/*
 * Copyright (C) 1999-2001 Tommi Komulainen <Tommi.Komulainen@iki.fi>
 *
 *     This program is free software; you can redistribute it and/or modify
 *     it under the terms of the GNU General Public License as published by
 *     the Free Software Foundation; either version 2 of the License, or
 *     (at your option) any later version.
 *
 *     This program is distributed in the hope that it will be useful,
 *     but WITHOUT ANY WARRANTY; without even the implied warranty of
 *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *     GNU General Public License for more details.
 *
 *     You should have received a copy of the GNU General Public License
 *     along with this program; if not, write to the Free Software
 *     Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */

#if HAVE_CONFIG_H
# include "config.h"
#endif

#include <openssl/ssl.h>
#include <openssl/x509.h>
#include <openssl/x509v3.h>
#include <openssl/x509_vfy.h>
#include <openssl/err.h>
#include <openssl/rand.h>
#include <openssl/evp.h>

/* LibreSSL defines OPENSSL_VERSION_NUMBER but sets it to 0x20000000L.
 * So technically we don't need the defined(OPENSSL_VERSION_NUMBER) check.
 */
#if (defined(OPENSSL_VERSION_NUMBER)  && OPENSSL_VERSION_NUMBER  < 0x10100000L) || \
    (defined(LIBRESSL_VERSION_NUMBER) && LIBRESSL_VERSION_NUMBER < 0x2070000fL)
#define X509_get0_notBefore X509_get_notBefore
#define X509_get0_notAfter X509_get_notAfter
#define X509_getm_notBefore X509_get_notBefore
#define X509_getm_notAfter X509_get_notAfter
#define X509_STORE_CTX_get0_chain X509_STORE_CTX_get_chain
#define SSL_has_pending SSL_pending
#endif

/* Unimplemented OpenSSL 1.1 api calls */
#if (defined(LIBRESSL_VERSION_NUMBER) && LIBRESSL_VERSION_NUMBER >= 0x2070000fL)
#define SSL_has_pending SSL_pending
#endif

#undef _

#include <string.h>

#include "mutt.h"
#include "mutt_socket.h"
#include "mutt_menu.h"
#include "mutt_curses.h"
#include "mutt_ssl.h"
#include "mutt_idna.h"

/* Just in case OpenSSL doesn't define DEVRANDOM */
#ifndef DEVRANDOM
#define DEVRANDOM "/dev/urandom"
#endif

#define HAVE_ENTROPY()	(RAND_status() == 1)

/* index for storing hostname as application specific data in SSL structure */
static int HostExDataIndex = -1;

/* Index for storing the "skip mode" state in SSL structure.  When the
 * user skips a certificate in the chain, the stored value will be
 * non-null. */
static int SkipModeExDataIndex = -1;

/* keep a handle on accepted certificates in case we want to
 * open up another connection to the same server in this session */
static STACK_OF(X509) *SslSessionCerts = NULL;

typedef struct
{
  SSL_CTX *ctx;
  SSL *ssl;
  unsigned char isopen;
}
sslsockdata;

/* local prototypes */
static int ssl_init (void);
static int add_entropy (const char *file);
static int ssl_socket_read (CONNECTION* conn, char* buf, size_t len);
static int ssl_socket_write (CONNECTION* conn, const char* buf, size_t len);
static int ssl_socket_poll (CONNECTION* conn, time_t wait_secs);
static int ssl_socket_open (CONNECTION * conn);
static int ssl_socket_close (CONNECTION * conn);
static int tls_close (CONNECTION* conn);
static void ssl_err (sslsockdata *data, int err);
static void ssl_dprint_err_stack (void);
static int ssl_cache_trusted_cert (X509 *cert);
static int ssl_verify_callback (int preverify_ok, X509_STORE_CTX *ctx);
static int interactive_check_cert (X509 *cert, int idx, int len, SSL *ssl, int allow_always);
static void ssl_get_client_cert(sslsockdata *ssldata, CONNECTION *conn);
static int ssl_passwd_cb(char *buf, int size, int rwflag, void *userdata);
static int ssl_negotiate (CONNECTION *conn, sslsockdata*);

/* ssl certificate verification can behave strangely if there are expired
 * certs loaded into the trusted store.  This function filters out expired
 * certs.
 * Previously the code used this form:
 *     SSL_CTX_load_verify_locations (ssldata->ctx, SslCertFile, NULL);
 */
static int ssl_load_certificates (SSL_CTX *ctx)
{
  FILE *fp;
  X509 *cert = NULL;
  X509_STORE *store;
  int rv = 1;
#ifdef DEBUG
  char buf[STRING];
#endif

  dprint (2, (debugfile, "ssl_load_certificates: loading trusted certificates\n"));
  store = SSL_CTX_get_cert_store (ctx);
  if (!store)
  {
    store = X509_STORE_new ();
    SSL_CTX_set_cert_store (ctx, store);
  }

  if ((fp = fopen (SslCertFile, "rt")) == NULL)
    return 0;

  while (NULL != PEM_read_X509 (fp, &cert, NULL, NULL))
  {
    if ((X509_cmp_current_time (X509_get0_notBefore (cert)) >= 0) ||
        (X509_cmp_current_time (X509_get0_notAfter (cert)) <= 0))
    {
      dprint (2, (debugfile, "ssl_load_certificates: filtering expired cert: %s\n",
                  X509_NAME_oneline (X509_get_subject_name (cert), buf, sizeof (buf))));
    }
    else
    {
      X509_STORE_add_cert (store, cert);
    }
  }
  /* PEM_read_X509 sets the error NO_START_LINE on eof */
  if (ERR_GET_REASON(ERR_peek_last_error()) != PEM_R_NO_START_LINE)
    rv = 0;
  ERR_clear_error();

  X509_free (cert);
  safe_fclose (&fp);

  return rv;
}

static int ssl_set_verify_partial (SSL_CTX *ctx)
{
  int rv = 0;
#ifdef HAVE_SSL_PARTIAL_CHAIN
  X509_VERIFY_PARAM *param;

  if (option (OPTSSLVERIFYPARTIAL))
  {
    param = X509_VERIFY_PARAM_new();
    if (param)
    {
      X509_VERIFY_PARAM_set_flags(param, X509_V_FLAG_PARTIAL_CHAIN);
      if (0 == SSL_CTX_set1_param(ctx, param))
      {
        dprint (2, (debugfile, "ssl_set_verify_partial: SSL_CTX_set1_param() failed."));
        rv = -1;
      }
      X509_VERIFY_PARAM_free(param);
    }
    else
    {
      dprint (2, (debugfile, "ssl_set_verify_partial: X509_VERIFY_PARAM_new() failed."));
      rv = -1;
    }
  }
#endif
  return rv;
}

/* Reset the min/max proto version allowed so that enabling old
 * (insecure) protocols inside Mutt will actually use them.
 *
 * SSL_CTX_set_min/max_proto_version were added in OpenSSL 1.1 and
 * LibreSSL 2.6.1.
 */
static void reset_allowed_proto_version_range (sslsockdata *ssldata)
{
#if (!defined(LIBRESSL_VERSION_NUMBER) &&       \
     defined(OPENSSL_VERSION_NUMBER) &&         \
     OPENSSL_VERSION_NUMBER >= 0x10100000L)     \
  ||                                            \
  (defined(LIBRESSL_VERSION_NUMBER) &&          \
   LIBRESSL_VERSION_NUMBER >= 0x2060100fL)

  /* 0 is magic for lowest/highest possible value in these calls */
  SSL_CTX_set_min_proto_version (ssldata->ctx, 0);
  SSL_CTX_set_max_proto_version (ssldata->ctx, 0);

#endif
}

/* mutt_ssl_starttls: Negotiate TLS over an already opened connection.
 *   TODO: Merge this code better with ssl_socket_open. */
int mutt_ssl_starttls (CONNECTION* conn)
{
  sslsockdata* ssldata;
  int maxbits;
  long ssl_options = 0;

  if (mutt_socket_has_buffered_input (conn))
  {
    /* L10N:
       The server is not supposed to send data immediately after
       confirming STARTTLS.  This warns the user that something
       weird is going on.
    */
    mutt_error _("Warning: clearing unexpected server data before TLS negotiation");
    mutt_sleep (0);
    mutt_socket_clear_buffered_input (conn);
  }

  if (ssl_init())
    goto bail;

  ssldata = (sslsockdata*) safe_calloc (1, sizeof (sslsockdata));
  /* the ssl_use_xxx protocol options don't apply. We must use TLS in TLS.
   *
   * However, we need to be able to negotiate amongst various TLS versions,
   * which at present can only be done with the SSLv23_client_method;
   * TLSv1_client_method gives us explicitly TLSv1.0, not 1.1 or 1.2 (True as
   * of OpenSSL 1.0.1c)
   */
  if (! (ssldata->ctx = SSL_CTX_new (SSLv23_client_method())))
  {
    dprint (1, (debugfile, "mutt_ssl_starttls: Error allocating SSL_CTX\n"));
    goto bail_ssldata;
  }

  reset_allowed_proto_version_range (ssldata);

#ifdef SSL_OP_NO_TLSv1_3
  if (!option(OPTTLSV1_3))
    ssl_options |= SSL_OP_NO_TLSv1_3;
#endif
#ifdef SSL_OP_NO_TLSv1_2
  if (!option(OPTTLSV1_2))
    ssl_options |= SSL_OP_NO_TLSv1_2;
#endif
#ifdef SSL_OP_NO_TLSv1_1
  if (!option(OPTTLSV1_1))
    ssl_options |= SSL_OP_NO_TLSv1_1;
#endif
#ifdef SSL_OP_NO_TLSv1
  if (!option(OPTTLSV1))
    ssl_options |= SSL_OP_NO_TLSv1;
#endif
  /* these are always set */
#ifdef SSL_OP_NO_SSLv3
  ssl_options |= SSL_OP_NO_SSLv3;
#endif
#ifdef SSL_OP_NO_SSLv2
  ssl_options |= SSL_OP_NO_SSLv2;
#endif
  if (! SSL_CTX_set_options(ssldata->ctx, ssl_options))
  {
    dprint(1, (debugfile, "mutt_ssl_starttls: Error setting options to %ld\n", ssl_options));
    goto bail_ctx;
  }

  if (option (OPTSSLSYSTEMCERTS))
  {
    if (! SSL_CTX_set_default_verify_paths (ssldata->ctx))
    {
      dprint (1, (debugfile, "mutt_ssl_starttls: Error setting default verify paths\n"));
      goto bail_ctx;
    }
  }

  if (SslCertFile && !ssl_load_certificates (ssldata->ctx))
    dprint (1, (debugfile, "mutt_ssl_starttls: Error loading trusted certificates\n"));

  ssl_get_client_cert(ssldata, conn);

  if (SslCiphers)
  {
    if (!SSL_CTX_set_cipher_list (ssldata->ctx, SslCiphers))
    {
      dprint (1, (debugfile, "mutt_ssl_starttls: Could not select preferred ciphers\n"));
      goto bail_ctx;
    }
  }

  if (ssl_set_verify_partial (ssldata->ctx))
  {
    mutt_error (_("Warning: error enabling ssl_verify_partial_chains"));
    mutt_sleep (2);
  }

  if (! (ssldata->ssl = SSL_new (ssldata->ctx)))
  {
    dprint (1, (debugfile, "mutt_ssl_starttls: Error allocating SSL\n"));
    goto bail_ctx;
  }

  if (SSL_set_fd (ssldata->ssl, conn->fd) != 1)
  {
    dprint (1, (debugfile, "mutt_ssl_starttls: Error setting fd\n"));
    goto bail_ssl;
  }

  if (ssl_negotiate (conn, ssldata))
    goto bail_ssl;

  ssldata->isopen = 1;

  /* hmm. watch out if we're starting TLS over any method other than raw. */
  conn->sockdata = ssldata;
  conn->conn_read = ssl_socket_read;
  conn->conn_write = ssl_socket_write;
  conn->conn_close = tls_close;
  conn->conn_poll = ssl_socket_poll;

  conn->ssf = SSL_CIPHER_get_bits (SSL_get_current_cipher (ssldata->ssl),
                                   &maxbits);

  return 0;

bail_ssl:
  SSL_free (ssldata->ssl);
  ssldata->ssl = 0;
bail_ctx:
  SSL_CTX_free (ssldata->ctx);
  ssldata->ctx = 0;
bail_ssldata:
  FREE (&ssldata);
bail:
  return -1;
}

/*
 * OpenSSL library needs to be fed with sufficient entropy. On systems
 * with /dev/urandom, this is done transparently by the library itself,
 * on other systems we need to fill the entropy pool ourselves.
 *
 * Even though only OpenSSL 0.9.5 and later will complain about the
 * lack of entropy, we try to our best and fill the pool with older
 * versions also. (That's the reason for the ugly #ifdefs and macros,
 * otherwise I could have simply #ifdef'd the whole ssl_init function)
 */
static int ssl_init (void)
{
  BUFFER *path = NULL;
  static unsigned char init_complete = 0;

  if (init_complete)
    return 0;

  if (! HAVE_ENTROPY())
  {
    /* load entropy from files */
    add_entropy (SslEntropyFile);

    path = mutt_buffer_pool_get ();
    add_entropy (RAND_file_name (path->data, path->dsize));

    /* load entropy from egd sockets */
#ifdef HAVE_RAND_EGD
    add_entropy (getenv ("EGDSOCKET"));
    mutt_buffer_printf (path, "%s/.entropy", NONULL(Homedir));
    add_entropy (mutt_b2s (path));
    add_entropy ("/tmp/entropy");
#endif

    /* shuffle $RANDFILE (or ~/.rnd if unset) */
    RAND_write_file (RAND_file_name (path->data, path->dsize));
    mutt_buffer_pool_release (&path);

    mutt_clear_error ();
    if (! HAVE_ENTROPY())
    {
      mutt_error (_("Failed to find enough entropy on your system"));
      mutt_sleep (2);
      return -1;
    }
  }

/* OpenSSL performs automatic initialization as of 1.1.
 * However LibreSSL does not (as of 2.8.3). */
#if (defined(OPENSSL_VERSION_NUMBER) && OPENSSL_VERSION_NUMBER < 0x10100000L) || \
    (defined(LIBRESSL_VERSION_NUMBER))
  /* I don't think you can do this just before reading the error. The call
   * itself might clobber the last SSL error. */
  SSL_load_error_strings();
  SSL_library_init();
#endif
  init_complete = 1;
  return 0;
}

static int add_entropy (const char *file)
{
  struct stat st;
  int n = -1;

  if (!file) return 0;

  if (stat (file, &st) == -1)
    return errno == ENOENT ? 0 : -1;

  mutt_message (_("Filling entropy pool: %s...\n"),
		file);

  /* check that the file permissions are secure */
  if (st.st_uid != getuid () ||
      ((st.st_mode & (S_IWGRP | S_IRGRP)) != 0) ||
      ((st.st_mode & (S_IWOTH | S_IROTH)) != 0))
  {
    mutt_error (_("%s has insecure permissions!"), file);
    mutt_sleep (2);
    return -1;
  }

#ifdef HAVE_RAND_EGD
  n = RAND_egd (file);
#endif
  if (n <= 0)
    n = RAND_load_file (file, -1);

  return n;
}

static int ssl_socket_open_err (CONNECTION *conn)
{
  mutt_error (_("SSL disabled due to the lack of entropy"));
  mutt_sleep (2);
  return -1;
}


int mutt_ssl_socket_setup (CONNECTION * conn)
{
  if (ssl_init() < 0)
  {
    conn->conn_open = ssl_socket_open_err;
    return -1;
  }

  conn->conn_open	= ssl_socket_open;
  conn->conn_read	= ssl_socket_read;
  conn->conn_write	= ssl_socket_write;
  conn->conn_close	= ssl_socket_close;
  conn->conn_poll       = ssl_socket_poll;

  return 0;
}

static int ssl_socket_read (CONNECTION* conn, char* buf, size_t len)
{
  sslsockdata *data = conn->sockdata;
  int rc;

  rc = SSL_read (data->ssl, buf, len);
  if (rc <= 0)
  {
    data->isopen = 0;
    ssl_err (data, rc);
  }

  return rc;
}

static int ssl_socket_write (CONNECTION* conn, const char* buf, size_t len)
{
  sslsockdata *data = conn->sockdata;
  int rc;

  rc = SSL_write (data->ssl, buf, len);
  if (rc <= 0)
    ssl_err (data, rc);

  return rc;
}

static int ssl_socket_poll (CONNECTION* conn, time_t wait_secs)
{
  sslsockdata *data = conn->sockdata;

  if (!data)
    return -1;

  if (SSL_has_pending (data->ssl))
    return 1;
  else
    return raw_socket_poll (conn, wait_secs);
}

static int ssl_socket_open (CONNECTION * conn)
{
  sslsockdata *data;
  int maxbits;

  if (raw_socket_open (conn) < 0)
    return -1;

  data = (sslsockdata *) safe_calloc (1, sizeof (sslsockdata));
  conn->sockdata = data;

  if (! (data->ctx = SSL_CTX_new (SSLv23_client_method ())))
  {
    /* L10N: an SSL context is a data structure returned by the OpenSSL
     *       function SSL_CTX_new().  In this case it returned NULL: an
     *       error condition.
     */
    mutt_error (_("Unable to create SSL context"));
    ssl_dprint_err_stack ();
    mutt_socket_close (conn);
    return -1;
  }

  reset_allowed_proto_version_range (data);

  /* disable SSL protocols as needed */
  if (!option(OPTTLSV1))
  {
    SSL_CTX_set_options(data->ctx, SSL_OP_NO_TLSv1);
  }
  /* TLSv1.1/1.2 support was added in OpenSSL 1.0.1, but some OS distros such
   * as Fedora 17 are on OpenSSL 1.0.0.
   */
#ifdef SSL_OP_NO_TLSv1_1
  if (!option(OPTTLSV1_1))
  {
    SSL_CTX_set_options(data->ctx, SSL_OP_NO_TLSv1_1);
  }
#endif
#ifdef SSL_OP_NO_TLSv1_2
  if (!option(OPTTLSV1_2))
  {
    SSL_CTX_set_options(data->ctx, SSL_OP_NO_TLSv1_2);
  }
#endif
#ifdef SSL_OP_NO_TLSv1_3
  if (!option(OPTTLSV1_3))
  {
    SSL_CTX_set_options(data->ctx, SSL_OP_NO_TLSv1_3);
  }
#endif
  if (!option(OPTSSLV2))
  {
    SSL_CTX_set_options(data->ctx, SSL_OP_NO_SSLv2);
  }
  if (!option(OPTSSLV3))
  {
    SSL_CTX_set_options(data->ctx, SSL_OP_NO_SSLv3);
  }

  if (option (OPTSSLSYSTEMCERTS))
  {
    if (! SSL_CTX_set_default_verify_paths (data->ctx))
    {
      dprint (1, (debugfile, "ssl_socket_open: Error setting default verify paths\n"));
      mutt_socket_close (conn);
      return -1;
    }
  }

  if (SslCertFile && !ssl_load_certificates (data->ctx))
    dprint (1, (debugfile, "ssl_socket_open: Error loading trusted certificates\n"));

  ssl_get_client_cert(data, conn);

  if (SslCiphers)
  {
    SSL_CTX_set_cipher_list (data->ctx, SslCiphers);
  }

  if (ssl_set_verify_partial (data->ctx))
  {
    mutt_error (_("Warning: error enabling ssl_verify_partial_chains"));
    mutt_sleep (2);
  }

  data->ssl = SSL_new (data->ctx);
  SSL_set_fd (data->ssl, conn->fd);

  if (ssl_negotiate(conn, data))
  {
    mutt_socket_close (conn);
    return -1;
  }

  data->isopen = 1;

  conn->ssf = SSL_CIPHER_get_bits (SSL_get_current_cipher (data->ssl),
                                   &maxbits);

  return 0;
}

/* ssl_negotiate: After SSL state has been initialized, attempt to negotiate
 *   SSL over the wire, including certificate checks. */
static int ssl_negotiate (CONNECTION *conn, sslsockdata* ssldata)
{
  int err;
  const char *errmsg;
  char *hostname;

  hostname = SslVerifyHostOverride ? SslVerifyHostOverride : conn->account.host;

  if (HostExDataIndex == -1)
  {
    if ((HostExDataIndex = SSL_get_ex_new_index (0, "host", NULL, NULL, NULL)) == -1)
    {
      dprint (1, (debugfile, "failed to get index for application specific data\n"));
      return -1;
    }
  }

  if (! SSL_set_ex_data (ssldata->ssl, HostExDataIndex, hostname))
  {
    dprint (1, (debugfile, "failed to save hostname in SSL structure\n"));
    return -1;
  }

  if (SkipModeExDataIndex == -1)
  {
    if ((SkipModeExDataIndex = SSL_get_ex_new_index (0, "skip", NULL, NULL, NULL)) == -1)
    {
      dprint (1, (debugfile, "failed to get index for application specific data\n"));
      return -1;
    }
  }

  if (! SSL_set_ex_data (ssldata->ssl, SkipModeExDataIndex, NULL))
  {
    dprint (1, (debugfile, "failed to save skip mode in SSL structure\n"));
    return -1;
  }

  SSL_set_verify (ssldata->ssl, SSL_VERIFY_PEER, ssl_verify_callback);
  SSL_set_mode (ssldata->ssl, SSL_MODE_AUTO_RETRY);

  if (!SSL_set_tlsext_host_name (ssldata->ssl, hostname))
  {
    /* L10N: This is a warning when trying to set the host name for
     * TLS Server Name Indication (SNI).  This allows the server to present
     * the correct certificate if it supports multiple hosts. */
    mutt_error _("Warning: unable to set TLS SNI host name");
    mutt_sleep (1);
  }

  ERR_clear_error ();

  if ((err = SSL_connect (ssldata->ssl)) != 1)
  {
    switch (SSL_get_error (ssldata->ssl, err))
    {
      case SSL_ERROR_SYSCALL:
        errmsg = _("I/O error");
        break;
      case SSL_ERROR_SSL:
        errmsg = ERR_error_string (ERR_get_error (), NULL);
        break;
      default:
        errmsg = _("unknown error");
    }

    mutt_error (_("SSL failed: %s"), errmsg);
    mutt_sleep (1);

    return -1;
  }

  /* L10N:
     %1$s is version (e.g. "TLSv1.2")
     %2$s is cipher_version (e.g. "TLSv1/SSLv3")
     %3$s is cipher_name (e.g. "ECDHE-RSA-AES128-GCM-SHA256") */
  mutt_message (_("%s connection using %s (%s)"),
                SSL_get_version(ssldata->ssl),
                SSL_get_cipher_version (ssldata->ssl),
                SSL_get_cipher_name (ssldata->ssl));
  mutt_sleep (0);

  return 0;
}

static int ssl_socket_close (CONNECTION * conn)
{
  sslsockdata *data = conn->sockdata;
  if (data)
  {
    if (data->isopen)
      SSL_shutdown (data->ssl);

    SSL_free (data->ssl);
    SSL_CTX_free (data->ctx);
    FREE (&conn->sockdata);
  }

  return raw_socket_close (conn);
}

static int tls_close (CONNECTION* conn)
{
  int rc;

  rc = ssl_socket_close (conn);
  conn->conn_read = raw_socket_read;
  conn->conn_write = raw_socket_write;
  conn->conn_close = raw_socket_close;
  conn->conn_poll = raw_socket_poll;

  return rc;
}

static void ssl_err (sslsockdata *data, int err)
{
  const char* errmsg;
  unsigned long sslerr;

  switch (SSL_get_error (data->ssl, err))
  {
    case SSL_ERROR_NONE:
      return;
    case SSL_ERROR_ZERO_RETURN:
      errmsg = "SSL connection closed";
      data->isopen = 0;
      break;
    case SSL_ERROR_WANT_READ:
      errmsg = "retry read";
      break;
    case SSL_ERROR_WANT_WRITE:
      errmsg = "retry write";
      break;
    case SSL_ERROR_WANT_CONNECT:
      errmsg = "retry connect";
      break;
    case SSL_ERROR_WANT_ACCEPT:
      errmsg = "retry accept";
      break;
    case SSL_ERROR_WANT_X509_LOOKUP:
      errmsg = "retry x509 lookup";
      break;
    case SSL_ERROR_SYSCALL:
      errmsg = "I/O error";
      data->isopen = 0;
      break;
    case SSL_ERROR_SSL:
      sslerr = ERR_get_error ();
      switch (sslerr)
      {
        case 0:
          switch (err)
          {
            case 0:
              errmsg = "EOF";
              break;
            default:
              errmsg = strerror(errno);
          }
          break;
        default:
          errmsg = ERR_error_string (sslerr, NULL);
      }
      break;
    default:
      errmsg = "unknown error";
  }

  dprint (1, (debugfile, "SSL error: %s\n", errmsg));
}

static void ssl_dprint_err_stack (void)
{
#ifdef DEBUG
  BIO *bio;
  char *buf = NULL;
  long buflen;
  char *output;

  if (! (bio = BIO_new (BIO_s_mem ())))
    return;
  ERR_print_errors (bio);
  if ((buflen = BIO_get_mem_data (bio, &buf)) > 0)
  {
    output = safe_malloc (buflen + 1);
    memcpy (output, buf, buflen);
    output[buflen] = '\0';
    dprint (1, (debugfile, "SSL error stack: %s\n", output));
    FREE (&output);
  }
  BIO_free (bio);
#endif
}


static char *x509_get_part (X509_NAME *name, int nid)
{
  static char ret[SHORT_STRING];

  if (!name ||
      X509_NAME_get_text_by_NID (name, nid, ret, sizeof (ret)) < 0)
    strfcpy (ret, _("Unknown"), sizeof (ret));

  return ret;
}

static void x509_fingerprint (char *s, int l, X509 * cert, const EVP_MD *(*hashfunc)(void))
{
  unsigned char md[EVP_MAX_MD_SIZE];
  unsigned int n;
  int j;

  if (!X509_digest (cert, hashfunc(), md, &n))
  {
    snprintf (s, l, "%s", _("[unable to calculate]"));
  }
  else
  {
    for (j = 0; j < (int) n; j++)
    {
      char ch[8];
      snprintf (ch, 8, "%02X%s", md[j], (j % 2 ? " " : ""));
      safe_strcat (s, l, ch);
    }
  }
}

static char *asn1time_to_string (ASN1_UTCTIME *tm)
{
  static char buf[64];
  BIO *bio;

  strfcpy (buf, _("[invalid date]"), sizeof (buf));

  bio = BIO_new (BIO_s_mem());
  if (bio)
  {
    if (ASN1_TIME_print (bio, tm))
      (void) BIO_read (bio, buf, sizeof (buf));
    BIO_free (bio);
  }

  return buf;
}

static int compare_certificates (X509 *cert, X509 *peercert,
                                 unsigned char *peermd, unsigned int peermdlen)
{
  unsigned char md[EVP_MAX_MD_SIZE];
  unsigned int mdlen;

  /* Avoid CPU-intensive digest calculation if the certificates are
   * not even remotely equal.
   */
  if (X509_subject_name_cmp (cert, peercert) != 0 ||
      X509_issuer_name_cmp (cert, peercert) != 0)
    return -1;

  if (!X509_digest (cert, EVP_sha256(), md, &mdlen) || peermdlen != mdlen)
    return -1;

  if (memcmp(peermd, md, mdlen) != 0)
    return -1;

  return 0;
}

static int check_certificate_cache (X509 *peercert)
{
  unsigned char peermd[EVP_MAX_MD_SIZE];
  unsigned int peermdlen;
  X509 *cert;
  int i;

  if (!X509_digest (peercert, EVP_sha256(), peermd, &peermdlen)
      || !SslSessionCerts)
  {
    return 0;
  }

  for (i = sk_X509_num (SslSessionCerts); i-- > 0;)
  {
    cert = sk_X509_value (SslSessionCerts, i);
    if (!compare_certificates (cert, peercert, peermd, peermdlen))
    {
      return 1;
    }
  }

  return 0;
}

static int check_certificate_expiration (X509 *peercert, int silent)
{
  if (option (OPTSSLVERIFYDATES) != MUTT_NO)
  {
    if (X509_cmp_current_time (X509_get0_notBefore (peercert)) >= 0)
    {
      if (!silent)
      {
        dprint (2, (debugfile, "Server certificate is not yet valid\n"));
        mutt_error (_("Server certificate is not yet valid"));
        mutt_sleep (2);
      }
      return 0;
    }
    if (X509_cmp_current_time (X509_get0_notAfter (peercert)) <= 0)
    {
      if (!silent)
      {
        dprint (2, (debugfile, "Server certificate has expired\n"));
        mutt_error (_("Server certificate has expired"));
        mutt_sleep (2);
      }
      return 0;
    }
  }

  return 1;
}

static int check_certificate_file (X509 *peercert)
{
  unsigned char peermd[EVP_MAX_MD_SIZE];
  unsigned int peermdlen;
  X509 *cert = NULL;
  int pass = 0;
  FILE *fp;

  if (!SslCertFile)
    return 0;

  if ((fp = fopen (SslCertFile, "rt")) == NULL)
    return 0;

  if (!X509_digest (peercert, EVP_sha256(), peermd, &peermdlen))
  {
    safe_fclose (&fp);
    return 0;
  }

  while (PEM_read_X509 (fp, &cert, NULL, NULL) != NULL)
  {
    if ((compare_certificates (cert, peercert, peermd, peermdlen) == 0) &&
        check_certificate_expiration (cert, 1))
    {
      pass = 1;
      break;
    }
  }
  /* PEM_read_X509 sets an error on eof */
  if (!pass)
    ERR_clear_error();
  X509_free (cert);
  safe_fclose (&fp);

  return pass;
}

static int check_certificate_by_digest (X509 *peercert)
{
  return check_certificate_expiration (peercert, 0) &&
    check_certificate_file (peercert);
}

/* port to mutt from msmtp's tls.c */
static int hostname_match (const char *hostname, const char *certname)
{
  const char *cmp1, *cmp2;

  if (strncmp(certname, "*.", 2) == 0)
  {
    cmp1 = certname + 2;
    cmp2 = strchr(hostname, '.');
    if (!cmp2)
    {
      return 0;
    }
    else
    {
      cmp2++;
    }
  }
  else
  {
    cmp1 = certname;
    cmp2 = hostname;
  }

  if (*cmp1 == '\0' || *cmp2 == '\0')
  {
    return 0;
  }

  if (strcasecmp(cmp1, cmp2) != 0)
  {
    return 0;
  }

  return 1;
}

/* port to mutt from msmtp's tls.c */
static int check_host (X509 *x509cert, const char *hostname, char *err, size_t errlen)
{
  int i, rc = 0;
  /* hostname in ASCII format: */
  char *hostname_ascii = NULL;
  /* needed to get the common name: */
  X509_NAME *x509_subject;
  char *buf = NULL;
  int bufsize;
  /* needed to get the DNS subjectAltNames: */
  STACK_OF(GENERAL_NAME) *subj_alt_names;
  int subj_alt_names_count;
  GENERAL_NAME *subj_alt_name;
  /* did we find a name matching hostname? */
  int match_found;

  /* Check if 'hostname' matches the one of the subjectAltName extensions of
   * type DNS or the Common Name (CN). */

#if defined(HAVE_LIBIDN) || defined(HAVE_LIBIDN2)
  if (idna_to_ascii_lz(hostname, &hostname_ascii, 0) != IDNA_SUCCESS)
  {
    hostname_ascii = safe_strdup(hostname);
  }
#else
  hostname_ascii = safe_strdup(hostname);
#endif

  /* Try the DNS subjectAltNames. */
  match_found = 0;
  if ((subj_alt_names = X509_get_ext_d2i(x509cert, NID_subject_alt_name,
					 NULL, NULL)))
  {
    subj_alt_names_count = sk_GENERAL_NAME_num(subj_alt_names);
    for (i = 0; i < subj_alt_names_count; i++)
    {
      subj_alt_name = sk_GENERAL_NAME_value(subj_alt_names, i);
      if (subj_alt_name->type == GEN_DNS)
      {
	if (subj_alt_name->d.ia5->length >= 0 &&
	    mutt_strlen((char *)subj_alt_name->d.ia5->data) == (size_t)subj_alt_name->d.ia5->length &&
	    (match_found = hostname_match(hostname_ascii,
					  (char *)(subj_alt_name->d.ia5->data))))
	{
	  break;
	}
      }
    }
    GENERAL_NAMES_free(subj_alt_names);
  }

  if (!match_found)
  {
    /* Try the common name */
    if (!(x509_subject = X509_get_subject_name(x509cert)))
    {
      if (err && errlen)
	strfcpy (err, _("cannot get certificate subject"), errlen);
      goto out;
    }

    /* first get the space requirements */
    bufsize = X509_NAME_get_text_by_NID(x509_subject, NID_commonName,
					NULL, 0);
    if (bufsize == -1)
    {
      if (err && errlen)
	strfcpy (err, _("cannot get certificate common name"), errlen);
      goto out;
    }
    bufsize++; /* space for the terminal nul char */
    buf = safe_malloc((size_t)bufsize);
    if (X509_NAME_get_text_by_NID(x509_subject, NID_commonName,
				  buf, bufsize) == -1)
    {
      if (err && errlen)
	strfcpy (err, _("cannot get certificate common name"), errlen);
      goto out;
    }
    /* cast is safe since bufsize is incremented above, so bufsize-1 is always
     * zero or greater.
     */
    if (mutt_strlen(buf) == (size_t)bufsize - 1)
    {
      match_found = hostname_match(hostname_ascii, buf);
    }
  }

  if (!match_found)
  {
    if (err && errlen)
      snprintf (err, errlen, _("certificate owner does not match hostname %s"),
		hostname);
    goto out;
  }

  rc = 1;

out:
  FREE(&buf);
  FREE(&hostname_ascii);

  return rc;
}

static int ssl_cache_trusted_cert (X509 *c)
{
  dprint (1, (debugfile, "ssl_cache_trusted_cert: trusted\n"));
  if (!SslSessionCerts)
    SslSessionCerts = sk_X509_new_null();
  return (sk_X509_push (SslSessionCerts, X509_dup(c)));
}

/* certificate verification callback, called for each certificate in the chain
 * sent by the peer, starting from the root; returning 1 means that the given
 * certificate is trusted, returning 0 immediately aborts the SSL connection */
static int ssl_verify_callback (int preverify_ok, X509_STORE_CTX *ctx)
{
  char buf[STRING];
  const char *host;
  int len, pos;
  X509 *cert;
  SSL *ssl;
  int skip_mode;
#ifdef HAVE_SSL_PARTIAL_CHAIN
  static int last_pos = 0;
  static X509 *last_cert = NULL;
  unsigned char last_cert_md[EVP_MAX_MD_SIZE];
  unsigned int last_cert_mdlen;
#endif

  if (! (ssl = X509_STORE_CTX_get_ex_data (ctx, SSL_get_ex_data_X509_STORE_CTX_idx ())))
  {
    dprint (1, (debugfile, "ssl_verify_callback: failed to retrieve SSL structure from X509_STORE_CTX\n"));
    return 0;
  }
  if (! (host = SSL_get_ex_data (ssl, HostExDataIndex)))
  {
    dprint (1, (debugfile, "ssl_verify_callback: failed to retrieve hostname from SSL structure\n"));
    return 0;
  }

  /* This is true when a previous entry in the certificate chain did
   * not verify and the user manually chose to skip it via the
   * $ssl_verify_partial_chains option.
   * In this case, all following certificates need to be treated as non-verified
   * until one is actually verified.
   */
  skip_mode = (SSL_get_ex_data (ssl, SkipModeExDataIndex) != NULL);

  cert = X509_STORE_CTX_get_current_cert (ctx);
  pos = X509_STORE_CTX_get_error_depth (ctx);
  len = sk_X509_num (X509_STORE_CTX_get0_chain (ctx));

  dprint (1, (debugfile,
              "ssl_verify_callback: checking cert chain entry %s (preverify: %d skipmode: %d)\n",
              X509_NAME_oneline (X509_get_subject_name (cert), buf, sizeof (buf)),
              preverify_ok, skip_mode));

#ifdef HAVE_SSL_PARTIAL_CHAIN
  /* Sometimes, when a certificate is (s)kipped, OpenSSL will pass it
   * a second time with preverify_ok = 1.  Don't show it or the user
   * will think their "s" key is broken.
   */
  if (option (OPTSSLVERIFYPARTIAL))
  {
    if (skip_mode && preverify_ok && (pos == last_pos) && last_cert)
    {
      if (X509_digest (last_cert, EVP_sha256(), last_cert_md, &last_cert_mdlen) &&
          !compare_certificates (cert, last_cert, last_cert_md, last_cert_mdlen))
      {
        dprint (2, (debugfile,
                    "ssl_verify_callback: ignoring duplicate skipped certificate.\n"));
        return 1;
      }
    }

    last_pos = pos;
    if (last_cert)
      X509_free (last_cert);
    last_cert = X509_dup (cert);
  }
#endif

  /* check session cache first */
  if (check_certificate_cache (cert))
  {
    dprint (2, (debugfile, "ssl_verify_callback: using cached certificate\n"));
    SSL_set_ex_data (ssl, SkipModeExDataIndex, NULL);
    return 1;
  }

  /* check hostname only for the leaf certificate */
  buf[0] = 0;
  if (pos == 0 && option (OPTSSLVERIFYHOST) != MUTT_NO)
  {
    if (!check_host (cert, host, buf, sizeof (buf)))
    {
      mutt_error (_("Certificate host check failed: %s"), buf);
      mutt_sleep (2);
      /* we disallow (a)ccept always in the prompt, because it will have no effect
       * for hostname mismatches. */
      return interactive_check_cert (cert, pos, len, ssl, 0);
    }
    dprint (2, (debugfile, "ssl_verify_callback: hostname check passed\n"));
  }

  if (!preverify_ok || skip_mode)
  {
    /* automatic check from user's database */
    if (SslCertFile && check_certificate_by_digest (cert))
    {
      dprint (2, (debugfile, "ssl_verify_callback: digest check passed\n"));
      SSL_set_ex_data (ssl, SkipModeExDataIndex, NULL);
      return 1;
    }

#ifdef DEBUG
    /* log verification error */
    {
      int err = X509_STORE_CTX_get_error (ctx);
      snprintf (buf, sizeof (buf), "%s (%d)",
                X509_verify_cert_error_string (err), err);
      dprint (2, (debugfile, "X509_verify_cert: %s\n", buf));
    }
#endif

    /* prompt user */
    return interactive_check_cert (cert, pos, len, ssl, 1);
  }

  return 1;
}

static int interactive_check_cert (X509 *cert, int idx, int len, SSL *ssl, int allow_always)
{
  static const int part[] =
    { NID_commonName,             /* CN */
      NID_pkcs9_emailAddress,     /* Email */
      NID_organizationName,       /* O */
      NID_organizationalUnitName, /* OU */
      NID_localityName,           /* L */
      NID_stateOrProvinceName,    /* ST */
      NID_countryName             /* C */ };
  X509_NAME *x509_subject;
  X509_NAME *x509_issuer;
  char helpstr[LONG_STRING];
  char buf[STRING];
  char title[STRING];
  MUTTMENU *menu;
  int done;
  BUFFER *drow = NULL;
  unsigned u;
  FILE *fp;
  int allow_skip = 0, reset_ignoremacro = 0;

  if (option (OPTNOCURSES))
  {
    dprint (1, (debugfile, "interactive_check_cert: unable to prompt for certificate in batch mode\n"));
    mutt_error _("Untrusted server certificate");
    return 0;
  }

  menu = mutt_new_menu (MENU_GENERIC);
  mutt_push_current_menu (menu);

  drow = mutt_buffer_pool_get ();

  mutt_menu_add_dialog_row (menu, _("This certificate belongs to:"));
  x509_subject = X509_get_subject_name (cert);
  for (u = 0; u < mutt_array_size (part); u++)
  {
    mutt_buffer_printf (drow, "   %s", x509_get_part (x509_subject, part[u]));
    mutt_menu_add_dialog_row (menu, mutt_b2s (drow));
  }

  mutt_menu_add_dialog_row (menu, "");
  mutt_menu_add_dialog_row (menu, _("This certificate was issued by:"));
  x509_issuer = X509_get_issuer_name (cert);
  for (u = 0; u < mutt_array_size (part); u++)
  {
    mutt_buffer_printf (drow, "   %s", x509_get_part (x509_issuer, part[u]));
    mutt_menu_add_dialog_row (menu, mutt_b2s (drow));
  }

  mutt_menu_add_dialog_row (menu, "");
  mutt_menu_add_dialog_row (menu, _("This certificate is valid"));
  mutt_buffer_printf (drow, _("   from %s"),
            asn1time_to_string (X509_getm_notBefore (cert)));
  mutt_menu_add_dialog_row (menu, mutt_b2s (drow));
  mutt_buffer_printf (drow, _("     to %s"),
            asn1time_to_string (X509_getm_notAfter (cert)));
  mutt_menu_add_dialog_row (menu, mutt_b2s (drow));

  mutt_menu_add_dialog_row (menu, "");
  buf[0] = '\0';
  x509_fingerprint (buf, sizeof (buf), cert, EVP_sha1);
  mutt_buffer_printf (drow, _("SHA1 Fingerprint: %s"), buf);
  mutt_menu_add_dialog_row (menu, mutt_b2s (drow));
  buf[0] = '\0';
  buf[40] = '\0';  /* Ensure the second printed line is null terminated */
  x509_fingerprint (buf, sizeof (buf), cert, EVP_sha256);
  buf[39] = '\0';  /* Divide into two lines of output */
  mutt_buffer_printf (drow, "%s%s", _("SHA256 Fingerprint: "), buf);
  mutt_menu_add_dialog_row (menu, mutt_b2s (drow));
  mutt_buffer_printf (drow, "%*s%s",
            (int)mutt_strlen (_("SHA256 Fingerprint: ")), "", buf + 40);
  mutt_menu_add_dialog_row (menu, mutt_b2s (drow));

  snprintf (title, sizeof (title),
	    _("SSL Certificate check (certificate %d of %d in chain)"),
	    len - idx, len);
  menu->title = title;

  /* The leaf/host certificate can't be skipped. */
#ifdef HAVE_SSL_PARTIAL_CHAIN
  if ((idx != 0) &&
      (option (OPTSSLVERIFYPARTIAL)))
    allow_skip = 1;
#endif

  /* Inside ssl_verify_callback(), this function is guarded by a call to
   * check_certificate_by_digest().  This means if check_certificate_expiration() is
   * true, then check_certificate_file() must be false.  Therefore we don't need
   * to also scan the certificate file here.
   */
  allow_always = allow_always &&
    SslCertFile &&
    check_certificate_expiration (cert, 1);

  /* L10N:
   * These four letters correspond to the choices in the next four strings:
   * (r)eject, accept (o)nce, (a)ccept always, (s)kip.
   * These prompts are the interactive certificate confirmation prompts for
   * an OpenSSL connection.
   */
  menu->keys = _("roas");
  if (allow_always)
  {
    if (allow_skip)
      menu->prompt = _("(r)eject, accept (o)nce, (a)ccept always, (s)kip");
    else
      menu->prompt = _("(r)eject, accept (o)nce, (a)ccept always");
  }
  else
  {
    if (allow_skip)
      menu->prompt = _("(r)eject, accept (o)nce, (s)kip");
    else
      menu->prompt = _("(r)eject, accept (o)nce");
  }

  helpstr[0] = '\0';
  mutt_make_help (buf, sizeof (buf), _("Exit  "), MENU_GENERIC, OP_EXIT);
  safe_strcat (helpstr, sizeof (helpstr), buf);
  mutt_make_help (buf, sizeof (buf), _("Help"), MENU_GENERIC, OP_HELP);
  safe_strcat (helpstr, sizeof (helpstr), buf);
  menu->help = helpstr;

  done = 0;

  if (!option (OPTIGNOREMACROEVENTS))
  {
    set_option (OPTIGNOREMACROEVENTS);
    reset_ignoremacro = 1;
  }
  while (!done)
  {
    switch (mutt_menuLoop (menu))
    {
      case -1:			/* abort */
      case OP_MAX + 1:		/* reject */
      case OP_EXIT:
        done = 1;
        break;
      case OP_MAX + 3:		/* accept always */
        if (!allow_always)
          break;
        done = 0;
        if ((fp = fopen (SslCertFile, "a")))
	{
	  if (PEM_write_X509 (fp, cert))
	    done = 1;
	  safe_fclose (&fp);
	}
	if (!done)
        {
	  mutt_error (_("Warning: Couldn't save certificate"));
	  mutt_sleep (2);
	}
	else
        {
	  mutt_message (_("Certificate saved"));
	  mutt_sleep (0);
	}
        /* fall through */
      case OP_MAX + 2:		/* accept once */
        done = 2;
        SSL_set_ex_data (ssl, SkipModeExDataIndex, NULL);
	ssl_cache_trusted_cert (cert);
        break;
      case OP_MAX + 4:          /* skip */
        if (!allow_skip)
          break;
        done = 2;
        SSL_set_ex_data (ssl, SkipModeExDataIndex, &SkipModeExDataIndex);
        break;
    }
  }
  if (reset_ignoremacro)
    unset_option (OPTIGNOREMACROEVENTS);

  mutt_buffer_pool_release (&drow);
  mutt_pop_current_menu (menu);
  mutt_menuDestroy (&menu);
  dprint (2, (debugfile, "ssl interactive_check_cert: done=%d\n", done));
  return (done == 2);
}

static void ssl_get_client_cert(sslsockdata *ssldata, CONNECTION *conn)
{
  if (SslClientCert)
  {
    dprint (2, (debugfile, "Using client certificate %s\n", SslClientCert));
    SSL_CTX_set_default_passwd_cb_userdata(ssldata->ctx, &conn->account);
    SSL_CTX_set_default_passwd_cb(ssldata->ctx, ssl_passwd_cb);
    SSL_CTX_use_certificate_file(ssldata->ctx, SslClientCert, SSL_FILETYPE_PEM);
    SSL_CTX_use_PrivateKey_file(ssldata->ctx, SslClientCert, SSL_FILETYPE_PEM);

#if 0
    /* This interferes with SMTP client-cert authentication that doesn't
     * use AUTH EXTERNAL. (see gitlab #336)
     *
     * The mutt_sasl.c code sets up callbacks to get the login or
     * user, and it looks like the Cyrus SASL external code calls
     * those.
     *
     * Brendan doesn't recall if this really was necessary at one time, so
     * I'm disabling it.
     */

    /* if we are using a client cert, SASL may expect an external auth name */
    mutt_account_getuser (&conn->account);
#endif
  }
}

static void client_cert_prompt (char *prompt, size_t prompt_size, ACCOUNT *account)
{
  /* L10N:
     When using a $ssl_client_cert, OpenSSL may prompt for the password
     to decrypt the cert.  %s is the hostname.
  */
  snprintf (prompt, prompt_size, _("Password for %s client cert: "),
            account->host);
}

static int ssl_passwd_cb(char *buf, int size, int rwflag, void *userdata)
{
  ACCOUNT *account;

  if (!buf || size <= 0 || !userdata)
    return 0;

  account = (ACCOUNT *) userdata;

  if (_mutt_account_getpass (account, client_cert_prompt))
    return 0;

  return snprintf(buf, size, "%s", account->pass);
}
