securetransport.c

/*****************************************************************************
 * securetransport.c
 *****************************************************************************
 * Copyright (C) 2013 David Fuhrmann
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 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 Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser 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.
 *****************************************************************************/

/*****************************************************************************
 * Preamble
 *****************************************************************************/

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

#include <vlc_common.h>
#include <vlc_plugin.h>
#include <vlc_tls.h>
#include <vlc_dialog.h>

#include <Security/Security.h>
#include <Security/SecureTransport.h>
#include <TargetConditionals.h>

/* From MacErrors.h (cannot be included because it isn't present in iOS: */
#ifndef ioErr
# define ioErr -36
#endif

/*****************************************************************************
 * ALPN helper functions
 *****************************************************************************/

/* Converts the VLC ALPN C array (null-terminated) to a ALPN
 * CFMutableArray as expected by the Secure Transport API
 * Returns CFMutableArrayRef on success, else NULL.
 */
static CFMutableArrayRef alpnToCFArray(const char *const *alpn)
{
    CFMutableArrayRef alpnValues =
            CFArrayCreateMutable(kCFAllocatorDefault, 0, &kCFTypeArrayCallBacks);

    for (size_t i = 0; alpn[i] != NULL; i++) {
        CFStringRef alpnVal =
                CFStringCreateWithCString(kCFAllocatorDefault, alpn[i], kCFStringEncodingASCII);
        if (alpnVal == NULL) {
            // Failed to convert the ALPN value to CFString, error out.
            CFRelease(alpnValues);
            return NULL;
        }
        CFArrayAppendValue(alpnValues, alpnVal);
        CFRelease(alpnVal);
    }
    return alpnValues;
}

/* Obtains a copy of the contents of a CFString in ASCII encoding.
 * Returns char* (must be freed by caller) or NULL on failure.
 */
static char* CFStringCopyASCIICString(CFStringRef cfString)
{
    // Try the quick way to obtain the buffer
    const char *tmpBuffer = CFStringGetCStringPtr(cfString, kCFStringEncodingASCII);

    if (tmpBuffer != NULL) {
       return strdup(tmpBuffer);
    }

    // The quick way did not work, try the long way
    CFIndex length = CFStringGetLength(cfString);
    CFIndex maxSize =
        CFStringGetMaximumSizeForEncoding(length, kCFStringEncodingASCII);

    // If result would exceed LONG_MAX, kCFNotFound is returned
    if (unlikely(maxSize == kCFNotFound)) {
        return NULL;
    }

    // Account for the null terminator
    maxSize++;

    char *buffer = (char *)malloc(maxSize);
    Boolean success = CFStringGetCString(cfString, buffer, maxSize, kCFStringEncodingASCII);

    if (!success)
        FREENULL(buffer);
    return buffer;
}

/* Returns the first entry copy of the ALPN array as char*
 * or NULL on failure.
 */
static char* CFArrayALPNCopyFirst(CFArrayRef alpnArray)
{
    CFIndex count = CFArrayGetCount(alpnArray);

    if (count <= 0)
        return NULL;

    CFStringRef alpnVal = CFArrayGetValueAtIndex(alpnArray, 0);
    return CFStringCopyASCIICString(alpnVal);
}

/*****************************************************************************
 * Module descriptor
 *****************************************************************************/
static int  OpenClient  (vlc_tls_creds_t *);
static void CloseClient (vlc_tls_creds_t *);

#if !TARGET_OS_IPHONE
    static int  OpenServer  (vlc_tls_creds_t *crd, const char *cert, const char *key);
    static void CloseServer (vlc_tls_creds_t *);
#endif

vlc_module_begin ()
    set_description(N_("TLS support for OS X and iOS"))
    set_capability("tls client", 2)
    set_callbacks(OpenClient, CloseClient)
    set_category(CAT_ADVANCED)
    set_subcategory(SUBCAT_ADVANCED_NETWORK)

    /*
     * The server module currently uses an OSX only API, to be compatible with 10.6.
      If the module is needed on iOS, then the "modern" keychain lookup API need to be
     * implemented.
     */
#if !TARGET_OS_IPHONE
    add_submodule()
        set_description(N_("TLS server support for OS X"))
        set_capability("tls server", 2)
        set_callbacks(OpenServer, CloseServer)
        set_category(CAT_ADVANCED)
        set_subcategory(SUBCAT_ADVANCED_NETWORK)
#endif /* !TARGET_OS_IPHONE */

vlc_module_end ()


#define cfKeyHost CFSTR("host")
#define cfKeyCertificate CFSTR("certificate")

typedef struct {
    CFMutableArrayRef whitelist;

    /* valid in server mode */
    CFArrayRef server_cert_chain;
} vlc_tls_creds_sys_t;

typedef struct {
    vlc_tls_t tls;
    SSLContextRef p_context;
    vlc_tls_creds_sys_t *p_cred;
    size_t i_send_buffered_bytes;
    vlc_tls_t *sock;
    vlc_object_t *obj;

    bool b_blocking_send;
    bool b_handshaked;
    bool b_server_mode;

    vlc_mutex_t lock;
} vlc_tls_st_t;

static int st_Error (vlc_tls_t *obj, int val)
{
    vlc_tls_st_t *sys = (vlc_tls_st_t *)obj;

    switch (val)
    {
        case errSSLWouldBlock:
            errno = EAGAIN;
            break;

        case errSSLClosedGraceful:
        case errSSLClosedAbort:
            msg_Dbg(sys->obj, "Connection closed with code %d", val);
            errno = ECONNRESET;
            break;
        default:
            msg_Err(sys->obj, "Found error %d", val);
            errno = ECONNRESET;
    }
    return -1;
}

/*
 * Read function called by secure transport for socket read.
 *
 * Function is based on Apples SSLSample sample code.
 */
static OSStatus st_SocketReadFunc (SSLConnectionRef connection,
                                   void *data,
                                   size_t *dataLength) {

    vlc_tls_t *session = (vlc_tls_t *)connection;
    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;
    struct iovec iov = {
        .iov_base = data,
        .iov_len = *dataLength,
    };
    OSStatus retValue = noErr;

    while (iov.iov_len > 0) {
        ssize_t val = sys->sock->readv(sys->sock, &iov, 1);
        if (val <= 0) {
            if (val == 0) {
                msg_Dbg(sys->obj, "found eof");
                retValue = errSSLClosedGraceful;
            } else { /* do the switch */
                switch (errno) {
                    case ENOENT:
                        /* connection closed */
                        retValue = errSSLClosedGraceful;
                        break;
                    case ECONNRESET:
                        retValue = errSSLClosedAbort;
                        break;
                    case EAGAIN:
                        retValue = errSSLWouldBlock;
                        sys->b_blocking_send = false;
                        break;
                    default:
                        msg_Err(sys->obj, "try to read %zu bytes, "
                                "got error %d", iov.iov_len, errno);
                        retValue = ioErr;
                        break;
                }
            }
            break;
        }

        iov.iov_base = (char *)iov.iov_base + val;
        iov.iov_len -= val;
    }

    *dataLength -= iov.iov_len;
    return retValue;
}

/*
 * Write function called by secure transport for socket read.
 *
 * Function is based on Apples SSLSample sample code.
 */
static OSStatus st_SocketWriteFunc (SSLConnectionRef connection,
                                    const void *data,
                                    size_t *dataLength) {

    vlc_tls_t *session = (vlc_tls_t *)connection;
    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;
    struct iovec iov = {
        .iov_base = (void *)data,
        .iov_len = *dataLength,
    };
    OSStatus retValue = noErr;

    while (iov.iov_len > 0) {
        ssize_t val = sys->sock->writev(sys->sock, &iov, 1);
        if (val < 0) {
            switch (errno) {
                case EAGAIN:
                    retValue = errSSLWouldBlock;
                    sys->b_blocking_send = true;
                    break;

                case EPIPE:
                case ECONNRESET:
                    retValue = errSSLClosedAbort;
                    break;

                default:
                    msg_Err(sys->obj, "error while writing: %d", errno);
                    retValue = ioErr;
                    break;
            }
            break;
        }

        iov.iov_base = (char *)iov.iov_base + val;
        iov.iov_len -= val;
    }

    *dataLength -= iov.iov_len;
    return retValue;
}

static int st_validateServerCertificate (vlc_tls_t *session, const char *hostname) {

    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;
    int result = -1;
    SecCertificateRef leaf_cert = NULL;

    SecTrustRef trust = NULL;
    OSStatus ret = SSLCopyPeerTrust(sys->p_context, &trust);
    if (ret != noErr || trust == NULL) {
        msg_Err(sys->obj, "error getting certifictate chain");
        return -1;
    }

    CFStringRef cfHostname = CFStringCreateWithCString(kCFAllocatorDefault,
                                                       hostname,
                                                       kCFStringEncodingUTF8);


    /* enable default root / anchor certificates */
    ret = SecTrustSetAnchorCertificates(trust, NULL);
    if (ret != noErr) {
        msg_Err(sys->obj, "error setting anchor certificates");
        result = -1;
        goto out;
    }

    SecTrustResultType trust_eval_result = 0;

    ret = SecTrustEvaluate(trust, &trust_eval_result);
    if (ret != noErr) {
        msg_Err(sys->obj, "error calling SecTrustEvaluate");
        result = -1;
        goto out;
    }

    switch (trust_eval_result) {
        case kSecTrustResultUnspecified:
        case kSecTrustResultProceed:
            msg_Dbg(sys->obj, "cerfificate verification successful, result is %d", trust_eval_result);
            result = 0;
            goto out;

        case kSecTrustResultRecoverableTrustFailure:
        case kSecTrustResultDeny:
        default:
            msg_Warn(sys->obj, "cerfificate verification failed, result is %d", trust_eval_result);
    }

    /* get leaf certificate */
    /* SSLCopyPeerCertificates is only available on OSX 10.5 or later */
#if !TARGET_OS_IPHONE
    CFArrayRef cert_chain = NULL;
    ret = SSLCopyPeerCertificates(sys->p_context, &cert_chain);
    if (ret != noErr || !cert_chain) {
        result = -1;
        goto out;
    }

    if (CFArrayGetCount(cert_chain) == 0) {
        CFRelease(cert_chain);
        result = -1;
        goto out;
    }

    leaf_cert = (SecCertificateRef)CFArrayGetValueAtIndex(cert_chain, 0);
    CFRetain(leaf_cert);
    CFRelease(cert_chain);
#else
    /* SecTrustGetCertificateAtIndex is only available on 10.7 or iOS */
    if (SecTrustGetCertificateCount(trust) == 0) {
        result = -1;
        goto out;
    }

    leaf_cert = SecTrustGetCertificateAtIndex(trust, 0);
    CFRetain(leaf_cert);
#endif


    /* check if leaf already accepted */
    CFIndex max = CFArrayGetCount(sys->p_cred->whitelist);
    for (CFIndex i = 0; i < max; ++i) {
        CFDictionaryRef dict = CFArrayGetValueAtIndex(sys->p_cred->whitelist, i);
        CFStringRef knownHost = (CFStringRef)CFDictionaryGetValue(dict, cfKeyHost);
        SecCertificateRef knownCert = (SecCertificateRef)CFDictionaryGetValue(dict, cfKeyCertificate);

        if (!knownHost || !knownCert)
            continue;

        if (CFEqual(knownHost, cfHostname) && CFEqual(knownCert, leaf_cert)) {
            msg_Warn(sys->obj, "certificate already accepted, continuing");
            result = 0;
            goto out;
        }
    }

    /* We do not show more certificate details yet because there is no proper API to get
       a summary of the certificate. SecCertificateCopySubjectSummary is the only method
       available on iOS and 10.6. More promising API functions such as
       SecCertificateCopyLongDescription also print out the subject only, more or less.
       But only showing the certificate subject is of no real help for the user.
       We could use SecCertificateCopyValues, but then we need to parse all OID values for
       ourself. This is too mad for just printing information the user will never check
       anyway.
     */

    const char *msg = N_("You attempted to reach %s. "
             "However the security certificate presented by the server "
             "is unknown and could not be authenticated by any trusted "
             "Certification Authority. "
             "This problem may be caused by a configuration error "
             "or an attempt to breach your security or your privacy.\n\n"
             "If in doubt, abort now.\n");
    int answer = vlc_dialog_wait_question(sys->obj,
                                          VLC_DIALOG_QUESTION_WARNING, _("Abort"),
                                          _("Accept certificate temporarily"),
                                          NULL, _("Insecure site"),
                                          vlc_gettext (msg), hostname);
    if (answer == 1) {
        msg_Warn(sys->obj, "Proceeding despite of failed certificate validation");

        /* save leaf certificate in whitelist */
        const void *keys[] = {cfKeyHost, cfKeyCertificate};
        const void *values[] = {cfHostname, leaf_cert};
        CFDictionaryRef dict = CFDictionaryCreate(kCFAllocatorDefault,
                                                   keys, values, 2,
                                                   &kCFTypeDictionaryKeyCallBacks,
                                                   &kCFTypeDictionaryValueCallBacks);
        if (!dict) {
            msg_Err(sys->obj, "error creating dict");
            result = -1;
            goto out;
        }

        CFArrayAppendValue(sys->p_cred->whitelist, dict);
        CFRelease(dict);

        result = 0;
        goto out;

    } else {
        result = -1;
        goto out;
    }

out:
    CFRelease(trust);

    if (cfHostname)
        CFRelease(cfHostname);
    if (leaf_cert)
        CFRelease(leaf_cert);

    return result;
}

/*
 * @return -1 on fatal error, 0 on successful handshake completion,
 * 1 if more would-be blocking recv is needed,
 * 2 if more would-be blocking send is required.
 */
static int st_Handshake (vlc_tls_creds_t *crd, vlc_tls_t *session,
                         const char *host, const char *service,
                         char **restrict alp) {

    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;

    VLC_UNUSED(service);

    OSStatus retValue = SSLHandshake(sys->p_context);

// Only try to use ALPN on recent enough SDKs
// macOS 10.13.2, iOS 11, tvOS 11, watchOS 4
#if (TARGET_OS_OSX    && MAC_OS_X_VERSION_MAX_ALLOWED     >= 101302) || \
    (TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED  >= 110000) || \
    (TARGET_OS_TV     && __TV_OS_VERSION_MAX_ALLOWED      >= 110000) || \
    (TARGET_OS_WATCH  && __WATCH_OS_VERSION_MAX_ALLOWED   >= 40000)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wpartial-availability"

    /* Handle ALPN data */
    if (alp != NULL) {
        if (SSLCopyALPNProtocols != NULL) {
            CFArrayRef alpnArray = NULL;
            OSStatus res = SSLCopyALPNProtocols(sys->p_context, &alpnArray);
            if (res == noErr && alpnArray) {
                *alp = CFArrayALPNCopyFirst(alpnArray);
                if (unlikely(*alp == NULL))
                    return -1;
            } else {
                *alp = NULL;
            }
        } else {
            *alp = NULL;
        }
    }

#pragma clang diagnostic pop
#else

    /* No ALPN support */
    if (alp != NULL) {
        *alp = NULL;
    }

#endif

    if (retValue == errSSLWouldBlock) {
        msg_Dbg(crd, "handshake is blocked, try again later");
        return 1 + (sys->b_blocking_send ? 1 : 0);
    }

    switch (retValue) {
        case noErr:
            if (sys->b_server_mode == false && st_validateServerCertificate(session, host) != 0) {
                return -1;
            }
            msg_Dbg(crd, "handshake completed successfully");
            sys->b_handshaked = true;
            return 0;

        case errSSLServerAuthCompleted:
            msg_Dbg(crd, "SSLHandshake returned errSSLServerAuthCompleted, continuing handshake");
            return st_Handshake(crd, session, host, service, alp);

        case errSSLConnectionRefused:
            msg_Err(crd, "connection was refused");
            return -1;
        case errSSLNegotiation:
            msg_Err(crd, "cipher suite negotiation failed");
            return -1;
        case errSSLFatalAlert:
            msg_Err(crd, "fatal error occurred during handshake");
            return -1;

        default:
            msg_Err(crd, "handshake returned error %d", (int)retValue);
            return -1;
    }
}

static int st_GetFD (vlc_tls_t *session)
{
    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;
    vlc_tls_t *sock = sys->sock;

    return vlc_tls_GetFD(sock);
}

/**
 * Sends data through a TLS session.
 */
static ssize_t st_Send (vlc_tls_t *session, const struct iovec *iov,
                        unsigned count)
{
    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;
    OSStatus ret = noErr;

    if (unlikely(count == 0))
        return 0;

    vlc_mutex_lock(&sys->lock);

    /*
     * SSLWrite does not return the number of bytes actually written to
     * the socket, but the number of bytes written to the internal cache.
     *
     * If return value is errSSLWouldBlock, the underlying socket cannot
     * send all data, but the data is already cached. In this situation,
     * we need to call SSLWrite again. To ensure this call even for the
     * last bytes, we return EAGAIN. On the next call, we give no new data
     * to SSLWrite until the error is not errSSLWouldBlock anymore.
     *
     * This code is adapted the same way as done in curl.
     * (https://github.com/bagder/curl/blob/master/lib/curl_darwinssl.c#L2067)
     */

    /* EAGAIN is not expected by net_Write in this situation,
       so use EINTR here */
    int againErr = sys->b_server_mode ? EAGAIN : EINTR;

    size_t actualSize;
    if (sys->i_send_buffered_bytes > 0) {
        ret = SSLWrite(sys->p_context, NULL, 0, &actualSize);

        if (ret == noErr) {
            /* actualSize remains zero because no new data send */
            actualSize = sys->i_send_buffered_bytes;
            sys->i_send_buffered_bytes = 0;

        } else if (ret == errSSLWouldBlock) {
            vlc_mutex_unlock(&sys->lock);
            errno = againErr;
            return -1;
        }

    } else {
        ret = SSLWrite(sys->p_context, iov->iov_base, iov->iov_len,
                       &actualSize);

        if (ret == errSSLWouldBlock) {
            sys->i_send_buffered_bytes = iov->iov_len;
            errno = againErr;
            vlc_mutex_unlock(&sys->lock);
            return -1;
        }
    }

    vlc_mutex_unlock(&sys->lock);
    return ret != noErr ? st_Error(session, ret) : actualSize;
}

/**
 * Receives data through a TLS session.
 */
static ssize_t st_Recv (vlc_tls_t *session, struct iovec *iov, unsigned count)
{
    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;

    if (unlikely(count == 0))
        return 0;

    vlc_mutex_lock(&sys->lock);

    size_t actualSize;
    OSStatus ret = SSLRead(sys->p_context, iov->iov_base, iov->iov_len,
                           &actualSize);

    if (ret == errSSLWouldBlock && actualSize) {
        vlc_mutex_unlock(&sys->lock);
        return actualSize;
    }

    /* peer performed shutdown */
    if (ret == errSSLClosedNoNotify || ret == errSSLClosedGraceful) {
        msg_Dbg(sys->obj, "Got close notification with code %i", (int)ret);
        vlc_mutex_unlock(&sys->lock);
        return 0;
    }

    vlc_mutex_unlock(&sys->lock);
    return ret != noErr ? st_Error(session, ret) : actualSize;
}

/**
 * Closes a TLS session.
 */

static int st_SessionShutdown (vlc_tls_t *session, bool duplex) {

    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;

    msg_Dbg(sys->obj, "shutdown TLS session");

    vlc_mutex_destroy(&sys->lock);

    OSStatus ret = noErr;
    VLC_UNUSED(duplex);

    if (sys->b_handshaked) {
        ret = SSLClose(sys->p_context);
    }

    if (ret != noErr) {
        msg_Warn(sys->obj, "Cannot close ssl context (%i)", (int)ret);
        return ret;
    }

    return 0;
}

static void st_SessionClose (vlc_tls_t *session) {

    vlc_tls_st_t *sys = (vlc_tls_st_t *)session;
    msg_Dbg(sys->obj, "close TLS session");

    if (sys->p_context) {
#if TARGET_OS_IPHONE
        CFRelease(sys->p_context);
#else
        if (SSLDisposeContext(sys->p_context) != noErr) {
            msg_Err(sys->obj, "error deleting context");
        }
#endif
    }
    free(sys);
}

/**
 * Initializes a client-side TLS session.
 */

static vlc_tls_t *st_SessionOpenCommon(vlc_tls_creds_t *crd, vlc_tls_t *sock,
                                       bool b_server)
{
    vlc_tls_st_t *sys = malloc(sizeof (*sys));
    if (unlikely(sys == NULL))
        return NULL;

    sys->p_cred = crd->sys;
    sys->b_handshaked = false;
    sys->b_blocking_send = false;
    sys->i_send_buffered_bytes = 0;
    sys->p_context = NULL;
    sys->sock = sock;
    sys->b_server_mode = b_server;
    vlc_mutex_init(&sys->lock);
    sys->obj = VLC_OBJECT(crd);

    vlc_tls_t *tls = &sys->tls;

    tls->get_fd = st_GetFD;
    tls->readv = st_Recv;
    tls->writev = st_Send;
    tls->shutdown = st_SessionShutdown;
    tls->close = st_SessionClose;
    crd->handshake = st_Handshake;

    SSLContextRef p_context = NULL;
#if TARGET_OS_IPHONE
    p_context = SSLCreateContext(NULL, b_server ? kSSLServerSide : kSSLClientSide, kSSLStreamType);
    if (p_context == NULL) {
        msg_Err(crd, "cannot create ssl context");
        goto error;
    }
#else
    if (SSLNewContext(b_server, &p_context) != noErr) {
        msg_Err(crd, "error calling SSLNewContext");
        goto error;
    }
#endif

    sys->p_context = p_context;

    OSStatus ret = SSLSetIOFuncs(p_context, st_SocketReadFunc, st_SocketWriteFunc);
    if (ret != noErr) {
        msg_Err(crd, "cannot set io functions");
        goto error;
    }

    ret = SSLSetConnection(p_context, tls);
    if (ret != noErr) {
        msg_Err(crd, "cannot set connection");
        goto error;
    }

    return tls;

error:
    st_SessionClose(tls);
    return NULL;
}

static vlc_tls_t *st_ClientSessionOpen(vlc_tls_creds_t *crd, vlc_tls_t *sock,
                                 const char *hostname, const char *const *alpn)
{
    msg_Dbg(crd, "open TLS session for %s", hostname);

    vlc_tls_t *tls = st_SessionOpenCommon(crd, sock, false);
    if (tls == NULL)
        return NULL;

    vlc_tls_st_t *sys = (vlc_tls_st_t *)tls;

    OSStatus ret = SSLSetPeerDomainName(sys->p_context, hostname, strlen(hostname));
    if (ret != noErr) {
        msg_Err(crd, "cannot set peer domain name");
        goto error;
    }

// Only try to use ALPN on recent enough SDKs
// macOS 10.13.2, iOS 11, tvOS 11, watchOS 4
#if (TARGET_OS_OSX    && MAC_OS_X_VERSION_MAX_ALLOWED     >= 101302) || \
    (TARGET_OS_IPHONE && __IPHONE_OS_VERSION_MAX_ALLOWED  >= 110000) || \
    (TARGET_OS_TV     && __TV_OS_VERSION_MAX_ALLOWED      >= 110000) || \
    (TARGET_OS_WATCH  && __WATCH_OS_VERSION_MAX_ALLOWED   >= 40000)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wpartial-availability"

    /* Handle ALPN */
    if (alpn != NULL) {
        if (SSLSetALPNProtocols != NULL) {
            CFMutableArrayRef alpnValues = alpnToCFArray(alpn);

            if (alpnValues == NULL) {
                msg_Err(crd, "cannot create CFMutableArray for ALPN values");
                goto error;
            }

            OSStatus ret = SSLSetALPNProtocols(sys->p_context, alpnValues);
            if (ret != noErr){
                msg_Err(crd, "failed setting ALPN protocols (%i)", ret);
            }
            CFRelease(alpnValues);
        } else {
            msg_Warn(crd, "Ignoring ALPN request due to lack of support in the backend. Proxy behavior potentially undefined.");
        }
    }

#pragma clang diagnostic pop
#else

    /* No ALPN support */
    if (alpn != NULL) {
        // Fallback if SDK does not has SSLSetALPNProtocols
        msg_Warn(crd, "Compiled in SDK without ALPN support. Proxy behavior potentially undefined.");
        #warning ALPN support in your SDK version missing (need 10.13.2), proxy behavior potentially undefined (rdar://29127318, #17721)
    }

#endif

    /* disable automatic validation. We do so manually to also handle invalid
       certificates */

    /* this has effect only on iOS 5 and OSX 10.8 or later ... */
    ret = SSLSetSessionOption(sys->p_context, kSSLSessionOptionBreakOnServerAuth, true);
    if (ret != noErr) {
        msg_Err (crd, "cannot set session option");
        goto error;
    }
#if !TARGET_OS_IPHONE
    /* ... thus calling this for earlier osx versions, which is not available on iOS in turn */
    ret = SSLSetEnableCertVerify(sys->p_context, false);
    if (ret != noErr) {
        msg_Err(crd, "error setting enable cert verify");
        goto error;
    }
#endif

    return tls;

error:
    st_SessionShutdown(tls, true);
    st_SessionClose(tls);
    return NULL;
}

/**
 * Initializes a client-side TLS credentials.
 */
static int OpenClient (vlc_tls_creds_t *crd) {

    msg_Dbg(crd, "open st client");

    vlc_tls_creds_sys_t *sys = malloc (sizeof (*sys));
    if (unlikely(sys == NULL))
        return VLC_ENOMEM;

    sys->whitelist = CFArrayCreateMutable(kCFAllocatorDefault, 0, &kCFTypeArrayCallBacks);
    sys->server_cert_chain = NULL;

    crd->sys = sys;
    crd->open = st_ClientSessionOpen;

    return VLC_SUCCESS;
}

static void CloseClient (vlc_tls_creds_t *crd) {
    msg_Dbg(crd, "close secure transport client");

    vlc_tls_creds_sys_t *sys = crd->sys;

    if (sys->whitelist)
        CFRelease(sys->whitelist);

    free(sys);
}

/* Begin of server-side methods */
#if !TARGET_OS_IPHONE

/**
 * Initializes a server-side TLS session.
 */
static vlc_tls_t *st_ServerSessionOpen (vlc_tls_creds_t *crd, vlc_tls_t *sock,
                               const char *hostname, const char *const *alpn) {

    VLC_UNUSED(hostname);
    VLC_UNUSED(alpn);
    msg_Dbg(crd, "open TLS server session");

    vlc_tls_t *tls = st_SessionOpenCommon(crd, sock, true);
    if (tls != NULL)
        return NULL;

    vlc_tls_st_t *sys = (vlc_tls_st_t *)tls;
    vlc_tls_creds_sys_t *p_cred_sys = crd->sys;

    OSStatus ret = SSLSetCertificate(sys->p_context, p_cred_sys->server_cert_chain);
    if (ret != noErr) {
        msg_Err(crd, "cannot set server certificate");
        goto error;
    }

    return tls;

error:
    st_SessionShutdown(tls, true);
    st_SessionClose(tls);
    return NULL;
}

/**
 * Initializes server-side TLS credentials.
 */
static int OpenServer (vlc_tls_creds_t *crd, const char *cert, const char *key) {

    /*
     * This function expects the label of the certificate in "cert", stored
     * in the MacOS keychain. The appropriate private key is found automatically.
     */
    VLC_UNUSED(key);
    OSStatus ret;

    msg_Dbg(crd, "open st server");

    /*
     * Get the server certificate.
     *
     * This API is deprecated, but the replacement SecItemCopyMatching
     * only works on >= 10.7
     */
    SecKeychainAttribute attrib = { kSecLabelItemAttr, strlen(cert), (void *)cert };
    SecKeychainAttributeList attrList = { 1, &attrib };

    SecKeychainSearchRef searchReference = NULL;
    ret = SecKeychainSearchCreateFromAttributes(NULL, kSecCertificateItemClass,
                                                 &attrList, &searchReference);
    if (ret != noErr || searchReference == NULL) {
        msg_Err(crd, "Cannot find certificate with alias %s", cert);
        return VLC_EGENERIC;
    }

    SecKeychainItemRef itemRef = NULL;
    ret = SecKeychainSearchCopyNext(searchReference, &itemRef);
    if (ret != noErr) {
        msg_Err(crd, "Cannot get certificate with alias %s, error: %d", cert, ret);
        return VLC_EGENERIC;
    }
    CFRelease(searchReference);

    /* cast allowed according to documentation */
    SecCertificateRef certificate = (SecCertificateRef)itemRef;

    SecIdentityRef cert_identity = NULL;
    ret = SecIdentityCreateWithCertificate(NULL, certificate, &cert_identity);
    if (ret != noErr) {
        msg_Err(crd, "Cannot get private key for certificate");
        CFRelease(certificate);
        return VLC_EGENERIC;
    }

    /*
     * We try to validate the server certificate, but do not care about the result.
     * The only aim is to get the certificate chain.
     */
    SecPolicyRef policy = SecPolicyCreateSSL(true, NULL);
    SecTrustRef trust_ref = NULL;
    int result = VLC_SUCCESS;

    /* According to docu its fine to pass just one certificate */
    ret = SecTrustCreateWithCertificates((CFArrayRef)certificate, policy, &trust_ref);
    if (ret != noErr) {
        msg_Err(crd, "Cannot create trust");
        result = VLC_EGENERIC;
        goto out;
    }

    SecTrustResultType status;
    ret = SecTrustEvaluate(trust_ref, &status);
    if (ret != noErr) {
        msg_Err(crd, "Error evaluating trust");
        result = VLC_EGENERIC;
        goto out;
    }

    CFArrayRef cert_chain = NULL;
    CSSM_TP_APPLE_EVIDENCE_INFO *status_chain;
    ret = SecTrustGetResult(trust_ref, &status, &cert_chain, &status_chain);
    if (ret != noErr || !cert_chain) {
        msg_Err(crd, "error while getting certificate chain");
        result = VLC_EGENERIC;
        goto out;
    }

    CFIndex num_cert_chain = CFArrayGetCount(cert_chain);

    /* Build up the certificate chain array expected by SSLSetCertificate */
    CFMutableArrayRef server_cert_chain = CFArrayCreateMutable(kCFAllocatorDefault, num_cert_chain, &kCFTypeArrayCallBacks);
    CFArrayAppendValue(server_cert_chain, cert_identity);

    msg_Dbg(crd, "Found certificate chain with %ld entries for server certificate", num_cert_chain);
    if (num_cert_chain > 1)
        CFArrayAppendArray(server_cert_chain, cert_chain, CFRangeMake(1, num_cert_chain - 1));
    CFRelease(cert_chain);

    vlc_tls_creds_sys_t *sys = malloc(sizeof(*sys));
    if (unlikely(sys == NULL)) {
        CFRelease(server_cert_chain);
        result = VLC_ENOMEM;
        goto out;
    }

    sys->server_cert_chain = server_cert_chain;
    sys->whitelist = NULL;

    crd->sys = sys;
    crd->open = st_ServerSessionOpen;

out:
    if (policy)
        CFRelease(policy);
    if (trust_ref)
        CFRelease(trust_ref);

    if (certificate)
        CFRelease(certificate);
    if (cert_identity)
        CFRelease(cert_identity);

    return result;
}

static void CloseServer (vlc_tls_creds_t *crd) {
    msg_Dbg(crd, "close secure transport server");

    vlc_tls_creds_sys_t *sys = crd->sys;

    if (sys->server_cert_chain)
        CFRelease(sys->server_cert_chain);

    free(sys);
}

#endif /* !TARGET_OS_IPHONE */