/* $OpenBSD: ssh-ecdsa-sk.c,v 1.21 2026/02/06 22:59:18 dtucker 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 "openbsd-compat/openssl-compat.h"

#include <openssl/bn.h>

#include <openssl/ec.h>

#include <openssl/ecdsa.h>

#include <openssl/evp.h>

#include <string.h>

#include <stdio.h> /* 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 preamble:\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;

  EVP_MD_CTX *md_ctx = NULL;

  BIGNUM *sig_r = NULL, *sig_s = NULL;

  u_char sig_flags;

  u_char msghash[32], apphash[32];

  u_int sig_counter;

  u_char *sigb = NULL, *cp;

  int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR, len = 0;

  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->pkey == 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 ||

      strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256-cert-v01@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__, dlen);

  /* 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;

  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);

#endif

  if ((md_ctx = EVP_MD_CTX_new()) == NULL) {

    ret = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  if ((len = i2d_ECDSA_SIG(esig, NULL)) <= 0) {

    len = 0;

    ret = SSH_ERR_LIBCRYPTO_ERROR;

    goto out;

  }

  if ((sigb = calloc(1, len)) == NULL) {

    ret = SSH_ERR_ALLOC_FAIL;

    goto out;

  }

  cp = sigb; /* ASN1_item_i2d increments the pointer past the object */

  if (i2d_ECDSA_SIG(esig, &cp) != len) {

    ret = SSH_ERR_LIBCRYPTO_ERROR;

    goto out;

  }

#ifdef DEBUG_SK

  fprintf(stderr, "%s: signed hash:\n", __func__);

  sshbuf_dump_data(sigb, len, stderr);

#endif

  /* Verify it */

  if (EVP_DigestVerifyInit(md_ctx, NULL, EVP_sha256(), NULL,

      key->pkey) != 1) {

    ret = SSH_ERR_LIBCRYPTO_ERROR;

    goto out;

  }

  switch (EVP_DigestVerify(md_ctx, sigb, len,

      sshbuf_ptr(original_signed), sshbuf_len(original_signed))) {

  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(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);

  freezero(sigb, len);

  EVP_MD_CTX_free(md_ctx);

  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,

};

const struct sshkey_impl sshkey_ecdsa_sk_webauthn_cert_impl = {

  /* .name = */   "webauthn-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 = */  1,

  /* .keybits = */  256,

  /* .funcs = */    &sshkey_ecdsa_sk_funcs,

};

#endif /* WITH_OPENSSL */