CMake/Utilities/cmcurl/lib/vtls/nss.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 1998 - 2019, Daniel Stenberg, <[email protected]>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at https://curl.haxx.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 ***************************************************************************/

/*
 * Source file for all NSS-specific code for the TLS/SSL layer. No code
 * but vtls.c should ever call or use these functions.
 */

#include "curl_setup.h"

#ifdef USE_NSS

#include "urldata.h"
#include "sendf.h"
#include "formdata.h" /* for the boundary function */
#include "url.h" /* for the ssl config check function */
#include "connect.h"
#include "strcase.h"
#include "select.h"
#include "vtls.h"
#include "llist.h"
#include "multiif.h"
#include "curl_printf.h"
#include "nssg.h"
#include <nspr.h>
#include <nss.h>
#include <ssl.h>
#include <sslerr.h>
#include <secerr.h>
#include <secmod.h>
#include <sslproto.h>
#include <prtypes.h>
#include <pk11pub.h>
#include <prio.h>
#include <secitem.h>
#include <secport.h>
#include <certdb.h>
#include <base64.h>
#include <cert.h>
#include <prerror.h>
#include <keyhi.h>         /* for SECKEY_DestroyPublicKey() */
#include <private/pprio.h> /* for PR_ImportTCPSocket */

#define NSSVERNUM ((NSS_VMAJOR<<16)|(NSS_VMINOR<<8)|NSS_VPATCH)

#if NSSVERNUM >= 0x030f00 /* 3.15.0 */
#include <ocsp.h>
#endif

#include "strcase.h"
#include "warnless.h"
#include "x509asn1.h"

/* The last #include files should be: */
#include "curl_memory.h"
#include "memdebug.h"

#define SSL_DIR "/etc/pki/nssdb"

/* enough to fit the string "PEM Token #[0|1]" */
#define SLOTSIZE 13

struct ssl_backend_data {
  PRFileDesc *handle;
  char *client_nickname;
  struct Curl_easy *data;
  struct curl_llist obj_list;
  PK11GenericObject *obj_clicert;
};

#define BACKEND connssl->backend

static PRLock *nss_initlock = NULL;
static PRLock *nss_crllock = NULL;
static PRLock *nss_findslot_lock = NULL;
static PRLock *nss_trustload_lock = NULL;
static struct curl_llist nss_crl_list;
static NSSInitContext *nss_context = NULL;
static volatile int initialized = 0;

/* type used to wrap pointers as list nodes */
struct ptr_list_wrap {
  void *ptr;
  struct curl_llist_element node;
};

typedef struct {
  const char *name;
  int num;
} cipher_s;

#define PK11_SETATTRS(_attr, _idx, _type, _val, _len) do {  \
  CK_ATTRIBUTE *ptr = (_attr) + ((_idx)++);                 \
  ptr->type = (_type);                                      \
  ptr->pValue = (_val);                                     \
  ptr->ulValueLen = (_len);                                 \
} WHILE_FALSE

#define CERT_NewTempCertificate __CERT_NewTempCertificate

#define NUM_OF_CIPHERS sizeof(cipherlist)/sizeof(cipherlist[0])
static const cipher_s cipherlist[] = {
  /* SSL2 cipher suites */
  {"rc4",                        SSL_EN_RC4_128_WITH_MD5},
  {"rc4-md5",                    SSL_EN_RC4_128_WITH_MD5},
  {"rc4export",                  SSL_EN_RC4_128_EXPORT40_WITH_MD5},
  {"rc2",                        SSL_EN_RC2_128_CBC_WITH_MD5},
  {"rc2export",                  SSL_EN_RC2_128_CBC_EXPORT40_WITH_MD5},
  {"des",                        SSL_EN_DES_64_CBC_WITH_MD5},
  {"desede3",                    SSL_EN_DES_192_EDE3_CBC_WITH_MD5},
  /* SSL3/TLS cipher suites */
  {"rsa_rc4_128_md5",            SSL_RSA_WITH_RC4_128_MD5},
  {"rsa_rc4_128_sha",            SSL_RSA_WITH_RC4_128_SHA},
  {"rsa_3des_sha",               SSL_RSA_WITH_3DES_EDE_CBC_SHA},
  {"rsa_des_sha",                SSL_RSA_WITH_DES_CBC_SHA},
  {"rsa_rc4_40_md5",             SSL_RSA_EXPORT_WITH_RC4_40_MD5},
  {"rsa_rc2_40_md5",             SSL_RSA_EXPORT_WITH_RC2_CBC_40_MD5},
  {"rsa_null_md5",               SSL_RSA_WITH_NULL_MD5},
  {"rsa_null_sha",               SSL_RSA_WITH_NULL_SHA},
  {"fips_3des_sha",              SSL_RSA_FIPS_WITH_3DES_EDE_CBC_SHA},
  {"fips_des_sha",               SSL_RSA_FIPS_WITH_DES_CBC_SHA},
  {"fortezza",                   SSL_FORTEZZA_DMS_WITH_FORTEZZA_CBC_SHA},
  {"fortezza_rc4_128_sha",       SSL_FORTEZZA_DMS_WITH_RC4_128_SHA},
  {"fortezza_null",              SSL_FORTEZZA_DMS_WITH_NULL_SHA},
  /* TLS 1.0: Exportable 56-bit Cipher Suites. */
  {"rsa_des_56_sha",             TLS_RSA_EXPORT1024_WITH_DES_CBC_SHA},
  {"rsa_rc4_56_sha",             TLS_RSA_EXPORT1024_WITH_RC4_56_SHA},
  /* AES ciphers. */
  {"dhe_dss_aes_128_cbc_sha",    TLS_DHE_DSS_WITH_AES_128_CBC_SHA},
  {"dhe_dss_aes_256_cbc_sha",    TLS_DHE_DSS_WITH_AES_256_CBC_SHA},
  {"dhe_rsa_aes_128_cbc_sha",    TLS_DHE_RSA_WITH_AES_128_CBC_SHA},
  {"dhe_rsa_aes_256_cbc_sha",    TLS_DHE_RSA_WITH_AES_256_CBC_SHA},
  {"rsa_aes_128_sha",            TLS_RSA_WITH_AES_128_CBC_SHA},
  {"rsa_aes_256_sha",            TLS_RSA_WITH_AES_256_CBC_SHA},
  /* ECC ciphers. */
  {"ecdh_ecdsa_null_sha",        TLS_ECDH_ECDSA_WITH_NULL_SHA},
  {"ecdh_ecdsa_rc4_128_sha",     TLS_ECDH_ECDSA_WITH_RC4_128_SHA},
  {"ecdh_ecdsa_3des_sha",        TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA},
  {"ecdh_ecdsa_aes_128_sha",     TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA},
  {"ecdh_ecdsa_aes_256_sha",     TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA},
  {"ecdhe_ecdsa_null_sha",       TLS_ECDHE_ECDSA_WITH_NULL_SHA},
  {"ecdhe_ecdsa_rc4_128_sha",    TLS_ECDHE_ECDSA_WITH_RC4_128_SHA},
  {"ecdhe_ecdsa_3des_sha",       TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA},
  {"ecdhe_ecdsa_aes_128_sha",    TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA},
  {"ecdhe_ecdsa_aes_256_sha",    TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA},
  {"ecdh_rsa_null_sha",          TLS_ECDH_RSA_WITH_NULL_SHA},
  {"ecdh_rsa_128_sha",           TLS_ECDH_RSA_WITH_RC4_128_SHA},
  {"ecdh_rsa_3des_sha",          TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA},
  {"ecdh_rsa_aes_128_sha",       TLS_ECDH_RSA_WITH_AES_128_CBC_SHA},
  {"ecdh_rsa_aes_256_sha",       TLS_ECDH_RSA_WITH_AES_256_CBC_SHA},
  {"ecdhe_rsa_null",             TLS_ECDHE_RSA_WITH_NULL_SHA},
  {"ecdhe_rsa_rc4_128_sha",      TLS_ECDHE_RSA_WITH_RC4_128_SHA},
  {"ecdhe_rsa_3des_sha",         TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA},
  {"ecdhe_rsa_aes_128_sha",      TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA},
  {"ecdhe_rsa_aes_256_sha",      TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA},
  {"ecdh_anon_null_sha",         TLS_ECDH_anon_WITH_NULL_SHA},
  {"ecdh_anon_rc4_128sha",       TLS_ECDH_anon_WITH_RC4_128_SHA},
  {"ecdh_anon_3des_sha",         TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA},
  {"ecdh_anon_aes_128_sha",      TLS_ECDH_anon_WITH_AES_128_CBC_SHA},
  {"ecdh_anon_aes_256_sha",      TLS_ECDH_anon_WITH_AES_256_CBC_SHA},
#ifdef TLS_RSA_WITH_NULL_SHA256
  /* new HMAC-SHA256 cipher suites specified in RFC */
  {"rsa_null_sha_256",                TLS_RSA_WITH_NULL_SHA256},
  {"rsa_aes_128_cbc_sha_256",         TLS_RSA_WITH_AES_128_CBC_SHA256},
  {"rsa_aes_256_cbc_sha_256",         TLS_RSA_WITH_AES_256_CBC_SHA256},
  {"dhe_rsa_aes_128_cbc_sha_256",     TLS_DHE_RSA_WITH_AES_128_CBC_SHA256},
  {"dhe_rsa_aes_256_cbc_sha_256",     TLS_DHE_RSA_WITH_AES_256_CBC_SHA256},
  {"ecdhe_ecdsa_aes_128_cbc_sha_256", TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256},
  {"ecdhe_rsa_aes_128_cbc_sha_256",   TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256},
#endif
#ifdef TLS_RSA_WITH_AES_128_GCM_SHA256
  /* AES GCM cipher suites in RFC 5288 and RFC 5289 */
  {"rsa_aes_128_gcm_sha_256",         TLS_RSA_WITH_AES_128_GCM_SHA256},
  {"dhe_rsa_aes_128_gcm_sha_256",     TLS_DHE_RSA_WITH_AES_128_GCM_SHA256},
  {"dhe_dss_aes_128_gcm_sha_256",     TLS_DHE_DSS_WITH_AES_128_GCM_SHA256},
  {"ecdhe_ecdsa_aes_128_gcm_sha_256", TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256},
  {"ecdh_ecdsa_aes_128_gcm_sha_256",  TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256},
  {"ecdhe_rsa_aes_128_gcm_sha_256",   TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256},
  {"ecdh_rsa_aes_128_gcm_sha_256",    TLS_ECDH_RSA_WITH_AES_128_GCM_SHA256},
#endif
#ifdef TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
  /* cipher suites using SHA384 */
  {"rsa_aes_256_gcm_sha_384",         TLS_RSA_WITH_AES_256_GCM_SHA384},
  {"dhe_rsa_aes_256_gcm_sha_384",     TLS_DHE_RSA_WITH_AES_256_GCM_SHA384},
  {"dhe_dss_aes_256_gcm_sha_384",     TLS_DHE_DSS_WITH_AES_256_GCM_SHA384},
  {"ecdhe_ecdsa_aes_256_sha_384",     TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384},
  {"ecdhe_rsa_aes_256_sha_384",       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384},
  {"ecdhe_ecdsa_aes_256_gcm_sha_384", TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384},
  {"ecdhe_rsa_aes_256_gcm_sha_384",   TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384},
#endif
#ifdef TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256
  /* chacha20-poly1305 cipher suites */
 {"ecdhe_rsa_chacha20_poly1305_sha_256",
     TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256},
 {"ecdhe_ecdsa_chacha20_poly1305_sha_256",
     TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256},
 {"dhe_rsa_chacha20_poly1305_sha_256",
     TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256},
#endif
};

#ifdef WIN32
static const char *pem_library = "nsspem.dll";
static const char *trust_library = "nssckbi.dll";
#else
static const char *pem_library = "libnsspem.so";
static const char *trust_library = "libnssckbi.so";
#endif

static SECMODModule *pem_module = NULL;
static SECMODModule *trust_module = NULL;

/* NSPR I/O layer we use to detect blocking direction during SSL handshake */
static PRDescIdentity nspr_io_identity = PR_INVALID_IO_LAYER;
static PRIOMethods nspr_io_methods;

static const char *nss_error_to_name(PRErrorCode code)
{
  const char *name = PR_ErrorToName(code);
  if(name)
    return name;

  return "unknown error";
}

static void nss_print_error_message(struct Curl_easy *data, PRUint32 err)
{
  failf(data, "%s", PR_ErrorToString(err, PR_LANGUAGE_I_DEFAULT));
}

static char *nss_sslver_to_name(PRUint16 nssver)
{
  switch(nssver) {
  case SSL_LIBRARY_VERSION_2:
    return strdup("SSLv2");
  case SSL_LIBRARY_VERSION_3_0:
    return strdup("SSLv3");
  case SSL_LIBRARY_VERSION_TLS_1_0:
    return strdup("TLSv1.0");
#ifdef SSL_LIBRARY_VERSION_TLS_1_1
  case SSL_LIBRARY_VERSION_TLS_1_1:
    return strdup("TLSv1.1");
#endif
#ifdef SSL_LIBRARY_VERSION_TLS_1_2
  case SSL_LIBRARY_VERSION_TLS_1_2:
    return strdup("TLSv1.2");
#endif
#ifdef SSL_LIBRARY_VERSION_TLS_1_3
  case SSL_LIBRARY_VERSION_TLS_1_3:
    return strdup("TLSv1.3");
#endif
  default:
    return curl_maprintf("0x%04x", nssver);
  }
}

static SECStatus set_ciphers(struct Curl_easy *data, PRFileDesc * model,
                             char *cipher_list)
{
  unsigned int i;
  PRBool cipher_state[NUM_OF_CIPHERS];
  PRBool found;
  char *cipher;

  /* use accessors to avoid dynamic linking issues after an update of NSS */
  const PRUint16 num_implemented_ciphers = SSL_GetNumImplementedCiphers();
  const PRUint16 *implemented_ciphers = SSL_GetImplementedCiphers();
  if(!implemented_ciphers)
    return SECFailure;

  /* First disable all ciphers. This uses a different max value in case
   * NSS adds more ciphers later we don't want them available by
   * accident
   */
  for(i = 0; i < num_implemented_ciphers; i++) {
    SSL_CipherPrefSet(model, implemented_ciphers[i], PR_FALSE);
  }

  /* Set every entry in our list to false */
  for(i = 0; i < NUM_OF_CIPHERS; i++) {
    cipher_state[i] = PR_FALSE;
  }

  cipher = cipher_list;

  while(cipher_list && (cipher_list[0])) {
    while((*cipher) && (ISSPACE(*cipher)))
      ++cipher;

    cipher_list = strchr(cipher, ',');
    if(cipher_list) {
      *cipher_list++ = '\0';
    }

    found = PR_FALSE;

    for(i = 0; i<NUM_OF_CIPHERS; i++) {
      if(strcasecompare(cipher, cipherlist[i].name)) {
        cipher_state[i] = PR_TRUE;
        found = PR_TRUE;
        break;
      }
    }

    if(found == PR_FALSE) {
      failf(data, "Unknown cipher in list: %s", cipher);
      return SECFailure;
    }

    if(cipher_list) {
      cipher = cipher_list;
    }
  }

  /* Finally actually enable the selected ciphers */
  for(i = 0; i<NUM_OF_CIPHERS; i++) {
    if(!cipher_state[i])
      continue;

    if(SSL_CipherPrefSet(model, cipherlist[i].num, PR_TRUE) != SECSuccess) {
      failf(data, "cipher-suite not supported by NSS: %s", cipherlist[i].name);
      return SECFailure;
    }
  }

  return SECSuccess;
}

/*
 * Return true if at least one cipher-suite is enabled. Used to determine
 * if we need to call NSS_SetDomesticPolicy() to enable the default ciphers.
 */
static bool any_cipher_enabled(void)
{
  unsigned int i;

  for(i = 0; i<NUM_OF_CIPHERS; i++) {
    PRInt32 policy = 0;
    SSL_CipherPolicyGet(cipherlist[i].num, &policy);
    if(policy)
      return TRUE;
  }

  return FALSE;
}

/*
 * Determine whether the nickname passed in is a filename that needs to
 * be loaded as a PEM or a regular NSS nickname.
 *
 * returns 1 for a file
 * returns 0 for not a file (NSS nickname)
 */
static int is_file(const char *filename)
{
  struct_stat st;

  if(filename == NULL)
    return 0;

  if(stat(filename, &st) == 0)
    if(S_ISREG(st.st_mode) || S_ISFIFO(st.st_mode) || S_ISCHR(st.st_mode))
      return 1;

  return 0;
}

/* Check if the given string is filename or nickname of a certificate.  If the
 * given string is recognized as filename, return NULL.  If the given string is
 * recognized as nickname, return a duplicated string.  The returned string
 * should be later deallocated using free().  If the OOM failure occurs, we
 * return NULL, too.
 */
static char *dup_nickname(struct Curl_easy *data, const char *str)
{
  const char *n;

  if(!is_file(str))
    /* no such file exists, use the string as nickname */
    return strdup(str);

  /* search the first slash; we require at least one slash in a file name */
  n = strchr(str, '/');
  if(!n) {
    infof(data, "warning: certificate file name \"%s\" handled as nickname; "
          "please use \"./%s\" to force file name\n", str, str);
    return strdup(str);
  }

  /* we'll use the PEM reader to read the certificate from file */
  return NULL;
}

/* Lock/unlock wrapper for PK11_FindSlotByName() to work around race condition
 * in nssSlot_IsTokenPresent() causing spurious SEC_ERROR_NO_TOKEN.  For more
 * details, go to <https://bugzilla.mozilla.org/1297397>.
 */
static PK11SlotInfo* nss_find_slot_by_name(const char *slot_name)
{
  PK11SlotInfo *slot;
  PR_Lock(nss_findslot_lock);
  slot = PK11_FindSlotByName(slot_name);
  PR_Unlock(nss_findslot_lock);
  return slot;
}

/* wrap 'ptr' as list node and tail-insert into 'list' */
static CURLcode insert_wrapped_ptr(struct curl_llist *list, void *ptr)
{
  struct ptr_list_wrap *wrap = malloc(sizeof(*wrap));
  if(!wrap)
    return CURLE_OUT_OF_MEMORY;

  wrap->ptr = ptr;
  Curl_llist_insert_next(list, list->tail, wrap, &wrap->node);
  return CURLE_OK;
}

/* Call PK11_CreateGenericObject() with the given obj_class and filename.  If
 * the call succeeds, append the object handle to the list of objects so that
 * the object can be destroyed in Curl_nss_close(). */
static CURLcode nss_create_object(struct ssl_connect_data *connssl,
                                  CK_OBJECT_CLASS obj_class,
                                  const char *filename, bool cacert)
{
  PK11SlotInfo *slot;
  PK11GenericObject *obj;
  CK_BBOOL cktrue = CK_TRUE;
  CK_BBOOL ckfalse = CK_FALSE;
  CK_ATTRIBUTE attrs[/* max count of attributes */ 4];
  int attr_cnt = 0;
  CURLcode result = (cacert)
    ? CURLE_SSL_CACERT_BADFILE
    : CURLE_SSL_CERTPROBLEM;

  const int slot_id = (cacert) ? 0 : 1;
  char *slot_name = aprintf("PEM Token #%d", slot_id);
  if(!slot_name)
    return CURLE_OUT_OF_MEMORY;

  slot = nss_find_slot_by_name(slot_name);
  free(slot_name);
  if(!slot)
    return result;

  PK11_SETATTRS(attrs, attr_cnt, CKA_CLASS, &obj_class, sizeof(obj_class));
  PK11_SETATTRS(attrs, attr_cnt, CKA_TOKEN, &cktrue, sizeof(CK_BBOOL));
  PK11_SETATTRS(attrs, attr_cnt, CKA_LABEL, (unsigned char *)filename,
                (CK_ULONG)strlen(filename) + 1);

  if(CKO_CERTIFICATE == obj_class) {
    CK_BBOOL *pval = (cacert) ? (&cktrue) : (&ckfalse);
    PK11_SETATTRS(attrs, attr_cnt, CKA_TRUST, pval, sizeof(*pval));
  }

  /* PK11_CreateManagedGenericObject() was introduced in NSS 3.34 because
   * PK11_DestroyGenericObject() does not release resources allocated by
   * PK11_CreateGenericObject() early enough.  */
  obj =
#ifdef HAVE_PK11_CREATEMANAGEDGENERICOBJECT
    PK11_CreateManagedGenericObject
#else
    PK11_CreateGenericObject
#endif
    (slot, attrs, attr_cnt, PR_FALSE);

  PK11_FreeSlot(slot);
  if(!obj)
    return result;

  if(insert_wrapped_ptr(&BACKEND->obj_list, obj) != CURLE_OK) {
    PK11_DestroyGenericObject(obj);
    return CURLE_OUT_OF_MEMORY;
  }

  if(!cacert && CKO_CERTIFICATE == obj_class)
    /* store reference to a client certificate */
    BACKEND->obj_clicert = obj;

  return CURLE_OK;
}

/* Destroy the NSS object whose handle is given by ptr.  This function is
 * a callback of Curl_llist_alloc() used by Curl_llist_destroy() to destroy
 * NSS objects in Curl_nss_close() */
static void nss_destroy_object(void *user, void *ptr)
{
  struct ptr_list_wrap *wrap = (struct ptr_list_wrap *) ptr;
  PK11GenericObject *obj = (PK11GenericObject *) wrap->ptr;
  (void) user;
  PK11_DestroyGenericObject(obj);
  free(wrap);
}

/* same as nss_destroy_object() but for CRL items */
static void nss_destroy_crl_item(void *user, void *ptr)
{
  struct ptr_list_wrap *wrap = (struct ptr_list_wrap *) ptr;
  SECItem *crl_der = (SECItem *) wrap->ptr;
  (void) user;
  SECITEM_FreeItem(crl_der, PR_TRUE);
  free(wrap);
}

static CURLcode nss_load_cert(struct ssl_connect_data *ssl,
                              const char *filename, PRBool cacert)
{
  CURLcode result = (cacert)
    ? CURLE_SSL_CACERT_BADFILE
    : CURLE_SSL_CERTPROBLEM;

  /* libnsspem.so leaks memory if the requested file does not exist.  For more
   * details, go to <https://bugzilla.redhat.com/734760>. */
  if(is_file(filename))
    result = nss_create_object(ssl, CKO_CERTIFICATE, filename, cacert);

  if(!result && !cacert) {
    /* we have successfully loaded a client certificate */
    CERTCertificate *cert;
    char *nickname = NULL;
    char *n = strrchr(filename, '/');
    if(n)
      n++;

    /* The following undocumented magic helps to avoid a SIGSEGV on call
     * of PK11_ReadRawAttribute() from SelectClientCert() when using an
     * immature version of libnsspem.so.  For more details, go to
     * <https://bugzilla.redhat.com/733685>. */
    nickname = aprintf("PEM Token #1:%s", n);
    if(nickname) {
      cert = PK11_FindCertFromNickname(nickname, NULL);
      if(cert)
        CERT_DestroyCertificate(cert);

      free(nickname);
    }
  }

  return result;
}

/* add given CRL to cache if it is not already there */
static CURLcode nss_cache_crl(SECItem *crl_der)
{
  CERTCertDBHandle *db = CERT_GetDefaultCertDB();
  CERTSignedCrl *crl = SEC_FindCrlByDERCert(db, crl_der, 0);
  if(crl) {
    /* CRL already cached */
    SEC_DestroyCrl(crl);
    SECITEM_FreeItem(crl_der, PR_TRUE);
    return CURLE_OK;
  }

  /* acquire lock before call of CERT_CacheCRL() and accessing nss_crl_list */
  PR_Lock(nss_crllock);

  /* store the CRL item so that we can free it in Curl_nss_cleanup() */
  if(insert_wrapped_ptr(&nss_crl_list, crl_der) != CURLE_OK) {
    SECITEM_FreeItem(crl_der, PR_TRUE);
    PR_Unlock(nss_crllock);
    return CURLE_OUT_OF_MEMORY;
  }

  if(SECSuccess != CERT_CacheCRL(db, crl_der)) {
    /* unable to cache CRL */
    PR_Unlock(nss_crllock);
    return CURLE_SSL_CRL_BADFILE;
  }

  /* we need to clear session cache, so that the CRL could take effect */
  SSL_ClearSessionCache();
  PR_Unlock(nss_crllock);
  return CURLE_OK;
}

static CURLcode nss_load_crl(const char *crlfilename)
{
  PRFileDesc *infile;
  PRFileInfo  info;
  SECItem filedata = { 0, NULL, 0 };
  SECItem *crl_der = NULL;
  char *body;

  infile = PR_Open(crlfilename, PR_RDONLY, 0);
  if(!infile)
    return CURLE_SSL_CRL_BADFILE;

  if(PR_SUCCESS != PR_GetOpenFileInfo(infile, &info))
    goto fail;

  if(!SECITEM_AllocItem(NULL, &filedata, info.size + /* zero ended */ 1))
    goto fail;

  if(info.size != PR_Read(infile, filedata.data, info.size))
    goto fail;

  crl_der = SECITEM_AllocItem(NULL, NULL, 0U);
  if(!crl_der)
    goto fail;

  /* place a trailing zero right after the visible data */
  body = (char *)filedata.data;
  body[--filedata.len] = '\0';

  body = strstr(body, "-----BEGIN");
  if(body) {
    /* assume ASCII */
    char *trailer;
    char *begin = PORT_Strchr(body, '\n');
    if(!begin)
      begin = PORT_Strchr(body, '\r');
    if(!begin)
      goto fail;

    trailer = strstr(++begin, "-----END");
    if(!trailer)
      goto fail;

    /* retrieve DER from ASCII */
    *trailer = '\0';
    if(ATOB_ConvertAsciiToItem(crl_der, begin))
      goto fail;

    SECITEM_FreeItem(&filedata, PR_FALSE);
  }
  else
    /* assume DER */
    *crl_der = filedata;

  PR_Close(infile);
  return nss_cache_crl(crl_der);

fail:
  PR_Close(infile);
  SECITEM_FreeItem(crl_der, PR_TRUE);
  SECITEM_FreeItem(&filedata, PR_FALSE);
  return CURLE_SSL_CRL_BADFILE;
}

static CURLcode nss_load_key(struct connectdata *conn, int sockindex,
                             char *key_file)
{
  PK11SlotInfo *slot, *tmp;
  SECStatus status;
  CURLcode result;
  struct ssl_connect_data *ssl = conn->ssl;
  struct Curl_easy *data = conn->data;

  (void)sockindex; /* unused */

  result = nss_create_object(ssl, CKO_PRIVATE_KEY, key_file, FALSE);
  if(result) {
    PR_SetError(SEC_ERROR_BAD_KEY, 0);
    return result;
  }

  slot = nss_find_slot_by_name("PEM Token #1");
  if(!slot)
    return CURLE_SSL_CERTPROBLEM;

  /* This will force the token to be seen as re-inserted */
  tmp = SECMOD_WaitForAnyTokenEvent(pem_module, 0, 0);
  if(tmp)
    PK11_FreeSlot(tmp);
  PK11_IsPresent(slot);

  status = PK11_Authenticate(slot, PR_TRUE, SSL_SET_OPTION(key_passwd));
  PK11_FreeSlot(slot);

  return (SECSuccess == status) ? CURLE_OK : CURLE_SSL_CERTPROBLEM;
}

static int display_error(struct connectdata *conn, PRInt32 err,
                         const char *filename)
{
  switch(err) {
  case SEC_ERROR_BAD_PASSWORD:
    failf(conn->data, "Unable to load client key: Incorrect password");
    return 1;
  case SEC_ERROR_UNKNOWN_CERT:
    failf(conn->data, "Unable to load certificate %s", filename);
    return 1;
  default:
    break;
  }
  return 0; /* The caller will print a generic error */
}

static CURLcode cert_stuff(struct connectdata *conn, int sockindex,
                           char *cert_file, char *key_file)
{
  struct Curl_easy *data = conn->data;
  CURLcode result;

  if(cert_file) {
    result = nss_load_cert(&conn->ssl[sockindex], cert_file, PR_FALSE);
    if(result) {
      const PRErrorCode err = PR_GetError();
      if(!display_error(conn, err, cert_file)) {
        const char *err_name = nss_error_to_name(err);
        failf(data, "unable to load client cert: %d (%s)", err, err_name);
      }

      return result;
    }
  }

  if(key_file || (is_file(cert_file))) {
    if(key_file)
      result = nss_load_key(conn, sockindex, key_file);
    else
      /* In case the cert file also has the key */
      result = nss_load_key(conn, sockindex, cert_file);
    if(result) {
      const PRErrorCode err = PR_GetError();
      if(!display_error(conn, err, key_file)) {
        const char *err_name = nss_error_to_name(err);
        failf(data, "unable to load client key: %d (%s)", err, err_name);
      }

      return result;
    }
  }

  return CURLE_OK;
}

static char *nss_get_password(PK11SlotInfo *slot, PRBool retry, void *arg)
{
  (void)slot; /* unused */

  if(retry || NULL == arg)
    return NULL;
  else
    return (char *)PORT_Strdup((char *)arg);
}

/* bypass the default SSL_AuthCertificate() hook in case we do not want to
 * verify peer */
static SECStatus nss_auth_cert_hook(void *arg, PRFileDesc *fd, PRBool checksig,
                                    PRBool isServer)
{
  struct connectdata *conn = (struct connectdata *)arg;

#ifdef SSL_ENABLE_OCSP_STAPLING
  if(SSL_CONN_CONFIG(verifystatus)) {
    SECStatus cacheResult;

    const SECItemArray *csa = SSL_PeerStapledOCSPResponses(fd);
    if(!csa) {
      failf(conn->data, "Invalid OCSP response");
      return SECFailure;
    }

    if(csa->len == 0) {
      failf(conn->data, "No OCSP response received");
      return SECFailure;
    }

    cacheResult = CERT_CacheOCSPResponseFromSideChannel(
      CERT_GetDefaultCertDB(), SSL_PeerCertificate(fd),
      PR_Now(), &csa->items[0], arg
    );

    if(cacheResult != SECSuccess) {
      failf(conn->data, "Invalid OCSP response");
      return cacheResult;
    }
  }
#endif

  if(!SSL_CONN_CONFIG(verifypeer)) {
    infof(conn->data, "skipping SSL peer certificate verification\n");
    return SECSuccess;
  }

  return SSL_AuthCertificate(CERT_GetDefaultCertDB(), fd, checksig, isServer);
}

/**
 * Inform the application that the handshake is complete.
 */
static void HandshakeCallback(PRFileDesc *sock, void *arg)
{
  struct connectdata *conn = (struct connectdata*) arg;
  unsigned int buflenmax = 50;
  unsigned char buf[50];
  unsigned int buflen;
  SSLNextProtoState state;

  if(!conn->bits.tls_enable_npn && !conn->bits.tls_enable_alpn) {
    return;
  }

  if(SSL_GetNextProto(sock, &state, buf, &buflen, buflenmax) == SECSuccess) {

    switch(state) {
#if NSSVERNUM >= 0x031a00 /* 3.26.0 */
    /* used by NSS internally to implement 0-RTT */
    case SSL_NEXT_PROTO_EARLY_VALUE:
      /* fall through! */
#endif
    case SSL_NEXT_PROTO_NO_SUPPORT:
    case SSL_NEXT_PROTO_NO_OVERLAP:
      infof(conn->data, "ALPN/NPN, server did not agree to a protocol\n");
      return;
#ifdef SSL_ENABLE_ALPN
    case SSL_NEXT_PROTO_SELECTED:
      infof(conn->data, "ALPN, server accepted to use %.*s\n", buflen, buf);
      break;
#endif
    case SSL_NEXT_PROTO_NEGOTIATED:
      infof(conn->data, "NPN, server accepted to use %.*s\n", buflen, buf);
      break;
    }

#ifdef USE_NGHTTP2
    if(buflen == NGHTTP2_PROTO_VERSION_ID_LEN &&
       !memcmp(NGHTTP2_PROTO_VERSION_ID, buf, NGHTTP2_PROTO_VERSION_ID_LEN)) {
      conn->negnpn = CURL_HTTP_VERSION_2;
    }
    else
#endif
    if(buflen == ALPN_HTTP_1_1_LENGTH &&
       !memcmp(ALPN_HTTP_1_1, buf, ALPN_HTTP_1_1_LENGTH)) {
      conn->negnpn = CURL_HTTP_VERSION_1_1;
    }
    Curl_multiuse_state(conn, conn->negnpn == CURL_HTTP_VERSION_2 ?
                        BUNDLE_MULTIPLEX : BUNDLE_NO_MULTIUSE);
  }
}

#if NSSVERNUM >= 0x030f04 /* 3.15.4 */
static SECStatus CanFalseStartCallback(PRFileDesc *sock, void *client_data,
                                       PRBool *canFalseStart)
{
  struct connectdata *conn = client_data;
  struct Curl_easy *data = conn->data;

  SSLChannelInfo channelInfo;
  SSLCipherSuiteInfo cipherInfo;

  SECStatus rv;
  PRBool negotiatedExtension;

  *canFalseStart = PR_FALSE;

  if(SSL_GetChannelInfo(sock, &channelInfo, sizeof(channelInfo)) != SECSuccess)
    return SECFailure;

  if(SSL_GetCipherSuiteInfo(channelInfo.cipherSuite, &cipherInfo,
                            sizeof(cipherInfo)) != SECSuccess)
    return SECFailure;

  /* Prevent version downgrade attacks from TLS 1.2, and avoid False Start for
   * TLS 1.3 and later. See https://bugzilla.mozilla.org/show_bug.cgi?id=861310
   */
  if(channelInfo.protocolVersion != SSL_LIBRARY_VERSION_TLS_1_2)
    goto end;

  /* Only allow ECDHE key exchange algorithm.
   * See https://bugzilla.mozilla.org/show_bug.cgi?id=952863 */
  if(cipherInfo.keaType != ssl_kea_ecdh)
    goto end;

  /* Prevent downgrade attacks on the symmetric cipher. We do not allow CBC
   * mode due to BEAST, POODLE, and other attacks on the MAC-then-Encrypt
   * design. See https://bugzilla.mozilla.org/show_bug.cgi?id=1109766 */
  if(cipherInfo.symCipher != ssl_calg_aes_gcm)
    goto end;

  /* Enforce ALPN or NPN to do False Start, as an indicator of server
   * compatibility. */
  rv = SSL_HandshakeNegotiatedExtension(sock, ssl_app_layer_protocol_xtn,
                                        &negotiatedExtension);
  if(rv != SECSuccess || !negotiatedExtension) {
    rv = SSL_HandshakeNegotiatedExtension(sock, ssl_next_proto_nego_xtn,
                                          &negotiatedExtension);
  }

  if(rv != SECSuccess || !negotiatedExtension)
    goto end;

  *canFalseStart = PR_TRUE;

  infof(data, "Trying TLS False Start\n");

end:
  return SECSuccess;
}
#endif

static void display_cert_info(struct Curl_easy *data,
                              CERTCertificate *cert)
{
  char *subject, *issuer, *common_name;
  PRExplodedTime printableTime;
  char timeString[256];
  PRTime notBefore, notAfter;

  subject = CERT_NameToAscii(&cert->subject);
  issuer = CERT_NameToAscii(&cert->issuer);
  common_name = CERT_GetCommonName(&cert->subject);
  infof(data, "\tsubject: %s\n", subject);

  CERT_GetCertTimes(cert, &notBefore, &notAfter);
  PR_ExplodeTime(notBefore, PR_GMTParameters, &printableTime);
  PR_FormatTime(timeString, 256, "%b %d %H:%M:%S %Y GMT", &printableTime);
  infof(data, "\tstart date: %s\n", timeString);
  PR_ExplodeTime(notAfter, PR_GMTParameters, &printableTime);
  PR_FormatTime(timeString, 256, "%b %d %H:%M:%S %Y GMT", &printableTime);
  infof(data, "\texpire date: %s\n", timeString);
  infof(data, "\tcommon name: %s\n", common_name);
  infof(data, "\tissuer: %s\n", issuer);

  PR_Free(subject);
  PR_Free(issuer);
  PR_Free(common_name);
}

static CURLcode display_conn_info(struct connectdata *conn, PRFileDesc *sock)
{
  CURLcode result = CURLE_OK;
  SSLChannelInfo channel;
  SSLCipherSuiteInfo suite;
  CERTCertificate *cert;
  CERTCertificate *cert2;
  CERTCertificate *cert3;
  PRTime now;
  int i;

  if(SSL_GetChannelInfo(sock, &channel, sizeof(channel)) ==
     SECSuccess && channel.length == sizeof(channel) &&
     channel.cipherSuite) {
    if(SSL_GetCipherSuiteInfo(channel.cipherSuite,
                              &suite, sizeof(suite)) == SECSuccess) {
      infof(conn->data, "SSL connection using %s\n", suite.cipherSuiteName);
    }
  }

  cert = SSL_PeerCertificate(sock);
  if(cert) {
    infof(conn->data, "Server certificate:\n");

    if(!conn->data->set.ssl.certinfo) {
      display_cert_info(conn->data, cert);
      CERT_DestroyCertificate(cert);
    }
    else {
      /* Count certificates in chain. */
      now = PR_Now();
      i = 1;
      if(!cert->isRoot) {
        cert2 = CERT_FindCertIssuer(cert, now, certUsageSSLCA);
        while(cert2) {
          i++;
          if(cert2->isRoot) {
            CERT_DestroyCertificate(cert2);
            break;
          }
          cert3 = CERT_FindCertIssuer(cert2, now, certUsageSSLCA);
          CERT_DestroyCertificate(cert2);
          cert2 = cert3;
        }
      }

      result = Curl_ssl_init_certinfo(conn->data, i);
      if(!result) {
        for(i = 0; cert; cert = cert2) {
          result = Curl_extract_certinfo(conn, i++, (char *)cert->derCert.data,
                                         (char *)cert->derCert.data +
                                                 cert->derCert.len);
          if(result)
            break;

          if(cert->isRoot) {
            CERT_DestroyCertificate(cert);
            break;
          }

          cert2 = CERT_FindCertIssuer(cert, now, certUsageSSLCA);
          CERT_DestroyCertificate(cert);
        }
      }
    }
  }

  return result;
}

static SECStatus BadCertHandler(void *arg, PRFileDesc *sock)
{
  struct connectdata *conn = (struct connectdata *)arg;
  struct Curl_easy *data = conn->data;
  PRErrorCode err = PR_GetError();
  CERTCertificate *cert;

  /* remember the cert verification result */
  if(SSL_IS_PROXY())
    data->set.proxy_ssl.certverifyresult = err;
  else
    data->set.ssl.certverifyresult = err;

  if(err == SSL_ERROR_BAD_CERT_DOMAIN && !SSL_CONN_CONFIG(verifyhost))
    /* we are asked not to verify the host name */
    return SECSuccess;

  /* print only info about the cert, the error is printed off the callback */
  cert = SSL_PeerCertificate(sock);
  if(cert) {
    infof(data, "Server certificate:\n");
    display_cert_info(data, cert);
    CERT_DestroyCertificate(cert);
  }

  return SECFailure;
}

/**
 *
 * Check that the Peer certificate's issuer certificate matches the one found
 * by issuer_nickname.  This is not exactly the way OpenSSL and GNU TLS do the
 * issuer check, so we provide comments that mimic the OpenSSL
 * X509_check_issued function (in x509v3/v3_purp.c)
 */
static SECStatus check_issuer_cert(PRFileDesc *sock,
                                   char *issuer_nickname)
{
  CERTCertificate *cert, *cert_issuer, *issuer;
  SECStatus res = SECSuccess;
  void *proto_win = NULL;

  cert = SSL_PeerCertificate(sock);
  cert_issuer = CERT_FindCertIssuer(cert, PR_Now(), certUsageObjectSigner);

  proto_win = SSL_RevealPinArg(sock);
  issuer = PK11_FindCertFromNickname(issuer_nickname, proto_win);

  if((!cert_issuer) || (!issuer))
    res = SECFailure;
  else if(SECITEM_CompareItem(&cert_issuer->derCert,
                              &issuer->derCert) != SECEqual)
    res = SECFailure;

  CERT_DestroyCertificate(cert);
  CERT_DestroyCertificate(issuer);
  CERT_DestroyCertificate(cert_issuer);
  return res;
}

static CURLcode cmp_peer_pubkey(struct ssl_connect_data *connssl,
                                const char *pinnedpubkey)
{
  CURLcode result = CURLE_SSL_PINNEDPUBKEYNOTMATCH;
  struct Curl_easy *data = BACKEND->data;
  CERTCertificate *cert;

  if(!pinnedpubkey)
    /* no pinned public key specified */
    return CURLE_OK;

  /* get peer certificate */
  cert = SSL_PeerCertificate(BACKEND->handle);
  if(cert) {
    /* extract public key from peer certificate */
    SECKEYPublicKey *pubkey = CERT_ExtractPublicKey(cert);
    if(pubkey) {
      /* encode the public key as DER */
      SECItem *cert_der = PK11_DEREncodePublicKey(pubkey);
      if(cert_der) {
        /* compare the public key with the pinned public key */
        result = Curl_pin_peer_pubkey(data, pinnedpubkey, cert_der->data,
                                      cert_der->len);
        SECITEM_FreeItem(cert_der, PR_TRUE);
      }
      SECKEY_DestroyPublicKey(pubkey);
    }
    CERT_DestroyCertificate(cert);
  }

  /* report the resulting status */
  switch(result) {
  case CURLE_OK:
    infof(data, "pinned public key verified successfully!\n");
    break;
  case CURLE_SSL_PINNEDPUBKEYNOTMATCH:
    failf(data, "failed to verify pinned public key");
    break;
  default:
    /* OOM, etc. */
    break;
  }

  return result;
}

/**
 *
 * Callback to pick the SSL client certificate.
 */
static SECStatus SelectClientCert(void *arg, PRFileDesc *sock,
                                  struct CERTDistNamesStr *caNames,
                                  struct CERTCertificateStr **pRetCert,
                                  struct SECKEYPrivateKeyStr **pRetKey)
{
  struct ssl_connect_data *connssl = (struct ssl_connect_data *)arg;
  struct Curl_easy *data = BACKEND->data;
  const char *nickname = BACKEND->client_nickname;
  static const char pem_slotname[] = "PEM Token #1";

  if(BACKEND->obj_clicert) {
    /* use the cert/key provided by PEM reader */
    SECItem cert_der = { 0, NULL, 0 };
    void *proto_win = SSL_RevealPinArg(sock);
    struct CERTCertificateStr *cert;
    struct SECKEYPrivateKeyStr *key;

    PK11SlotInfo *slot = nss_find_slot_by_name(pem_slotname);
    if(NULL == slot) {
      failf(data, "NSS: PK11 slot not found: %s", pem_slotname);
      return SECFailure;
    }

    if(PK11_ReadRawAttribute(PK11_TypeGeneric, BACKEND->obj_clicert, CKA_VALUE,
                             &cert_der) != SECSuccess) {
      failf(data, "NSS: CKA_VALUE not found in PK11 generic object");
      PK11_FreeSlot(slot);
      return SECFailure;
    }

    cert = PK11_FindCertFromDERCertItem(slot, &cert_der, proto_win);
    SECITEM_FreeItem(&cert_der, PR_FALSE);
    if(NULL == cert) {
      failf(data, "NSS: client certificate from file not found");
      PK11_FreeSlot(slot);
      return SECFailure;
    }

    key = PK11_FindPrivateKeyFromCert(slot, cert, NULL);
    PK11_FreeSlot(slot);
    if(NULL == key) {
      failf(data, "NSS: private key from file not found");
      CERT_DestroyCertificate(cert);
      return SECFailure;
    }

    infof(data, "NSS: client certificate from file\n");
    display_cert_info(data, cert);

    *pRetCert = cert;
    *pRetKey = key;
    return SECSuccess;
  }

  /* use the default NSS hook */
  if(SECSuccess != NSS_GetClientAuthData((void *)nickname, sock, caNames,
                                          pRetCert, pRetKey)
      || NULL == *pRetCert) {

    if(NULL == nickname)
      failf(data, "NSS: client certificate not found (nickname not "
            "specified)");
    else
      failf(data, "NSS: client certificate not found: %s", nickname);

    return SECFailure;
  }

  /* get certificate nickname if any */
  nickname = (*pRetCert)->nickname;
  if(NULL == nickname)
    nickname = "[unknown]";

  if(!strncmp(nickname, pem_slotname, sizeof(pem_slotname) - 1U)) {
    failf(data, "NSS: refusing previously loaded certificate from file: %s",
          nickname);
    return SECFailure;
  }

  if(NULL == *pRetKey) {
    failf(data, "NSS: private key not found for certificate: %s", nickname);
    return SECFailure;
  }

  infof(data, "NSS: using client certificate: %s\n", nickname);
  display_cert_info(data, *pRetCert);
  return SECSuccess;
}

/* update blocking direction in case of PR_WOULD_BLOCK_ERROR */
static void nss_update_connecting_state(ssl_connect_state state, void *secret)
{
  struct ssl_connect_data *connssl = (struct ssl_connect_data *)secret;
  if(PR_GetError() != PR_WOULD_BLOCK_ERROR)
    /* an unrelated error is passing by */
    return;

  switch(connssl->connecting_state) {
  case ssl_connect_2:
  case ssl_connect_2_reading:
  case ssl_connect_2_writing:
    break;
  default:
    /* we are not called from an SSL handshake */
    return;
  }

  /* update the state accordingly */
  connssl->connecting_state = state;
}

/* recv() wrapper we use to detect blocking direction during SSL handshake */
static PRInt32 nspr_io_recv(PRFileDesc *fd, void *buf, PRInt32 amount,
                            PRIntn flags, PRIntervalTime timeout)
{
  const PRRecvFN recv_fn = fd->lower->methods->recv;
  const PRInt32 rv = recv_fn(fd->lower, buf, amount, flags, timeout);
  if(rv < 0)
    /* check for PR_WOULD_BLOCK_ERROR and update blocking direction */
    nss_update_connecting_state(ssl_connect_2_reading, fd->secret);
  return rv;
}

/* send() wrapper we use to detect blocking direction during SSL handshake */
static PRInt32 nspr_io_send(PRFileDesc *fd, const void *buf, PRInt32 amount,
                            PRIntn flags, PRIntervalTime timeout)
{
  const PRSendFN send_fn = fd->lower->methods->send;
  const PRInt32 rv = send_fn(fd->lower, buf, amount, flags, timeout);
  if(rv < 0)
    /* check for PR_WOULD_BLOCK_ERROR and update blocking direction */
    nss_update_connecting_state(ssl_connect_2_writing, fd->secret);
  return rv;
}

/* close() wrapper to avoid assertion failure due to fd->secret != NULL */
static PRStatus nspr_io_close(PRFileDesc *fd)
{
  const PRCloseFN close_fn = PR_GetDefaultIOMethods()->close;
  fd->secret = NULL;
  return close_fn(fd);
}

/* load a PKCS #11 module */
static CURLcode nss_load_module(SECMODModule **pmod, const char *library,
                                const char *name)
{
  char *config_string;
  SECMODModule *module = *pmod;
  if(module)
    /* already loaded */
    return CURLE_OK;

  config_string = aprintf("library=%s name=%s", library, name);
  if(!config_string)
    return CURLE_OUT_OF_MEMORY;

  module = SECMOD_LoadUserModule(config_string, NULL, PR_FALSE);
  free(config_string);

  if(module && module->loaded) {
    /* loaded successfully */
    *pmod = module;
    return CURLE_OK;
  }

  if(module)
    SECMOD_DestroyModule(module);
  return CURLE_FAILED_INIT;
}

/* unload a PKCS #11 module */
static void nss_unload_module(SECMODModule **pmod)
{
  SECMODModule *module = *pmod;
  if(!module)
    /* not loaded */
    return;

  if(SECMOD_UnloadUserModule(module) != SECSuccess)
    /* unload failed */
    return;

  SECMOD_DestroyModule(module);
  *pmod = NULL;
}

/* data might be NULL */
static CURLcode nss_init_core(struct Curl_easy *data, const char *cert_dir)
{
  NSSInitParameters initparams;
  PRErrorCode err;
  const char *err_name;

  if(nss_context != NULL)
    return CURLE_OK;

  memset((void *) &initparams, '\0', sizeof(initparams));
  initparams.length = sizeof(initparams);

  if(cert_dir) {
    char *certpath = aprintf("sql:%s", cert_dir);
    if(!certpath)
      return CURLE_OUT_OF_MEMORY;

    infof(data, "Initializing NSS with certpath: %s\n", certpath);
    nss_context = NSS_InitContext(certpath, "", "", "", &initparams,
            NSS_INIT_READONLY | NSS_INIT_PK11RELOAD);
    free(certpath);

    if(nss_context != NULL)
      return CURLE_OK;

    err = PR_GetError();
    err_name = nss_error_to_name(err);
    infof(data, "Unable to initialize NSS database: %d (%s)\n", err, err_name);
  }

  infof(data, "Initializing NSS with certpath: none\n");
  nss_context = NSS_InitContext("", "", "", "", &initparams, NSS_INIT_READONLY
         | NSS_INIT_NOCERTDB   | NSS_INIT_NOMODDB       | NSS_INIT_FORCEOPEN
         | NSS_INIT_NOROOTINIT | NSS_INIT_OPTIMIZESPACE | NSS_INIT_PK11RELOAD);
  if(nss_context != NULL)
    return CURLE_OK;

  err = PR_GetError();
  err_name = nss_error_to_name(err);
  failf(data, "Unable to initialize NSS: %d (%s)", err, err_name);
  return CURLE_SSL_CACERT_BADFILE;
}

/* data might be NULL */
static CURLcode nss_init(struct Curl_easy *data)
{
  char *cert_dir;
  struct_stat st;
  CURLcode result;

  if(initialized)
    return CURLE_OK;

  /* list of all CRL items we need to destroy in Curl_nss_cleanup() */
  Curl_llist_init(&nss_crl_list, nss_destroy_crl_item);

  /* First we check if $SSL_DIR points to a valid dir */
  cert_dir = getenv("SSL_DIR");
  if(cert_dir) {
    if((stat(cert_dir, &st) != 0) ||
        (!S_ISDIR(st.st_mode))) {
      cert_dir = NULL;
    }
  }

  /* Now we check if the default location is a valid dir */
  if(!cert_dir) {
    if((stat(SSL_DIR, &st) == 0) &&
        (S_ISDIR(st.st_mode))) {
      cert_dir = (char *)SSL_DIR;
    }
  }

  if(nspr_io_identity == PR_INVALID_IO_LAYER) {
    /* allocate an identity for our own NSPR I/O layer */
    nspr_io_identity = PR_GetUniqueIdentity("libcurl");
    if(nspr_io_identity == PR_INVALID_IO_LAYER)
      return CURLE_OUT_OF_MEMORY;

    /* the default methods just call down to the lower I/O layer */
    memcpy(&nspr_io_methods, PR_GetDefaultIOMethods(),
           sizeof(nspr_io_methods));

    /* override certain methods in the table by our wrappers */
    nspr_io_methods.recv  = nspr_io_recv;
    nspr_io_methods.send  = nspr_io_send;
    nspr_io_methods.close = nspr_io_close;
  }

  result = nss_init_core(data, cert_dir);
  if(result)
    return result;

  if(!any_cipher_enabled())
    NSS_SetDomesticPolicy();

  initialized = 1;

  return CURLE_OK;
}

/**
 * Global SSL init
 *
 * @retval 0 error initializing SSL
 * @retval 1 SSL initialized successfully
 */
static int Curl_nss_init(void)
{
  /* curl_global_init() is not thread-safe so this test is ok */
  if(nss_initlock == NULL) {
    PR_Init(PR_USER_THREAD, PR_PRIORITY_NORMAL, 256);
    nss_initlock = PR_NewLock();
    nss_crllock = PR_NewLock();
    nss_findslot_lock = PR_NewLock();
    nss_trustload_lock = PR_NewLock();
  }

  /* We will actually initialize NSS later */

  return 1;
}

/* data might be NULL */
CURLcode Curl_nss_force_init(struct Curl_easy *data)
{
  CURLcode result;
  if(!nss_initlock) {
    if(data)
      failf(data, "unable to initialize NSS, curl_global_init() should have "
                  "been called with CURL_GLOBAL_SSL or CURL_GLOBAL_ALL");
    return CURLE_FAILED_INIT;
  }

  PR_Lock(nss_initlock);
  result = nss_init(data);
  PR_Unlock(nss_initlock);

  return result;
}

/* Global cleanup */
static void Curl_nss_cleanup(void)
{
  /* This function isn't required to be threadsafe and this is only done
   * as a safety feature.
   */
  PR_Lock(nss_initlock);
  if(initialized) {
    /* Free references to client certificates held in the SSL session cache.
     * Omitting this hampers destruction of the security module owning
     * the certificates. */
    SSL_ClearSessionCache();

    nss_unload_module(&pem_module);
    nss_unload_module(&trust_module);
    NSS_ShutdownContext(nss_context);
    nss_context = NULL;
  }

  /* destroy all CRL items */
  Curl_llist_destroy(&nss_crl_list, NULL);

  PR_Unlock(nss_initlock);

  PR_DestroyLock(nss_initlock);
  PR_DestroyLock(nss_crllock);
  PR_DestroyLock(nss_findslot_lock);
  PR_DestroyLock(nss_trustload_lock);
  nss_initlock = NULL;

  initialized = 0;
}

/*
 * This function uses SSL_peek to determine connection status.
 *
 * Return codes:
 *     1 means the connection is still in place
 *     0 means the connection has been closed
 *    -1 means the connection status is unknown
 */
static int Curl_nss_check_cxn(struct connectdata *conn)
{
  struct ssl_connect_data *connssl = &conn->ssl[FIRSTSOCKET];
  int rc;
  char buf;

  rc =
    PR_Recv(BACKEND->handle, (void *)&buf, 1, PR_MSG_PEEK,
            PR_SecondsToInterval(1));
  if(rc > 0)
    return 1; /* connection still in place */

  if(rc == 0)
    return 0; /* connection has been closed */

  return -1;  /* connection status unknown */
}

static void nss_close(struct ssl_connect_data *connssl)
{
  /* before the cleanup, check whether we are using a client certificate */
  const bool client_cert = (BACKEND->client_nickname != NULL)
    || (BACKEND->obj_clicert != NULL);

  free(BACKEND->client_nickname);
  BACKEND->client_nickname = NULL;

  /* destroy all NSS objects in order to avoid failure of NSS shutdown */
  Curl_llist_destroy(&BACKEND->obj_list, NULL);
  BACKEND->obj_clicert = NULL;

  if(BACKEND->handle) {
    if(client_cert)
      /* A server might require different authentication based on the
       * particular path being requested by the client.  To support this
       * scenario, we must ensure that a connection will never reuse the
       * authentication data from a previous connection. */
      SSL_InvalidateSession(BACKEND->handle);

    PR_Close(BACKEND->handle);
    BACKEND->handle = NULL;
  }
}

/*
 * This function is called when an SSL connection is closed.
 */
static void Curl_nss_close(struct connectdata *conn, int sockindex)
{
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  struct ssl_connect_data *connssl_proxy = &conn->proxy_ssl[sockindex];

  if(BACKEND->handle || connssl_proxy->backend->handle) {
    /* NSS closes the socket we previously handed to it, so we must mark it
       as closed to avoid double close */
    fake_sclose(conn->sock[sockindex]);
    conn->sock[sockindex] = CURL_SOCKET_BAD;
  }

  if(BACKEND->handle)
    /* nss_close(connssl) will transitively close also
       connssl_proxy->backend->handle if both are used. Clear it to avoid
       a double close leading to crash. */
    connssl_proxy->backend->handle = NULL;

  nss_close(connssl);
  nss_close(connssl_proxy);
}

/* return true if NSS can provide error code (and possibly msg) for the
   error */
static bool is_nss_error(CURLcode err)
{
  switch(err) {
  case CURLE_PEER_FAILED_VERIFICATION:
  case CURLE_SSL_CERTPROBLEM:
  case CURLE_SSL_CONNECT_ERROR:
  case CURLE_SSL_ISSUER_ERROR:
    return true;

  default:
    return false;
  }
}

/* return true if the given error code is related to a client certificate */
static bool is_cc_error(PRInt32 err)
{
  switch(err) {
  case SSL_ERROR_BAD_CERT_ALERT:
  case SSL_ERROR_EXPIRED_CERT_ALERT:
  case SSL_ERROR_REVOKED_CERT_ALERT:
    return true;

  default:
    return false;
  }
}

static Curl_recv nss_recv;
static Curl_send nss_send;

static CURLcode nss_load_ca_certificates(struct connectdata *conn,
                                         int sockindex)
{
  struct Curl_easy *data = conn->data;
  const char *cafile = SSL_CONN_CONFIG(CAfile);
  const char *capath = SSL_CONN_CONFIG(CApath);
  bool use_trust_module;
  CURLcode result = CURLE_OK;

  /* treat empty string as unset */
  if(cafile && !cafile[0])
    cafile = NULL;
  if(capath && !capath[0])
    capath = NULL;

  infof(data, "  CAfile: %s\n  CApath: %s\n",
      cafile ? cafile : "none",
      capath ? capath : "none");

  /* load libnssckbi.so if no other trust roots were specified */
  use_trust_module = !cafile && !capath;

  PR_Lock(nss_trustload_lock);
  if(use_trust_module && !trust_module) {
    /* libnssckbi.so needed but not yet loaded --> load it! */
    result = nss_load_module(&trust_module, trust_library, "trust");
    infof(data, "%s %s\n", (result) ? "failed to load" : "loaded",
          trust_library);
    if(result == CURLE_FAILED_INIT)
      /* If libnssckbi.so is not available (or fails to load), one can still
         use CA certificates stored in NSS database.  Ignore the failure. */
      result = CURLE_OK;
  }
  else if(!use_trust_module && trust_module) {
    /* libnssckbi.so not needed but already loaded --> unload it! */
    infof(data, "unloading %s\n", trust_library);
    nss_unload_module(&trust_module);
  }
  PR_Unlock(nss_trustload_lock);

  if(cafile)
    result = nss_load_cert(&conn->ssl[sockindex], cafile, PR_TRUE);

  if(result)
    return result;

  if(capath) {
    struct_stat st;
    if(stat(capath, &st) == -1)
      return CURLE_SSL_CACERT_BADFILE;

    if(S_ISDIR(st.st_mode)) {
      PRDirEntry *entry;
      PRDir *dir = PR_OpenDir(capath);
      if(!dir)
        return CURLE_SSL_CACERT_BADFILE;

      while((entry = PR_ReadDir(dir, PR_SKIP_BOTH | PR_SKIP_HIDDEN))) {
        char *fullpath = aprintf("%s/%s", capath, entry->name);
        if(!fullpath) {
          PR_CloseDir(dir);
          return CURLE_OUT_OF_MEMORY;
        }

        if(CURLE_OK != nss_load_cert(&conn->ssl[sockindex], fullpath, PR_TRUE))
          /* This is purposefully tolerant of errors so non-PEM files can
           * be in the same directory */
          infof(data, "failed to load '%s' from CURLOPT_CAPATH\n", fullpath);

        free(fullpath);
      }

      PR_CloseDir(dir);
    }
    else
      infof(data, "warning: CURLOPT_CAPATH not a directory (%s)\n", capath);
  }

  return CURLE_OK;
}

static CURLcode nss_sslver_from_curl(PRUint16 *nssver, long version)
{
  switch(version) {
  case CURL_SSLVERSION_SSLv2:
    *nssver = SSL_LIBRARY_VERSION_2;
    return CURLE_OK;

  case CURL_SSLVERSION_SSLv3:
    *nssver = SSL_LIBRARY_VERSION_3_0;
    return CURLE_OK;

  case CURL_SSLVERSION_TLSv1_0:
    *nssver = SSL_LIBRARY_VERSION_TLS_1_0;
    return CURLE_OK;

  case CURL_SSLVERSION_TLSv1_1:
#ifdef SSL_LIBRARY_VERSION_TLS_1_1
    *nssver = SSL_LIBRARY_VERSION_TLS_1_1;
    return CURLE_OK;
#else
    return CURLE_SSL_CONNECT_ERROR;
#endif

  case CURL_SSLVERSION_TLSv1_2:
#ifdef SSL_LIBRARY_VERSION_TLS_1_2
    *nssver = SSL_LIBRARY_VERSION_TLS_1_2;
    return CURLE_OK;
#else
    return CURLE_SSL_CONNECT_ERROR;
#endif

  case CURL_SSLVERSION_TLSv1_3:
#ifdef SSL_LIBRARY_VERSION_TLS_1_3
    *nssver = SSL_LIBRARY_VERSION_TLS_1_3;
    return CURLE_OK;
#else
    return CURLE_SSL_CONNECT_ERROR;
#endif

  default:
    return CURLE_SSL_CONNECT_ERROR;
  }
}

static CURLcode nss_init_sslver(SSLVersionRange *sslver,
                                struct Curl_easy *data,
                                struct connectdata *conn)
{
  CURLcode result;
  const long min = SSL_CONN_CONFIG(version);
  const long max = SSL_CONN_CONFIG(version_max);

  /* map CURL_SSLVERSION_DEFAULT to NSS default */
  if(min == CURL_SSLVERSION_DEFAULT || max == CURL_SSLVERSION_MAX_DEFAULT) {
    /* map CURL_SSLVERSION_DEFAULT to NSS default */
    if(SSL_VersionRangeGetDefault(ssl_variant_stream, sslver) != SECSuccess)
      return CURLE_SSL_CONNECT_ERROR;
    /* ... but make sure we use at least TLSv1.0 according to libcurl API */
    if(sslver->min < SSL_LIBRARY_VERSION_TLS_1_0)
      sslver->min = SSL_LIBRARY_VERSION_TLS_1_0;
  }

  switch(min) {
  case CURL_SSLVERSION_TLSv1:
  case CURL_SSLVERSION_DEFAULT:
    break;
  default:
    result = nss_sslver_from_curl(&sslver->min, min);
    if(result) {
      failf(data, "unsupported min version passed via CURLOPT_SSLVERSION");
      return result;
    }
  }

  switch(max) {
  case CURL_SSLVERSION_MAX_NONE:
  case CURL_SSLVERSION_MAX_DEFAULT:
    break;
  default:
    result = nss_sslver_from_curl(&sslver->max, max >> 16);
    if(result) {
      failf(data, "unsupported max version passed via CURLOPT_SSLVERSION");
      return result;
    }
  }

  return CURLE_OK;
}

static CURLcode nss_fail_connect(struct ssl_connect_data *connssl,
                                 struct Curl_easy *data,
                                 CURLcode curlerr)
{
  PRErrorCode err = 0;

  if(is_nss_error(curlerr)) {
    /* read NSPR error code */
    err = PR_GetError();
    if(is_cc_error(err))
      curlerr = CURLE_SSL_CERTPROBLEM;

    /* print the error number and error string */
    infof(data, "NSS error %d (%s)\n", err, nss_error_to_name(err));

    /* print a human-readable message describing the error if available */
    nss_print_error_message(data, err);
  }

  /* cleanup on connection failure */
  Curl_llist_destroy(&BACKEND->obj_list, NULL);

  return curlerr;
}

/* Switch the SSL socket into blocking or non-blocking mode. */
static CURLcode nss_set_blocking(struct ssl_connect_data *connssl,
                                 struct Curl_easy *data,
                                 bool blocking)
{
  static PRSocketOptionData sock_opt;
  sock_opt.option = PR_SockOpt_Nonblocking;
  sock_opt.value.non_blocking = !blocking;

  if(PR_SetSocketOption(BACKEND->handle, &sock_opt) != PR_SUCCESS)
    return nss_fail_connect(connssl, data, CURLE_SSL_CONNECT_ERROR);

  return CURLE_OK;
}

static CURLcode nss_setup_connect(struct connectdata *conn, int sockindex)
{
  PRFileDesc *model = NULL;
  PRFileDesc *nspr_io = NULL;
  PRFileDesc *nspr_io_stub = NULL;
  PRBool ssl_no_cache;
  PRBool ssl_cbc_random_iv;
  struct Curl_easy *data = conn->data;
  curl_socket_t sockfd = conn->sock[sockindex];
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  CURLcode result;
  bool second_layer = FALSE;
  SSLVersionRange sslver_supported;

  SSLVersionRange sslver = {
    SSL_LIBRARY_VERSION_TLS_1_0,  /* min */
#ifdef SSL_LIBRARY_VERSION_TLS_1_3
    SSL_LIBRARY_VERSION_TLS_1_3   /* max */
#elif defined SSL_LIBRARY_VERSION_TLS_1_2
    SSL_LIBRARY_VERSION_TLS_1_2
#elif defined SSL_LIBRARY_VERSION_TLS_1_1
    SSL_LIBRARY_VERSION_TLS_1_1
#else
    SSL_LIBRARY_VERSION_TLS_1_0
#endif
  };

  BACKEND->data = data;

  /* list of all NSS objects we need to destroy in Curl_nss_close() */
  Curl_llist_init(&BACKEND->obj_list, nss_destroy_object);

  PR_Lock(nss_initlock);
  result = nss_init(conn->data);
  if(result) {
    PR_Unlock(nss_initlock);
    goto error;
  }

  PK11_SetPasswordFunc(nss_get_password);

  result = nss_load_module(&pem_module, pem_library, "PEM");
  PR_Unlock(nss_initlock);
  if(result == CURLE_FAILED_INIT)
    infof(data, "WARNING: failed to load NSS PEM library %s. Using "
                "OpenSSL PEM certificates will not work.\n", pem_library);
  else if(result)
    goto error;

  result = CURLE_SSL_CONNECT_ERROR;

  model = PR_NewTCPSocket();
  if(!model)
    goto error;
  model = SSL_ImportFD(NULL, model);

  if(SSL_OptionSet(model, SSL_SECURITY, PR_TRUE) != SECSuccess)
    goto error;
  if(SSL_OptionSet(model, SSL_HANDSHAKE_AS_SERVER, PR_FALSE) != SECSuccess)
    goto error;
  if(SSL_OptionSet(model, SSL_HANDSHAKE_AS_CLIENT, PR_TRUE) != SECSuccess)
    goto error;

  /* do not use SSL cache if disabled or we are not going to verify peer */
  ssl_no_cache = (SSL_SET_OPTION(primary.sessionid)
                  && SSL_CONN_CONFIG(verifypeer)) ? PR_FALSE : PR_TRUE;
  if(SSL_OptionSet(model, SSL_NO_CACHE, ssl_no_cache) != SECSuccess)
    goto error;

  /* enable/disable the requested SSL version(s) */
  if(nss_init_sslver(&sslver, data, conn) != CURLE_OK)
    goto error;
  if(SSL_VersionRangeGetSupported(ssl_variant_stream,
                                  &sslver_supported) != SECSuccess)
    goto error;
  if(sslver_supported.max < sslver.max && sslver_supported.max >= sslver.min) {
    char *sslver_req_str, *sslver_supp_str;
    sslver_req_str = nss_sslver_to_name(sslver.max);
    sslver_supp_str = nss_sslver_to_name(sslver_supported.max);
    if(sslver_req_str && sslver_supp_str)
      infof(data, "Falling back from %s to max supported SSL version (%s)\n",
                  sslver_req_str, sslver_supp_str);
    free(sslver_req_str);
    free(sslver_supp_str);
    sslver.max = sslver_supported.max;
  }
  if(SSL_VersionRangeSet(model, &sslver) != SECSuccess)
    goto error;

  ssl_cbc_random_iv = !SSL_SET_OPTION(enable_beast);
#ifdef SSL_CBC_RANDOM_IV
  /* unless the user explicitly asks to allow the protocol vulnerability, we
     use the work-around */
  if(SSL_OptionSet(model, SSL_CBC_RANDOM_IV, ssl_cbc_random_iv) != SECSuccess)
    infof(data, "warning: failed to set SSL_CBC_RANDOM_IV = %d\n",
          ssl_cbc_random_iv);
#else
  if(ssl_cbc_random_iv)
    infof(data, "warning: support for SSL_CBC_RANDOM_IV not compiled in\n");
#endif

  if(SSL_CONN_CONFIG(cipher_list)) {
    if(set_ciphers(data, model, SSL_CONN_CONFIG(cipher_list)) != SECSuccess) {
      result = CURLE_SSL_CIPHER;
      goto error;
    }
  }

  if(!SSL_CONN_CONFIG(verifypeer) && SSL_CONN_CONFIG(verifyhost))
    infof(data, "warning: ignoring value of ssl.verifyhost\n");

  /* bypass the default SSL_AuthCertificate() hook in case we do not want to
   * verify peer */
  if(SSL_AuthCertificateHook(model, nss_auth_cert_hook, conn) != SECSuccess)
    goto error;

  /* not checked yet */
  if(SSL_IS_PROXY())
    data->set.proxy_ssl.certverifyresult = 0;
  else
    data->set.ssl.certverifyresult = 0;

  if(SSL_BadCertHook(model, BadCertHandler, conn) != SECSuccess)
    goto error;

  if(SSL_HandshakeCallback(model, HandshakeCallback, conn) != SECSuccess)
    goto error;

  {
    const CURLcode rv = nss_load_ca_certificates(conn, sockindex);
    if((rv == CURLE_SSL_CACERT_BADFILE) && !SSL_CONN_CONFIG(verifypeer))
      /* not a fatal error because we are not going to verify the peer */
      infof(data, "warning: CA certificates failed to load\n");
    else if(rv) {
      result = rv;
      goto error;
    }
  }

  if(SSL_SET_OPTION(CRLfile)) {
    const CURLcode rv = nss_load_crl(SSL_SET_OPTION(CRLfile));
    if(rv) {
      result = rv;
      goto error;
    }
    infof(data, "  CRLfile: %s\n", SSL_SET_OPTION(CRLfile));
  }

  if(SSL_SET_OPTION(cert)) {
    char *nickname = dup_nickname(data, SSL_SET_OPTION(cert));
    if(nickname) {
      /* we are not going to use libnsspem.so to read the client cert */
      BACKEND->obj_clicert = NULL;
    }
    else {
      CURLcode rv = cert_stuff(conn, sockindex, SSL_SET_OPTION(cert),
                               SSL_SET_OPTION(key));
      if(rv) {
        /* failf() is already done in cert_stuff() */
        result = rv;
        goto error;
      }
    }

    /* store the nickname for SelectClientCert() called during handshake */
    BACKEND->client_nickname = nickname;
  }
  else
    BACKEND->client_nickname = NULL;

  if(SSL_GetClientAuthDataHook(model, SelectClientCert,
                               (void *)connssl) != SECSuccess) {
    result = CURLE_SSL_CERTPROBLEM;
    goto error;
  }

  if(conn->proxy_ssl[sockindex].use) {
    DEBUGASSERT(ssl_connection_complete == conn->proxy_ssl[sockindex].state);
    DEBUGASSERT(conn->proxy_ssl[sockindex].backend->handle != NULL);
    nspr_io = conn->proxy_ssl[sockindex].backend->handle;
    second_layer = TRUE;
  }
  else {
    /* wrap OS file descriptor by NSPR's file descriptor abstraction */
    nspr_io = PR_ImportTCPSocket(sockfd);
    if(!nspr_io)
      goto error;
  }

  /* create our own NSPR I/O layer */
  nspr_io_stub = PR_CreateIOLayerStub(nspr_io_identity, &nspr_io_methods);
  if(!nspr_io_stub) {
    if(!second_layer)
      PR_Close(nspr_io);
    goto error;
  }

  /* make the per-connection data accessible from NSPR I/O callbacks */
  nspr_io_stub->secret = (void *)connssl;

  /* push our new layer to the NSPR I/O stack */
  if(PR_PushIOLayer(nspr_io, PR_TOP_IO_LAYER, nspr_io_stub) != PR_SUCCESS) {
    if(!second_layer)
      PR_Close(nspr_io);
    PR_Close(nspr_io_stub);
    goto error;
  }

  /* import our model socket onto the current I/O stack */
  BACKEND->handle = SSL_ImportFD(model, nspr_io);
  if(!BACKEND->handle) {
    if(!second_layer)
      PR_Close(nspr_io);
    goto error;
  }

  PR_Close(model); /* We don't need this any more */
  model = NULL;

  /* This is the password associated with the cert that we're using */
  if(SSL_SET_OPTION(key_passwd)) {
    SSL_SetPKCS11PinArg(BACKEND->handle, SSL_SET_OPTION(key_passwd));
  }

#ifdef SSL_ENABLE_OCSP_STAPLING
  if(SSL_CONN_CONFIG(verifystatus)) {
    if(SSL_OptionSet(BACKEND->handle, SSL_ENABLE_OCSP_STAPLING, PR_TRUE)
        != SECSuccess)
      goto error;
  }
#endif

#ifdef SSL_ENABLE_NPN
  if(SSL_OptionSet(BACKEND->handle, SSL_ENABLE_NPN, conn->bits.tls_enable_npn
                   ? PR_TRUE : PR_FALSE) != SECSuccess)
    goto error;
#endif

#ifdef SSL_ENABLE_ALPN
  if(SSL_OptionSet(BACKEND->handle, SSL_ENABLE_ALPN, conn->bits.tls_enable_alpn
                   ? PR_TRUE : PR_FALSE) != SECSuccess)
    goto error;
#endif

#if NSSVERNUM >= 0x030f04 /* 3.15.4 */
  if(data->set.ssl.falsestart) {
    if(SSL_OptionSet(BACKEND->handle, SSL_ENABLE_FALSE_START, PR_TRUE)
        != SECSuccess)
      goto error;

    if(SSL_SetCanFalseStartCallback(BACKEND->handle, CanFalseStartCallback,
        conn) != SECSuccess)
      goto error;
  }
#endif

#if defined(SSL_ENABLE_NPN) || defined(SSL_ENABLE_ALPN)
  if(conn->bits.tls_enable_npn || conn->bits.tls_enable_alpn) {
    int cur = 0;
    unsigned char protocols[128];

#ifdef USE_NGHTTP2
    if(data->set.httpversion >= CURL_HTTP_VERSION_2 &&
       (!SSL_IS_PROXY() || !conn->bits.tunnel_proxy)) {
      protocols[cur++] = NGHTTP2_PROTO_VERSION_ID_LEN;
      memcpy(&protocols[cur], NGHTTP2_PROTO_VERSION_ID,
          NGHTTP2_PROTO_VERSION_ID_LEN);
      cur += NGHTTP2_PROTO_VERSION_ID_LEN;
    }
#endif
    protocols[cur++] = ALPN_HTTP_1_1_LENGTH;
    memcpy(&protocols[cur], ALPN_HTTP_1_1, ALPN_HTTP_1_1_LENGTH);
    cur += ALPN_HTTP_1_1_LENGTH;

    if(SSL_SetNextProtoNego(BACKEND->handle, protocols, cur) != SECSuccess)
      goto error;
  }
#endif


  /* Force handshake on next I/O */
  if(SSL_ResetHandshake(BACKEND->handle, /* asServer */ PR_FALSE)
      != SECSuccess)
    goto error;

  /* propagate hostname to the TLS layer */
  if(SSL_SetURL(BACKEND->handle, SSL_IS_PROXY() ? conn->http_proxy.host.name :
                conn->host.name) != SECSuccess)
    goto error;

  /* prevent NSS from re-using the session for a different hostname */
  if(SSL_SetSockPeerID(BACKEND->handle, SSL_IS_PROXY() ?
                       conn->http_proxy.host.name : conn->host.name)
     != SECSuccess)
    goto error;

  return CURLE_OK;

error:
  if(model)
    PR_Close(model);

  return nss_fail_connect(connssl, data, result);
}

static CURLcode nss_do_connect(struct connectdata *conn, int sockindex)
{
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  struct Curl_easy *data = conn->data;
  CURLcode result = CURLE_SSL_CONNECT_ERROR;
  PRUint32 timeout;
  long * const certverifyresult = SSL_IS_PROXY() ?
    &data->set.proxy_ssl.certverifyresult : &data->set.ssl.certverifyresult;
  const char * const pinnedpubkey = SSL_IS_PROXY() ?
              data->set.str[STRING_SSL_PINNEDPUBLICKEY_PROXY] :
              data->set.str[STRING_SSL_PINNEDPUBLICKEY_ORIG];


  /* check timeout situation */
  const time_t time_left = Curl_timeleft(data, NULL, TRUE);
  if(time_left < 0) {
    failf(data, "timed out before SSL handshake");
    result = CURLE_OPERATION_TIMEDOUT;
    goto error;
  }

  /* Force the handshake now */
  timeout = PR_MillisecondsToInterval((PRUint32) time_left);
  if(SSL_ForceHandshakeWithTimeout(BACKEND->handle, timeout) != SECSuccess) {
    if(PR_GetError() == PR_WOULD_BLOCK_ERROR)
      /* blocking direction is updated by nss_update_connecting_state() */
      return CURLE_AGAIN;
    else if(*certverifyresult == SSL_ERROR_BAD_CERT_DOMAIN)
      result = CURLE_PEER_FAILED_VERIFICATION;
    else if(*certverifyresult != 0)
      result = CURLE_PEER_FAILED_VERIFICATION;
    goto error;
  }

  result = display_conn_info(conn, BACKEND->handle);
  if(result)
    goto error;

  if(SSL_SET_OPTION(issuercert)) {
    SECStatus ret = SECFailure;
    char *nickname = dup_nickname(data, SSL_SET_OPTION(issuercert));
    if(nickname) {
      /* we support only nicknames in case of issuercert for now */
      ret = check_issuer_cert(BACKEND->handle, nickname);
      free(nickname);
    }

    if(SECFailure == ret) {
      infof(data, "SSL certificate issuer check failed\n");
      result = CURLE_SSL_ISSUER_ERROR;
      goto error;
    }
    else {
      infof(data, "SSL certificate issuer check ok\n");
    }
  }

  result = cmp_peer_pubkey(connssl, pinnedpubkey);
  if(result)
    /* status already printed */
    goto error;

  return CURLE_OK;

error:
  return nss_fail_connect(connssl, data, result);
}

static CURLcode nss_connect_common(struct connectdata *conn, int sockindex,
                                   bool *done)
{
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  struct Curl_easy *data = conn->data;
  const bool blocking = (done == NULL);
  CURLcode result;

  if(connssl->state == ssl_connection_complete) {
    if(!blocking)
      *done = TRUE;
    return CURLE_OK;
  }

  if(connssl->connecting_state == ssl_connect_1) {
    result = nss_setup_connect(conn, sockindex);
    if(result)
      /* we do not expect CURLE_AGAIN from nss_setup_connect() */
      return result;

    connssl->connecting_state = ssl_connect_2;
  }

  /* enable/disable blocking mode before handshake */
  result = nss_set_blocking(connssl, data, blocking);
  if(result)
    return result;

  result = nss_do_connect(conn, sockindex);
  switch(result) {
  case CURLE_OK:
    break;
  case CURLE_AGAIN:
    if(!blocking)
      /* CURLE_AGAIN in non-blocking mode is not an error */
      return CURLE_OK;
    /* FALLTHROUGH */
  default:
    return result;
  }

  if(blocking) {
    /* in blocking mode, set NSS non-blocking mode _after_ SSL handshake */
    result = nss_set_blocking(connssl, data, /* blocking */ FALSE);
    if(result)
      return result;
  }
  else
    /* signal completed SSL handshake */
    *done = TRUE;

  connssl->state = ssl_connection_complete;
  conn->recv[sockindex] = nss_recv;
  conn->send[sockindex] = nss_send;

  /* ssl_connect_done is never used outside, go back to the initial state */
  connssl->connecting_state = ssl_connect_1;

  return CURLE_OK;
}

static CURLcode Curl_nss_connect(struct connectdata *conn, int sockindex)
{
  return nss_connect_common(conn, sockindex, /* blocking */ NULL);
}

static CURLcode Curl_nss_connect_nonblocking(struct connectdata *conn,
                                             int sockindex, bool *done)
{
  return nss_connect_common(conn, sockindex, done);
}

static ssize_t nss_send(struct connectdata *conn,  /* connection data */
                        int sockindex,             /* socketindex */
                        const void *mem,           /* send this data */
                        size_t len,                /* amount to write */
                        CURLcode *curlcode)
{
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  ssize_t rc;

  /* The SelectClientCert() hook uses this for infof() and failf() but the
     handle stored in nss_setup_connect() could have already been freed. */
  BACKEND->data = conn->data;

  rc = PR_Send(BACKEND->handle, mem, (int)len, 0, PR_INTERVAL_NO_WAIT);
  if(rc < 0) {
    PRInt32 err = PR_GetError();
    if(err == PR_WOULD_BLOCK_ERROR)
      *curlcode = CURLE_AGAIN;
    else {
      /* print the error number and error string */
      const char *err_name = nss_error_to_name(err);
      infof(conn->data, "SSL write: error %d (%s)\n", err, err_name);

      /* print a human-readable message describing the error if available */
      nss_print_error_message(conn->data, err);

      *curlcode = (is_cc_error(err))
        ? CURLE_SSL_CERTPROBLEM
        : CURLE_SEND_ERROR;
    }

    return -1;
  }

  return rc; /* number of bytes */
}

static ssize_t nss_recv(struct connectdata *conn,  /* connection data */
                        int sockindex,             /* socketindex */
                        char *buf,                 /* store read data here */
                        size_t buffersize,         /* max amount to read */
                        CURLcode *curlcode)
{
  struct ssl_connect_data *connssl = &conn->ssl[sockindex];
  ssize_t nread;

  /* The SelectClientCert() hook uses this for infof() and failf() but the
     handle stored in nss_setup_connect() could have already been freed. */
  BACKEND->data = conn->data;

  nread = PR_Recv(BACKEND->handle, buf, (int)buffersize, 0,
                  PR_INTERVAL_NO_WAIT);
  if(nread < 0) {
    /* failed SSL read */
    PRInt32 err = PR_GetError();

    if(err == PR_WOULD_BLOCK_ERROR)
      *curlcode = CURLE_AGAIN;
    else {
      /* print the error number and error string */
      const char *err_name = nss_error_to_name(err);
      infof(conn->data, "SSL read: errno %d (%s)\n", err, err_name);

      /* print a human-readable message describing the error if available */
      nss_print_error_message(conn->data, err);

      *curlcode = (is_cc_error(err))
        ? CURLE_SSL_CERTPROBLEM
        : CURLE_RECV_ERROR;
    }

    return -1;
  }

  return nread;
}

static size_t Curl_nss_version(char *buffer, size_t size)
{
  return msnprintf(buffer, size, "NSS/%s", NSS_VERSION);
}

/* data might be NULL */
static int Curl_nss_seed(struct Curl_easy *data)
{
  /* make sure that NSS is initialized */
  return !!Curl_nss_force_init(data);
}

/* data might be NULL */
static CURLcode Curl_nss_random(struct Curl_easy *data,
                                unsigned char *entropy,
                                size_t length)
{
  Curl_nss_seed(data);  /* Initiate the seed if not already done */

  if(SECSuccess != PK11_GenerateRandom(entropy, curlx_uztosi(length)))
    /* signal a failure */
    return CURLE_FAILED_INIT;

  return CURLE_OK;
}

static CURLcode Curl_nss_md5sum(unsigned char *tmp, /* input */
                                size_t tmplen,
                                unsigned char *md5sum, /* output */
                                size_t md5len)
{
  PK11Context *MD5pw = PK11_CreateDigestContext(SEC_OID_MD5);
  unsigned int MD5out;

  PK11_DigestOp(MD5pw, tmp, curlx_uztoui(tmplen));
  PK11_DigestFinal(MD5pw, md5sum, &MD5out, curlx_uztoui(md5len));
  PK11_DestroyContext(MD5pw, PR_TRUE);

  return CURLE_OK;
}

static CURLcode Curl_nss_sha256sum(const unsigned char *tmp, /* input */
                               size_t tmplen,
                               unsigned char *sha256sum, /* output */
                               size_t sha256len)
{
  PK11Context *SHA256pw = PK11_CreateDigestContext(SEC_OID_SHA256);
  unsigned int SHA256out;

  PK11_DigestOp(SHA256pw, tmp, curlx_uztoui(tmplen));
  PK11_DigestFinal(SHA256pw, sha256sum, &SHA256out, curlx_uztoui(sha256len));
  PK11_DestroyContext(SHA256pw, PR_TRUE);

  return CURLE_OK;
}

static bool Curl_nss_cert_status_request(void)
{
#ifdef SSL_ENABLE_OCSP_STAPLING
  return TRUE;
#else
  return FALSE;
#endif
}

static bool Curl_nss_false_start(void)
{
#if NSSVERNUM >= 0x030f04 /* 3.15.4 */
  return TRUE;
#else
  return FALSE;
#endif
}

static void *Curl_nss_get_internals(struct ssl_connect_data *connssl,
                                    CURLINFO info UNUSED_PARAM)
{
  (void)info;
  return BACKEND->handle;
}

const struct Curl_ssl Curl_ssl_nss = {
  { CURLSSLBACKEND_NSS, "nss" }, /* info */

  SSLSUPP_CA_PATH |
  SSLSUPP_CERTINFO |
  SSLSUPP_PINNEDPUBKEY |
  SSLSUPP_HTTPS_PROXY,

  sizeof(struct ssl_backend_data),

  Curl_nss_init,                /* init */
  Curl_nss_cleanup,             /* cleanup */
  Curl_nss_version,             /* version */
  Curl_nss_check_cxn,           /* check_cxn */
  /* NSS has no shutdown function provided and thus always fail */
  Curl_none_shutdown,           /* shutdown */
  Curl_none_data_pending,       /* data_pending */
  Curl_nss_random,              /* random */
  Curl_nss_cert_status_request, /* cert_status_request */
  Curl_nss_connect,             /* connect */
  Curl_nss_connect_nonblocking, /* connect_nonblocking */
  Curl_nss_get_internals,       /* get_internals */
  Curl_nss_close,               /* close_one */
  Curl_none_close_all,          /* close_all */
  /* NSS has its own session ID cache */
  Curl_none_session_free,       /* session_free */
  Curl_none_set_engine,         /* set_engine */
  Curl_none_set_engine_default, /* set_engine_default */
  Curl_none_engines_list,       /* engines_list */
  Curl_nss_false_start,         /* false_start */
  Curl_nss_md5sum,              /* md5sum */
  Curl_nss_sha256sum            /* sha256sum */
};

#endif /* USE_NSS */