1 files changed, 468 insertions, 0 deletions

diff --git a/ssh-ecdsa-sk.c b/ssh-ecdsa-sk.c
new file mode 100644
index 0000000..729e548
--- /dev/null
+++ b/ssh-ecdsa-sk.c
@@ -0,0 +1,468 @@
+/* $OpenBSD: ssh-ecdsa-sk.c,v 1.17 2022/10/28 00:44:44 djm Exp $ */
+/*
+ * Copyright (c) 2000 Markus Friedl.  All rights reserved.
+ * Copyright (c) 2010 Damien Miller.  All rights reserved.
+ * Copyright (c) 2019 Google Inc.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/* #define DEBUG_SK 1 */
+
+#include "includes.h"
+
+#include <sys/types.h>
+
+#ifdef WITH_OPENSSL
+#include <openssl/bn.h>
+#include <openssl/ec.h>
+#include <openssl/ecdsa.h>
+#include <openssl/evp.h>
+#endif
+
+#include <string.h>
+#include <stdio.h> /* needed for DEBUG_SK only */
+
+#include "openbsd-compat/openssl-compat.h"
+
+#include "sshbuf.h"
+#include "ssherr.h"
+#include "digest.h"
+#define SSHKEY_INTERNAL
+#include "sshkey.h"
+
+#ifndef OPENSSL_HAS_ECC
+/* ARGSUSED */
+int
+ssh_ecdsa_sk_verify(const struct sshkey *key,
+    const u_char *signature, size_t signaturelen,
+    const u_char *data, size_t datalen, u_int compat,
+    struct sshkey_sig_details **detailsp)
+{
+	return SSH_ERR_FEATURE_UNSUPPORTED;
+}
+#else /* OPENSSL_HAS_ECC */
+
+/* Reuse some ECDSA internals */
+extern struct sshkey_impl_funcs sshkey_ecdsa_funcs;
+
+static void
+ssh_ecdsa_sk_cleanup(struct sshkey *k)
+{
+	sshkey_sk_cleanup(k);
+	sshkey_ecdsa_funcs.cleanup(k);
+}
+
+static int
+ssh_ecdsa_sk_equal(const struct sshkey *a, const struct sshkey *b)
+{
+	if (!sshkey_sk_fields_equal(a, b))
+		return 0;
+	if (!sshkey_ecdsa_funcs.equal(a, b))
+		return 0;
+	return 1;
+}
+
+static int
+ssh_ecdsa_sk_serialize_public(const struct sshkey *key, struct sshbuf *b,
+    enum sshkey_serialize_rep opts)
+{
+	int r;
+
+	if ((r = sshkey_ecdsa_funcs.serialize_public(key, b, opts)) != 0)
+		return r;
+	if ((r = sshkey_serialize_sk(key, b)) != 0)
+		return r;
+
+	return 0;
+}
+
+static int
+ssh_ecdsa_sk_serialize_private(const struct sshkey *key, struct sshbuf *b,
+    enum sshkey_serialize_rep opts)
+{
+	int r;
+
+	if (!sshkey_is_cert(key)) {
+		if ((r = sshkey_ecdsa_funcs.serialize_public(key,
+		    b, opts)) != 0)
+			return r;
+	}
+	if ((r = sshkey_serialize_private_sk(key, b)) != 0)
+		return r;
+
+	return 0;
+}
+
+static int
+ssh_ecdsa_sk_copy_public(const struct sshkey *from, struct sshkey *to)
+{
+	int r;
+
+	if ((r = sshkey_ecdsa_funcs.copy_public(from, to)) != 0)
+		return r;
+	if ((r = sshkey_copy_public_sk(from, to)) != 0)
+		return r;
+	return 0;
+}
+
+static int
+ssh_ecdsa_sk_deserialize_public(const char *ktype, struct sshbuf *b,
+    struct sshkey *key)
+{
+	int r;
+
+	if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, b, key)) != 0)
+		return r;
+	if ((r = sshkey_deserialize_sk(b, key)) != 0)
+		return r;
+	return 0;
+}
+
+static int
+ssh_ecdsa_sk_deserialize_private(const char *ktype, struct sshbuf *b,
+    struct sshkey *key)
+{
+	int r;
+
+	if (!sshkey_is_cert(key)) {
+		if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype,
+		    b, key)) != 0)
+			return r;
+	}
+	if ((r = sshkey_private_deserialize_sk(b, key)) != 0)
+		return r;
+
+	return 0;
+}
+
+/*
+ * Check FIDO/W3C webauthn signatures clientData field against the expected
+ * format and prepare a hash of it for use in signature verification.
+ *
+ * webauthn signatures do not sign the hash of the message directly, but
+ * instead sign a JSON-like "clientData" wrapper structure that contains the
+ * message hash along with a other information.
+ *
+ * Fortunately this structure has a fixed format so it is possible to verify
+ * that the hash of the signed message is present within the clientData
+ * structure without needing to implement any JSON parsing.
+ */
+static int
+webauthn_check_prepare_hash(const u_char *data, size_t datalen,
+    const char *origin, const struct sshbuf *wrapper,
+    uint8_t flags, const struct sshbuf *extensions,
+    u_char *msghash, size_t msghashlen)
+{
+	int r = SSH_ERR_INTERNAL_ERROR;
+	struct sshbuf *chall = NULL, *m = NULL;
+
+	if ((m = sshbuf_new()) == NULL ||
+	    (chall = sshbuf_from(data, datalen)) == NULL) {
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	/*
+	 * Ensure origin contains no quote character and that the flags are
+	 * consistent with what we received
+	 */
+	if (strchr(origin, '\"') != NULL ||
+	    (flags & 0x40) != 0 /* AD */ ||
+	    ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) {
+		r = SSH_ERR_INVALID_FORMAT;
+		goto out;
+	}
+
+	/*
+	 * Prepare the preamble to clientData that we expect, poking the
+	 * challenge and origin into their canonical positions in the
+	 * structure. The crossOrigin flag and any additional extension
+	 * fields present are ignored.
+	 */
+#define WEBAUTHN_0	"{\"type\":\"webauthn.get\",\"challenge\":\""
+#define WEBAUTHN_1	"\",\"origin\":\""
+#define WEBAUTHN_2	"\""
+	if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 ||
+	    (r = sshbuf_dtourlb64(chall, m, 0)) != 0 ||
+	    (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 ||
+	    (r = sshbuf_put(m, origin, strlen(origin))) != 0 ||
+	    (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0)
+		goto out;
+#ifdef DEBUG_SK
+	fprintf(stderr, "%s: received origin: %s\n", __func__, origin);
+	fprintf(stderr, "%s: received clientData:\n", __func__);
+	sshbuf_dump(wrapper, stderr);
+	fprintf(stderr, "%s: expected clientData premable:\n", __func__);
+	sshbuf_dump(m, stderr);
+#endif
+	/* Check that the supplied clientData has the preamble we expect */
+	if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0)
+		goto out;
+
+	/* Prepare hash of clientData */
+	if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper,
+	    msghash, msghashlen)) != 0)
+		goto out;
+
+	/* success */
+	r = 0;
+ out:
+	sshbuf_free(chall);
+	sshbuf_free(m);
+	return r;
+}
+
+/* ARGSUSED */
+static int
+ssh_ecdsa_sk_verify(const struct sshkey *key,
+    const u_char *sig, size_t siglen,
+    const u_char *data, size_t dlen, const char *alg, u_int compat,
+    struct sshkey_sig_details **detailsp)
+{
+	ECDSA_SIG *esig = NULL;
+	BIGNUM *sig_r = NULL, *sig_s = NULL;
+	u_char sig_flags;
+	u_char msghash[32], apphash[32], sighash[32];
+	u_int sig_counter;
+	int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR;
+	struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL;
+	struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL;
+	char *ktype = NULL, *webauthn_origin = NULL;
+	struct sshkey_sig_details *details = NULL;
+#ifdef DEBUG_SK
+	char *tmp = NULL;
+#endif
+
+	if (detailsp != NULL)
+		*detailsp = NULL;
+	if (key == NULL || key->ecdsa == NULL ||
+	    sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
+	    sig == NULL || siglen == 0)
+		return SSH_ERR_INVALID_ARGUMENT;
+
+	if (key->ecdsa_nid != NID_X9_62_prime256v1)
+		return SSH_ERR_INTERNAL_ERROR;
+
+	/* fetch signature */
+	if ((b = sshbuf_from(sig, siglen)) == NULL)
+		return SSH_ERR_ALLOC_FAIL;
+	if ((details = calloc(1, sizeof(*details))) == NULL) {
+		ret = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if (sshbuf_get_cstring(b, &ktype, NULL) != 0) {
+		ret = SSH_ERR_INVALID_FORMAT;
+		goto out;
+	}
+	if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0)
+		is_webauthn = 1;
+	else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) {
+		ret = SSH_ERR_INVALID_FORMAT;
+		goto out;
+	}
+	if (sshbuf_froms(b, &sigbuf) != 0 ||
+	    sshbuf_get_u8(b, &sig_flags) != 0 ||
+	    sshbuf_get_u32(b, &sig_counter) != 0) {
+		ret = SSH_ERR_INVALID_FORMAT;
+		goto out;
+	}
+	if (is_webauthn) {
+		if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 ||
+		    sshbuf_froms(b, &webauthn_wrapper) != 0 ||
+		    sshbuf_froms(b, &webauthn_exts) != 0) {
+			ret = SSH_ERR_INVALID_FORMAT;
+			goto out;
+		}
+	}
+	if (sshbuf_len(b) != 0) {
+		ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
+		goto out;
+	}
+
+	/* parse signature */
+	if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
+	    sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
+		ret = SSH_ERR_INVALID_FORMAT;
+		goto out;
+	}
+	if (sshbuf_len(sigbuf) != 0) {
+		ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
+		goto out;
+	}
+
+#ifdef DEBUG_SK
+	fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen);
+	/* sshbuf_dump_data(data, datalen, stderr); */
+	fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r)));
+	free(tmp);
+	fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s)));
+	free(tmp);
+	fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
+	    __func__, sig_flags, sig_counter);
+	if (is_webauthn) {
+		fprintf(stderr, "%s: webauthn origin: %s\n", __func__,
+		    webauthn_origin);
+		fprintf(stderr, "%s: webauthn_wrapper:\n", __func__);
+		sshbuf_dump(webauthn_wrapper, stderr);
+	}
+#endif
+	if ((esig = ECDSA_SIG_new()) == NULL) {
+		ret = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if (!ECDSA_SIG_set0(esig, sig_r, sig_s)) {
+		ret = SSH_ERR_LIBCRYPTO_ERROR;
+		goto out;
+	}
+	sig_r = sig_s = NULL; /* transferred */
+
+	/* Reconstruct data that was supposedly signed */
+	if ((original_signed = sshbuf_new()) == NULL) {
+		ret = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if (is_webauthn) {
+		if ((ret = webauthn_check_prepare_hash(data, dlen,
+		    webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts,
+		    msghash, sizeof(msghash))) != 0)
+			goto out;
+	} else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, dlen,
+	    msghash, sizeof(msghash))) != 0)
+		goto out;
+	/* Application value is hashed before signature */
+	if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
+	    strlen(key->sk_application), apphash, sizeof(apphash))) != 0)
+		goto out;
+#ifdef DEBUG_SK
+	fprintf(stderr, "%s: hashed application:\n", __func__);
+	sshbuf_dump_data(apphash, sizeof(apphash), stderr);
+	fprintf(stderr, "%s: hashed message:\n", __func__);
+	sshbuf_dump_data(msghash, sizeof(msghash), stderr);
+#endif
+	if ((ret = sshbuf_put(original_signed,
+	    apphash, sizeof(apphash))) != 0 ||
+	    (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 ||
+	    (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 ||
+	    (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 ||
+	    (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0)
+		goto out;
+	/* Signature is over H(original_signed) */
+	if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed,
+	    sighash, sizeof(sighash))) != 0)
+		goto out;
+	details->sk_counter = sig_counter;
+	details->sk_flags = sig_flags;
+#ifdef DEBUG_SK
+	fprintf(stderr, "%s: signed buf:\n", __func__);
+	sshbuf_dump(original_signed, stderr);
+	fprintf(stderr, "%s: signed hash:\n", __func__);
+	sshbuf_dump_data(sighash, sizeof(sighash), stderr);
+#endif
+
+	/* Verify it */
+	switch (ECDSA_do_verify(sighash, sizeof(sighash), esig, key->ecdsa)) {
+	case 1:
+		ret = 0;
+		break;
+	case 0:
+		ret = SSH_ERR_SIGNATURE_INVALID;
+		goto out;
+	default:
+		ret = SSH_ERR_LIBCRYPTO_ERROR;
+		goto out;
+	}
+	/* success */
+	if (detailsp != NULL) {
+		*detailsp = details;
+		details = NULL;
+	}
+ out:
+	explicit_bzero(&sig_flags, sizeof(sig_flags));
+	explicit_bzero(&sig_counter, sizeof(sig_counter));
+	explicit_bzero(msghash, sizeof(msghash));
+	explicit_bzero(sighash, sizeof(msghash));
+	explicit_bzero(apphash, sizeof(apphash));
+	sshkey_sig_details_free(details);
+	sshbuf_free(webauthn_wrapper);
+	sshbuf_free(webauthn_exts);
+	free(webauthn_origin);
+	sshbuf_free(original_signed);
+	sshbuf_free(sigbuf);
+	sshbuf_free(b);
+	ECDSA_SIG_free(esig);
+	BN_clear_free(sig_r);
+	BN_clear_free(sig_s);
+	free(ktype);
+	return ret;
+}
+
+static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = {
+	/* .size = */		NULL,
+	/* .alloc = */		NULL,
+	/* .cleanup = */	ssh_ecdsa_sk_cleanup,
+	/* .equal = */		ssh_ecdsa_sk_equal,
+	/* .ssh_serialize_public = */ ssh_ecdsa_sk_serialize_public,
+	/* .ssh_deserialize_public = */ ssh_ecdsa_sk_deserialize_public,
+	/* .ssh_serialize_private = */ ssh_ecdsa_sk_serialize_private,
+	/* .ssh_deserialize_private = */ ssh_ecdsa_sk_deserialize_private,
+	/* .generate = */	NULL,
+	/* .copy_public = */	ssh_ecdsa_sk_copy_public,
+	/* .sign = */		NULL,
+	/* .verify = */		ssh_ecdsa_sk_verify,
+};
+
+const struct sshkey_impl sshkey_ecdsa_sk_impl = {
+	/* .name = */		"sk-ecdsa-sha2-nistp256@openssh.com",
+	/* .shortname = */	"ECDSA-SK",
+	/* .sigalg = */		NULL,
+	/* .type = */		KEY_ECDSA_SK,
+	/* .nid = */		NID_X9_62_prime256v1,
+	/* .cert = */		0,
+	/* .sigonly = */	0,
+	/* .keybits = */	256,
+	/* .funcs = */		&sshkey_ecdsa_sk_funcs,
+};
+
+const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = {
+	/* .name = */		"sk-ecdsa-sha2-nistp256-cert-v01@openssh.com",
+	/* .shortname = */	"ECDSA-SK-CERT",
+	/* .sigalg = */		NULL,
+	/* .type = */		KEY_ECDSA_SK_CERT,
+	/* .nid = */		NID_X9_62_prime256v1,
+	/* .cert = */		1,
+	/* .sigonly = */	0,
+	/* .keybits = */	256,
+	/* .funcs = */		&sshkey_ecdsa_sk_funcs,
+};
+
+const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = {
+	/* .name = */		"webauthn-sk-ecdsa-sha2-nistp256@openssh.com",
+	/* .shortname = */	"ECDSA-SK",
+	/* .sigalg = */		NULL,
+	/* .type = */		KEY_ECDSA_SK,
+	/* .nid = */		NID_X9_62_prime256v1,
+	/* .cert = */		0,
+	/* .sigonly = */	1,
+	/* .keybits = */	256,
+	/* .funcs = */		&sshkey_ecdsa_sk_funcs,
+};
+
+#endif /* OPENSSL_HAS_ECC */