| /* $OpenBSD: ssh-ecdsa-sk.c,v 1.8 2020/06/22 23:44:27 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 */ |
| |
| /* |
| * 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 */ |
| 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) |
| { |
| ECDSA_SIG *sig = 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 || |
| signature == NULL || signaturelen == 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(signature, signaturelen)) == 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 ((sig = ECDSA_SIG_new()) == NULL) { |
| ret = SSH_ERR_ALLOC_FAIL; |
| goto out; |
| } |
| if (!ECDSA_SIG_set0(sig, 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, datalen, |
| webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts, |
| msghash, sizeof(msghash))) != 0) |
| goto out; |
| } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, datalen, |
| 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), sig, 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(sig); |
| BN_clear_free(sig_r); |
| BN_clear_free(sig_s); |
| free(ktype); |
| return ret; |
| } |
| |
| #endif /* OPENSSL_HAS_ECC */ |