/* $NetBSD: ssh-ecdsa-sk.c,v 1.4.2.2 2023/12/25 12:31:07 martin Exp $ */ /* $OpenBSD: ssh-ecdsa-sk.c,v 1.18 2023/03/08 04:43:12 guenther 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. */ #include "includes.h" __RCSID("$NetBSD: ssh-ecdsa-sk.c,v 1.4.2.2 2023/12/25 12:31:07 martin Exp $"); /* #define DEBUG_SK 1 */ #include #include #include #include #include #include #include /* needed for DEBUG_SK only */ #include "sshbuf.h" #include "ssherr.h" #include "digest.h" #define SSHKEY_INTERNAL #include "sshkey.h" /* 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; } 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, };