diff --git a/PROTOCOL.u2f b/PROTOCOL.u2f
index 5b8a06277..f8ca56b11 100644
--- a/PROTOCOL.u2f
+++ b/PROTOCOL.u2f
@@ -154,6 +154,16 @@ by trusted hardware before it will issue a certificate. To support this
 case, OpenSSH optionally allows retaining the attestation information
 at the time of key generation. It will take the following format:
 
+	string		"ssh-sk-attest-v01"
+	string		attestation certificate
+	string		enrollment signature
+	string		authenticator data (CBOR encoded)
+	uint32		reserved flags
+	string		reserved string
+
+A previous version of this format, emitted prior to OpenSSH 8.4 omitted
+the authenticator data.
+
 	string		"ssh-sk-attest-v00"
 	string		attestation certificate
 	string		enrollment signature
@@ -267,87 +277,15 @@ regress testing. For this reason, OpenSSH shall support a dynamically-
 loaded middleware libraries to communicate with security keys, but offer
 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		0x00050000 /* API version */
-	#define SSH_SK_VERSION_MAJOR_MASK	0xffff0000
-
-	/* Flags */
-	#define SSH_SK_USER_PRESENCE_REQD	0x01
-	#define SSH_SK_USER_VERIFICATION_REQD	0x04
-	#define SSH_SK_RESIDENT_KEY		0x20
-
-	/* Algs */
-	#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
-	#define SSH_SK_ERR_DEVICE_NOT_FOUND	-4
-
-	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;
-	};
-
-	struct sk_resident_key {
-		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(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(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_option **options,
-	    struct sk_sign_response **sign_response);
-
-	/* Enumerate all resident keys */
-	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
+The middleware library need only expose a handful of functions and
+numbers listed in sk-api.h. Included in the defined numbers is a
+SSH_SK_VERSION_MAJOR that should be incremented for each incompatible
 API change.
 
-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
+miscellaneous options may be passed to the middleware as a NULL-
+terminated array of pointers to struct sk_option. The middleware may
+ignore unsupported or unknown options unless the "required" flag is set,
+in which case it should return failure if an unsupported option is
 requested.
 
 At present the following options names are supported:
@@ -368,4 +306,4 @@ 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.
 
-$OpenBSD: PROTOCOL.u2f,v 1.25 2020/08/31 00:17:41 djm Exp $
+$OpenBSD: PROTOCOL.u2f,v 1.26 2020/09/09 03:08:01 djm Exp $
diff --git a/sk-api.h b/sk-api.h
index cc32cd4cc..df17ca540 100644
--- a/sk-api.h
+++ b/sk-api.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: sk-api.h,v 1.10 2020/08/27 01:08:19 djm Exp $ */
+/* $OpenBSD: sk-api.h,v 1.11 2020/09/09 03:08:01 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -47,6 +47,8 @@ struct sk_enroll_response {
 	size_t signature_len;
 	uint8_t *attestation_cert;
 	size_t attestation_cert_len;
+	uint8_t *authdata;
+	size_t authdata_len;
 };
 
 struct sk_sign_response {
@@ -72,7 +74,7 @@ struct sk_option {
 	uint8_t required;
 };
 
-#define SSH_SK_VERSION_MAJOR		0x00060000 /* current API version */
+#define SSH_SK_VERSION_MAJOR		0x00070000 /* current API version */
 #define SSH_SK_VERSION_MAJOR_MASK	0xffff0000
 
 /* Return the version of the middleware API */
diff --git a/sk-usbhid.c b/sk-usbhid.c
index de85b2cb3..007c59644 100644
--- a/sk-usbhid.c
+++ b/sk-usbhid.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sk-usbhid.c,v 1.25 2020/08/31 00:17:41 djm Exp $ */
+/* $OpenBSD: sk-usbhid.c,v 1.26 2020/09/09 03:08:01 djm Exp $ */
 /*
  * Copyright (c) 2019 Markus Friedl
  * Copyright (c) 2020 Pedro Martelletto
@@ -822,6 +822,16 @@ sk_enroll(uint32_t alg, const uint8_t *challenge, size_t challenge_len,
 		memcpy(response->attestation_cert, ptr, len);
 		response->attestation_cert_len = len;
 	}
+	if ((ptr = fido_cred_authdata_ptr(cred)) != NULL) {
+		len = fido_cred_authdata_len(cred);
+		debug3("%s: authdata len=%zu", __func__, len);
+		if ((response->authdata = calloc(1, len)) == NULL) {
+			skdebug(__func__, "calloc authdata failed");
+			goto out;
+		}
+		memcpy(response->authdata, ptr, len);
+		response->authdata_len = len;
+	}
 	*enroll_response = response;
 	response = NULL;
 	ret = 0;
@@ -832,6 +842,7 @@ sk_enroll(uint32_t alg, const uint8_t *challenge, size_t challenge_len,
 		free(response->key_handle);
 		free(response->signature);
 		free(response->attestation_cert);
+		free(response->authdata);
 		free(response);
 	}
 	sk_close(sk);
diff --git a/ssh-keygen.1 b/ssh-keygen.1
index c51a0d9c8..3ae596caa 100644
--- a/ssh-keygen.1
+++ b/ssh-keygen.1
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: ssh-keygen.1,v 1.208 2020/08/27 06:15:22 jmc Exp $
+.\"	$OpenBSD: ssh-keygen.1,v 1.209 2020/09/09 03:08:01 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: August 27 2020 $
+.Dd $Mdocdate: September 9 2020 $
 .Dt SSH-KEYGEN 1
 .Os
 .Sh NAME
@@ -520,9 +520,10 @@ Not all FIDO tokens support this option.
 Currently PIN authentication is the only supported verification method,
 but other methods may be supported in the future.
 .It Cm write-attestation Ns = Ns Ar path
-May be used at key generation time to record the attestation certificate
+May be used at key generation time to record the attestation data
 returned from FIDO tokens during key generation.
-By default this information is discarded.
+Please note that this information is potentially sensitive.
+By default, this information is discarded.
 .El
 .Pp
 The
diff --git a/ssh-keygen.c b/ssh-keygen.c
index 64cee4de0..a12b79a56 100644
--- a/ssh-keygen.c
+++ b/ssh-keygen.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-keygen.c,v 1.419 2020/08/27 09:46:04 djm Exp $ */
+/* $OpenBSD: ssh-keygen.c,v 1.420 2020/09/09 03:08:01 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1994 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -3071,6 +3071,27 @@ do_download_sk(const char *skprovider, const char *device)
 	return ret;
 }
 
+static void
+save_attestation(struct sshbuf *attest, const char *path)
+{
+	mode_t omask;
+	int r;
+
+	if (path == NULL)
+		return; /* nothing to do */
+	if (attest == NULL || sshbuf_len(attest) == 0)
+		fatal("Enrollment did not return attestation data");
+	omask = umask(077);
+	r = sshbuf_write_file(path, attest);
+	umask(omask);
+	if (r != 0)
+		fatal("Unable to write attestation data \"%s\": %s", path,
+		    ssh_err(r));
+	if (!quiet)
+		printf("Your FIDO attestation certificate has been saved in "
+		    "%s\n", path);
+}
+
 static void
 usage(void)
 {
@@ -3137,7 +3158,7 @@ main(int argc, char **argv)
 	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;
-	char *sk_attestaion_path = NULL;
+	char *sk_attestation_path = NULL;
 	struct sshbuf *challenge = NULL, *attest = NULL;
 	size_t i, nopts = 0;
 	u_int32_t bits = 0;
@@ -3593,7 +3614,7 @@ main(int argc, char **argv)
 				}
 			} else if (strncasecmp(opts[i],
 			    "write-attestation=", 18) == 0) {
-				sk_attestaion_path = opts[i] + 18;
+				sk_attestation_path = opts[i] + 18;
 			} else if (strncasecmp(opts[i],
 			    "application=", 12) == 0) {
 				sk_application = xstrdup(opts[i] + 12);
@@ -3715,20 +3736,9 @@ main(int argc, char **argv)
 		free(fp);
 	}
 
-	if (sk_attestaion_path != NULL) {
-		if (attest == NULL || sshbuf_len(attest) == 0) {
-			fatal("Enrollment did not return attestation "
-			    "certificate");
-		}
-		if ((r = sshbuf_write_file(sk_attestaion_path, attest)) != 0) {
-			fatal("Unable to write attestation certificate "
-			    "\"%s\": %s", sk_attestaion_path, ssh_err(r));
-		}
-		if (!quiet) {
-			printf("Your FIDO attestation certificate has been "
-			    "saved in %s\n", sk_attestaion_path);
-		}
-	}
+	if (sk_attestation_path != NULL)
+		save_attestation(attest, sk_attestation_path);
+
 	sshbuf_free(attest);
 	sshkey_free(public);
 
diff --git a/ssh-sk.c b/ssh-sk.c
index 89478aff0..1455df635 100644
--- a/ssh-sk.c
+++ b/ssh-sk.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-sk.c,v 1.31 2020/08/27 01:08:19 djm Exp $ */
+/* $OpenBSD: ssh-sk.c,v 1.32 2020/09/09 03:08:02 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -174,6 +174,7 @@ sshsk_free_enroll_response(struct sk_enroll_response *r)
 	freezero(r->public_key, r->public_key_len);
 	freezero(r->signature, r->signature_len);
 	freezero(r->attestation_cert, r->attestation_cert_len);
+	freezero(r->authdata, r->authdata_len);
 	freezero(r, sizeof(*r));
 }
 
@@ -419,6 +420,31 @@ make_options(const char *device, const char *user_id,
 	return ret;
 }
 
+
+static int
+fill_attestation_blob(const struct sk_enroll_response *resp,
+    struct sshbuf *attest)
+{
+	int r;
+
+	if (attest == NULL)
+		return 0; /* nothing to do */
+	if ((r = sshbuf_put_cstring(attest, "ssh-sk-attest-v01")) != 0 ||
+	    (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_string(attest,
+	    resp->authdata, resp->authdata_len)) != 0 ||
+	    (r = sshbuf_put_u32(attest, 0)) != 0 || /* resvd flags */
+	    (r = sshbuf_put_string(attest, NULL, 0)) != 0 /* resvd */) {
+		error("%s: buffer error: %s", __func__, ssh_err(r));
+		return r;
+	}
+	/* success */
+	return 0;
+}
+
 int
 sshsk_enroll(int type, const char *provider_path, const char *device,
     const char *application, const char *userid, uint8_t flags,
@@ -506,19 +532,9 @@ sshsk_enroll(int type, const char *provider_path, const char *device,
 		goto out;
 
 	/* Optionally fill in the attestation information */
-	if (attest != NULL) {
-		if ((r = sshbuf_put_cstring(attest,
-		    "ssh-sk-attest-v00")) != 0 ||
-		    (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, 0)) != 0 || /* resvd flags */
-		    (r = sshbuf_put_string(attest, NULL, 0)) != 0 /* resvd */) {
-			error("%s: buffer error: %s", __func__, ssh_err(r));
-			goto out;
-		}
-	}
+	if ((r = fill_attestation_blob(resp, attest)) != 0)
+		goto out;
+
 	/* success */
 	*keyp = key;
 	key = NULL; /* transferred */