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Diffstat (limited to '')
| -rw-r--r-- | ssh-ecdsa-sk.c | 324 |
1 files changed, 324 insertions, 0 deletions
diff --git a/ssh-ecdsa-sk.c b/ssh-ecdsa-sk.c new file mode 100644 index 0000000..c6927ec --- /dev/null +++ b/ssh-ecdsa-sk.c @@ -0,0 +1,324 @@ +/* $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 */ |