ssh-ed25519.o ssh-sk.o digest-openssl.o digest-libc.o hmac.o \
 	sc25519.o ge25519.o fe25519.o ed25519.o verify.o hash.o \
 	kex.o kexdh.o kexgex.o kexecdh.o kexc25519.o \
 	kexgexc.o kexgexs.o \
diff --git a/sk-api.h b/sk-api.h
new file mode 100644
index 0000000..1de7334
--- /dev/null
+++ b/sk-api.h
@@ -0,0 +1,63 @@
+/* $OpenBSD: sk-api.h,v 1.1 2019/10/31 21:16:20 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.
+ */
+
+#ifndef _SK_API_H
+#define _SK_API_H 1
+
+#include <stddef.h>
+#include <stdint.h>
+
+/* Flags */
+#define SSH_SK_USER_PRESENCE_REQD	0x01
+
+struct sk_enroll_response {
+	uint8_t *public_key;
+	size_t public_key_len;
+	uint8_t *key_handle;
+	size_t key_handle_len;
+	uint8_t *signature;
+	size_t signature_len;
+	uint8_t *attestation_cert;
+	size_t attestation_cert_len;
+};
+
+struct sk_sign_response {
+	uint8_t flags;
+	uint32_t counter;
+	uint8_t *sig_r;
+	size_t sig_r_len;
+	uint8_t *sig_s;
+	size_t sig_s_len;
+};
+
+#define SSH_SK_VERSION_MAJOR		0x00010000 /* 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(const uint8_t *challenge, size_t challenge_len,
+    const char *application, uint8_t flags,
+    struct sk_enroll_response **enroll_response);
+
+/* Sign a challenge */
+int sk_sign(const uint8_t *message, size_t message_len,
+    const char *application, const uint8_t *key_handle, size_t key_handle_len,
+    uint8_t flags, struct sk_sign_response **sign_response);
+
+#endif /* _SK_API_H */
diff --git a/ssh-sk.c b/ssh-sk.c
new file mode 100644
index 0000000..7d313f5
--- /dev/null
+++ b/ssh-sk.c
@@ -0,0 +1,377 @@
+/* $OpenBSD: ssh-sk.c,v 1.1 2019/10/31 21:16:20 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.
+ */
+
+/* #define DEBUG_SK 1 */
+
+#include "includes.h"
+
+#include <dlfcn.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+#include <stdio.h>
+
+#include <openssl/objects.h>
+#include <openssl/ec.h>
+
+#include "log.h"
+#include "misc.h"
+#include "sshbuf.h"
+#include "sshkey.h"
+#include "ssherr.h"
+#include "digest.h"
+
+#include "ssh-sk.h"
+#include "sk-api.h"
+
+struct sshsk_provider {
+	char *path;
+	void *dlhandle;
+
+	/* Return the version of the middleware API */
+	uint32_t (*sk_api_version)(void);
+
+	/* Enroll a U2F key (private key generation) */
+	int (*sk_enroll)(const uint8_t *challenge, size_t challenge_len,
+	    const char *application, uint8_t flags,
+	    struct sk_enroll_response **enroll_response);
+
+	/* Sign a challenge */
+	int (*sk_sign)(const uint8_t *message, size_t message_len,
+	    const char *application,
+	    const uint8_t *key_handle, size_t key_handle_len,
+	    uint8_t flags, struct sk_sign_response **sign_response);
+};
+
+static void
+sshsk_free(struct sshsk_provider *p)
+{
+	if (p == NULL)
+		return;
+	free(p->path);
+	if (p->dlhandle != NULL)
+		dlclose(p->dlhandle);
+	free(p);
+}
+
+static struct sshsk_provider *
+sshsk_open(const char *path)
+{
+	struct sshsk_provider *ret = NULL;
+	uint32_t version;
+
+	if ((ret = calloc(1, sizeof(*ret))) == NULL) {
+		error("%s: calloc failed", __func__);
+		return NULL;
+	}
+	if ((ret->path = strdup(path)) == NULL) {
+		error("%s: strdup failed", __func__);
+		goto fail;
+	}
+	if ((ret->dlhandle = dlopen(path, RTLD_NOW)) == NULL) {
+		error("Security key provider %s dlopen failed: %s",
+		    path, dlerror());
+		goto fail;
+	}
+	if ((ret->sk_api_version = dlsym(ret->dlhandle,
+	    "sk_api_version")) == NULL) {
+		error("Security key provider %s dlsym(sk_api_version) "
+		    "failed: %s", path, dlerror());
+		goto fail;
+	}
+	version = ret->sk_api_version();
+	debug("%s: provider %s implements version 0x%08lx", __func__,
+	    ret->path, (u_long)version);
+	if ((version & SSH_SK_VERSION_MAJOR_MASK) != SSH_SK_VERSION_MAJOR) {
+		error("Security key provider %s implements unsupported version "
+		    "0x%08lx (supported: 0x%08lx)", path, (u_long)version,
+		    (u_long)SSH_SK_VERSION_MAJOR);
+		goto fail;
+	}
+	if ((ret->sk_enroll = dlsym(ret->dlhandle, "sk_enroll")) == NULL) {
+		error("Security key  provider %s dlsym(sk_enroll) "
+		    "failed: %s", path, dlerror());
+		goto fail;
+	}
+	if ((ret->sk_sign = dlsym(ret->dlhandle, "sk_sign")) == NULL) {
+		error("Security key provider %s dlsym(sk_sign) failed: %s",
+		    path, dlerror());
+		goto fail;
+	}
+	/* success */
+	return ret;
+fail:
+	sshsk_free(ret);
+	return NULL;
+}
+
+static void
+sshsk_free_enroll_response(struct sk_enroll_response *r)
+{
+	if (r == NULL)
+		return;
+	freezero(r->key_handle, r->key_handle_len);
+	freezero(r->public_key, r->public_key_len);
+	freezero(r->signature, r->signature_len);
+	freezero(r->attestation_cert, r->attestation_cert_len);
+	freezero(r, sizeof(*r));
+};
+
+static void
+sshsk_free_sign_response(struct sk_sign_response *r)
+{
+	if (r == NULL)
+		return;
+	freezero(r->sig_r, r->sig_r_len);
+	freezero(r->sig_s, r->sig_s_len);
+	freezero(r, sizeof(*r));
+};
+
+int
+sshsk_enroll(const char *provider_path, const char *application,
+    uint8_t flags, struct sshbuf *challenge_buf, struct sshkey **keyp,
+    struct sshbuf *attest)
+{
+	struct sshsk_provider *skp = NULL;
+	struct sshkey *key = NULL;
+	u_char randchall[32];
+	const u_char *challenge;
+	size_t challenge_len;
+	struct sk_enroll_response *resp = NULL;
+	int r = SSH_ERR_INTERNAL_ERROR;
+	struct sshbuf *b = NULL;
+	EC_POINT *q = NULL;
+
+	*keyp = NULL;
+	if (attest)
+		sshbuf_reset(attest);
+	if (provider_path == NULL) {
+		error("%s: missing provider", __func__);
+		r = SSH_ERR_INVALID_ARGUMENT;
+		goto out;
+	}
+	if (application == NULL || *application == '\0') {
+		error("%s: missing application", __func__);
+		r = SSH_ERR_INVALID_ARGUMENT;
+		goto out;
+	}
+	if (challenge_buf == NULL) {
+		debug("%s: using random challenge", __func__);
+		arc4random_buf(randchall, sizeof(randchall));
+		challenge = randchall;
+		challenge_len = sizeof(randchall);
+	} else if (sshbuf_len(challenge_buf) == 0) {
+		error("Missing enrollment challenge");
+		r = SSH_ERR_INVALID_ARGUMENT;
+		goto out;
+	} else {
+		challenge = sshbuf_ptr(challenge_buf);
+		challenge_len = sshbuf_len(challenge_buf);
+		debug3("%s: using explicit challenge len=%zd",
+		    __func__, challenge_len);
+	}
+	if ((skp = sshsk_open(provider_path)) == NULL) {
+		r = SSH_ERR_INVALID_FORMAT; /* XXX sshsk_open return code? */
+		goto out;
+	}
+	/* XXX validate flags? */
+	/* enroll key */
+	if ((r = skp->sk_enroll(challenge, challenge_len, application,
+	    flags, &resp)) != 0) {
+		error("Security key provider %s returned failure %d",
+		    provider_path, r);
+		r = SSH_ERR_INVALID_FORMAT; /* XXX error codes in API? */
+		goto out;
+	}
+	/* Check response validity */
+	if (resp->public_key == NULL || resp->key_handle == NULL ||
+	    resp->signature == NULL || resp->attestation_cert == NULL) {
+		error("%s: sk_enroll response invalid", __func__);
+		r = SSH_ERR_INVALID_FORMAT;
+		goto out;
+	}
+	/* Assemble key from response */
+	if ((key = sshkey_new(KEY_ECDSA_SK)) == NULL) {
+		error("%s: sshkey_new failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	key->ecdsa_nid = NID_X9_62_prime256v1;
+	key->sk_flags = flags;
+	if ((key->ecdsa = EC_KEY_new_by_curve_name(key->ecdsa_nid)) == NULL ||
+	    (q = EC_POINT_new(EC_KEY_get0_group(key->ecdsa))) == NULL ||
+	    (key->sk_key_handle = sshbuf_new()) == NULL ||
+	    (key->sk_reserved = sshbuf_new()) == NULL ||
+	    (b = sshbuf_new()) == NULL) {
+		error("%s: allocation failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if ((r = sshbuf_put_string(b,
+	    resp->public_key, resp->public_key_len)) != 0) {
+		error("%s: buffer error: %s", __func__, ssh_err(r));
+		goto out;
+	}
+	if ((key->sk_application = strdup(application)) == NULL) {
+		error("%s: strdup application failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if ((r = sshbuf_get_ec(b, q, EC_KEY_get0_group(key->ecdsa))) != 0) {
+		error("%s: parse key: %s", __func__, ssh_err(r));
+		r = SSH_ERR_INVALID_FORMAT;
+		goto out;
+	}
+	if (sshkey_ec_validate_public(EC_KEY_get0_group(key->ecdsa), q) != 0) {
+		error("Security key returned invalid ECDSA key");
+		r = SSH_ERR_KEY_INVALID_EC_VALUE;
+		goto out;
+	}
+	if (EC_KEY_set_public_key(key->ecdsa, q) != 1) {
+		/* XXX assume it is a allocation error */
+		error("%s: allocation failed", __func__);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if ((r = sshbuf_put(key->sk_key_handle, resp->key_handle,
+	    resp->key_handle_len)) != 0) {
+		error("%s: buffer error: %s", __func__, ssh_err(r));
+		goto out;
+	}
+	/* Optionally fill in the attestation information */
+	if (attest != NULL) {
+		if ((r = sshbuf_put_cstring(attest, "sk-attest-v00")) != 0 ||
+		    (r = sshbuf_put_u32(attest, 1)) != 0 || /* XXX U2F ver */
+		    (r = sshbuf_put_string(attest,
+		    resp->attestation_cert, resp->attestation_cert_len)) != 0 ||
+		    (r = sshbuf_put_string(attest,
+		    resp->signature, resp->signature_len)) != 0 ||
+		    (r = sshbuf_put_u32(attest, flags)) != 0 || /* XXX right? */
+		    (r = sshbuf_put_string(attest, NULL, 0)) != 0) {
+			error("%s: buffer error: %s", __func__, ssh_err(r));
+			goto out;
+		}
+	}
+	/* success */
+	*keyp = key;
+	key = NULL; /* transferred */
+	r = 0;
+ out:
+	EC_POINT_free(q);
+	sshsk_free(skp);
+	sshbuf_free(b);
+	sshkey_free(key);
+	sshsk_free_enroll_response(resp);
+	explicit_bzero(randchall, sizeof(randchall));
+	return r;
+}
+
+int
+sshsk_ecdsa_sign(const char *provider_path, const struct sshkey *key,
+    u_char **sigp, size_t *lenp, const u_char *data, size_t datalen,
+    u_int compat)
+{
+	struct sshsk_provider *skp = NULL;
+	int r = SSH_ERR_INTERNAL_ERROR;
+	struct sk_sign_response *resp = NULL;
+	struct sshbuf *inner_sig = NULL, *sig = NULL;
+	uint8_t message[32];
+
+	if (sigp != NULL)
+		*sigp = NULL;
+	if (lenp != NULL)
+		*lenp = 0;
+	if (provider_path == NULL ||
+	    sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
+	    key->sk_key_handle == NULL ||
+	    key->sk_application == NULL || *key->sk_application == '\0') {
+		r = SSH_ERR_INVALID_ARGUMENT;
+		goto out;
+	}
+	if ((skp = sshsk_open(provider_path)) == NULL) {
+		r = SSH_ERR_INVALID_FORMAT; /* XXX sshsk_open return code? */
+		goto out;
+	}
+
+	/* hash data to be signed before it goes to the security key */
+	if ((r = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen,
+	    message, sizeof(message))) != 0) {
+		error("%s: hash application failed: %s", __func__, ssh_err(r));
+		r = SSH_ERR_INTERNAL_ERROR;
+		goto out;
+	}
+	if ((r = skp->sk_sign(message, sizeof(message),
+	    key->sk_application,
+	    sshbuf_ptr(key->sk_key_handle), sshbuf_len(key->sk_key_handle),
+	    key->sk_flags, &resp)) != 0) {
+		debug("%s: sk_sign failed with code %d", __func__, r);
+		goto out;
+	}
+	if ((sig = sshbuf_new()) == NULL ||
+	    (inner_sig = sshbuf_new()) == NULL) {
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	/* Prepare inner signature object */
+	if ((r = sshbuf_put_bignum2_bytes(inner_sig,
+	    resp->sig_r, resp->sig_r_len)) != 0 ||
+	    (r = sshbuf_put_bignum2_bytes(inner_sig,
+	    resp->sig_s, resp->sig_s_len)) != 0 ||
+	    (r = sshbuf_put_u8(inner_sig, resp->flags)) != 0 ||
+	    (r = sshbuf_put_u32(inner_sig, resp->counter)) != 0) {
+		debug("%s: buffer error (inner): %s", __func__, ssh_err(r));
+		goto out;
+	}
+	/* Assemble outer signature */
+	if ((r = sshbuf_put_cstring(sig, sshkey_ssh_name_plain(key))) != 0 ||
+	    (r = sshbuf_put_stringb(sig, inner_sig)) != 0) {
+		debug("%s: buffer error (outer): %s", __func__, ssh_err(r));
+		goto out;
+	}
+#ifdef DEBUG_SK
+	fprintf(stderr, "%s: sig_r:\n", __func__);
+	sshbuf_dump_data(resp->sig_r, resp->sig_r_len, stderr);
+	fprintf(stderr, "%s: sig_s:\n", __func__);
+	sshbuf_dump_data(resp->sig_s, resp->sig_s_len, stderr);
+	fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
+	    __func__, resp->flags, resp->counter);
+	fprintf(stderr, "%s: hashed message:\n", __func__);
+	sshbuf_dump_data(message, sizeof(message), stderr);
+	fprintf(stderr, "%s: inner:\n", __func__);
+	sshbuf_dump(inner_sig, stderr);
+	fprintf(stderr, "%s: sigbuf:\n", __func__);
+	sshbuf_dump(sig, stderr);
+#endif
+	if (sigp != NULL) {
+		if ((*sigp = malloc(sshbuf_len(sig))) == NULL) {
+			r = SSH_ERR_ALLOC_FAIL;
+			goto out;
+		}
+		memcpy(*sigp, sshbuf_ptr(sig), sshbuf_len(sig));
+	}
+	if (lenp != NULL)
+		*lenp = sshbuf_len(sig);
+	/* success */
+	r = 0;
+ out:
+	explicit_bzero(message, sizeof(message));
+	sshsk_free(skp);
+	sshsk_free_sign_response(resp);
+	sshbuf_free(sig);
+	sshbuf_free(inner_sig);
+	return r;
+}
diff --git a/ssh-sk.h b/ssh-sk.h
new file mode 100644
index 0000000..7c1d2b9
--- /dev/null
+++ b/ssh-sk.h
@@ -0,0 +1,49 @@
+/* $OpenBSD: ssh-sk.h,v 1.1 2019/10/31 21:16:20 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.
+ */
+
+#ifndef _SSH_SK_H
+#define _SSH_SK_H 1
+
+struct sshbuf;
+struct sshkey;
+
+/*
+ * Enroll (generate) a new security-key hosted private key via the specified
+ * provider middleware.
+ * If challenge_buf is NULL then a random 256 bit challenge will be used.
+ *
+ * Returns 0 on success or a ssherr.h error code on failure.
+ *
+ * If successful and the attest_data buffer is not NULL then attestation
+ * information is placed there.
+ */
+int sshsk_enroll(const char *provider_path, const char *application,
+    uint8_t flags, struct sshbuf *challenge_buf, struct sshkey **keyp,
+    struct sshbuf *attest);
+
+/*
+ * Calculate an ECDSA_SK signature using the specified key and provider
+ * middleware.
+ *
+ * Returns 0 on success or a ssherr.h error code on failure.
+ */
+int sshsk_ecdsa_sign(const char *provider_path, const struct sshkey *key,
+    u_char **sigp, size_t *lenp, const u_char *data, size_t datalen,
+    u_int compat);
+
+#endif /* _SSH_SK_H */
+