changes to support FIDO attestation

Allow writing to disk the attestation certificate that is generated by
the FIDO token at key enrollment time. These certificates may be used
by an out-of-band workflow to prove that a particular key is held in
trustworthy hardware.

Allow passing in a challenge that will be sent to the card during
key enrollment. These are needed to build an attestation workflow
that resists replay attacks.

ok markus@

1 files changed, 33 insertions, 3 deletions

diff --git a/usr.bin/ssh/ssh-keygen.c b/usr.bin/ssh/ssh-keygen.c
index 16df93d1490..03145b4ddff 100644
--- a/usr.bin/ssh/ssh-keygen.c
+++ b/usr.bin/ssh/ssh-keygen.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-keygen.c,v 1.394 2020/01/25 23:13:09 djm Exp $ */
+/* $OpenBSD: ssh-keygen.c,v 1.395 2020/01/28 08:01:34 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1994 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -3092,6 +3092,8 @@ 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;
+	struct sshbuf *challenge = NULL, *attest = NULL;
 	size_t i, nopts = 0;
 	u_int32_t bits = 0;
 	uint8_t sk_flags = SSH_SK_USER_PRESENCE_REQD;
@@ -3532,6 +3534,16 @@ main(int argc, char **argv)
 				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], "challenge=", 10) == 0) {
+				if ((r = sshbuf_load_file(opts[i] + 10,
+				    &challenge)) != 0) {
+					fatal("Unable to load FIDO enrollment "
+					    "challenge \"%s\": %s",
+					    opts[i] + 10, ssh_err(r));
+				}
+			} else if (strncasecmp(opts[i],
+			    "write-attestation=", 18) == 0) {
+				sk_attestaion_path = opts[i] + 18;
 			} else if (strncasecmp(opts[i],
 			    "application=", 12) == 0) {
 				sk_application = xstrdup(opts[i] + 12);
@@ -3545,12 +3557,14 @@ main(int argc, char **argv)
 			    "to authorize key generation.\n");
 		}
 		passphrase = NULL;
+		if ((attest = sshbuf_new()) == NULL)
+			fatal("sshbuf_new failed");
 		for (i = 0 ; i < 3; i++) {
 			fflush(stdout);
 			r = sshsk_enroll(type, sk_provider, sk_device,
 			    sk_application == NULL ? "ssh:" : sk_application,
-			    sk_user, sk_flags, passphrase, NULL,
-			    &private, NULL);
+			    sk_user, sk_flags, passphrase, challenge,
+			    &private, attest);
 			if (r == 0)
 				break;
 			if (r != SSH_ERR_KEY_WRONG_PASSPHRASE)
@@ -3643,6 +3657,22 @@ 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);
+		}
+	}
+	sshbuf_free(attest);
 	sshkey_free(public);
+
 	exit(0);
 }