SSL连接建立过程分析(1)

Https协议:SSL建立过程分析

web訪问的两种方式:

http协议,我们普通情况下是通过它訪问web,由于它不要求太多的安全机制,使用起来也简单,非常多web网站也仅仅支持这样的方式下的訪问.

https协议(Hypertext Transfer Protocol over Secure Socket Layer),对于安全性要求比較高的情况,能够通过它訪问web,比方工商银行https://www.icbc.com.cn/icbc/(当然也能够通过http协议訪问,仅仅是没那么安全了).其安全基础是SSL协议.

SSL协议,当前版本号为3.1(SSL3.1就是TLS1.0)。它已被广泛地用于Web浏览器与server之间的身份认证和加密传输数据.它位于TCP/IP协议与各种应用层协议之间，为数据通讯提供安全支持。SSL协议可分为两层： SSL记录协议（SSL Record Protocol）：它建立在可靠的传输协议（如TCP）之上，为高层协议提供数据封装、压缩、加密等基本功能的支持。 SSL握手协议（SSL Handshake Protocol）：它建立在SSL记录协议之上，用于在实际的传输数据開始前，通讯两方进行身份认证、协商加密算法、交换加密密钥等。

为了了解具体过程,能够通过网络抓包工具(Commview,Iris)分析https协议,SSL连接建立过程中,数据包交换情况.

数据包分析过程用到的几个图.

图,SSL Protocol Stack

图.SSL Record Format

图.SSL Record Protocol Payload

图.Handshake Protocol Action

它们来之.Cryptography and Network Security Principles and Practices, Fourth Edition-Chapter 17. Web Security-17.2. Secure Socket Layer and Transport Layer Security(password学与网络安全原理与实践第四版,17章web安全,17.2节,SSL与TLS)详细细节參考本书.

以下跟踪握手过程(图Handshake Protocol Action)中,数据包的交换.

以为https方式訪问www.sun.com为样例,一般大型公司,银行的web都支持https訪问,如工商银行,sun,微软,IBM.

在IE中输入:https://wwww.sun.com,由于这是https协议,所以在实际訪问web前,会建立SSL连接.

通过Commview抓包工具,过滤443port(普通情况下,HTTPS使用port443，HTTP使用port80)能够得到数据包.

数据包大致情况和(图Handshake Protocol Action)相应.

SSL连接建立过程分析(1)

1. 应用程序接口
1.1 SSL初始化
SSL_CTX* InitSSL(int server, char *cert, char *key, char *pw)
{
    SSL_CTX* ctx;
    SSL_METHOD *meth;
    int status;
// 算法初始化
// 载入SSL错误信息
    SSL_load_error_strings();
// 加入SSL的加密/HASH算法
    SSLeay_add_ssl_algorithms();
// 服务器还是客户端
    If(server)
meth = SSLv23_server_method();
    else
meth = SSLv23_client_method();
// 建立新的SSL上下文
    ctx = SSL_CTX_new (meth);
    if(!ctx) return NULL;
// 设置证书文件的口令
    SSL_CTX_set_default_passwd_cb_userdata(ctx, pw);
//载入本地证书文件
    status=SSL_CTX_use_certificate_file(ctx, cert, SSL_FILETYPE_ASN1);
    if (status <= 0) {
        frintf(stderr, “Use cert fail, status=%d/n”, status);
        goto bad;
    }
// 载入私钥文件
    if (SSL_CTX_use_PrivateKey_file(ctx, key, SSL_FILETYPE_PEM) <= 0) {
        fprintf(stderr, “Use private key fail/n”);
        goto bad;
    }
// 检查证书和私钥是否匹配
    if (!SSL_CTX_check_private_key(ctx)) {
        fprintf(“Private key does not match the certificate public key/n”);
        goto bad;
    }
    fprintf(“Cert and key OK/n”);
    return ctx;
bad:
    SSL_CTX_free (ctx);
    return NULL;
}
1.2 建立SSL新连接
server：
// 建立SSL
ssl = SSL_new (ctx);
// 将SSL与TCP socket连接
SSL_set_fd (ssl, sd);
//接受新SSL连接
err = SSL_accept (ssl);
client：
// 建立SSL
ssl = SSL_new (ctx);
// 将SSL与TCP socket连接
SSL_set_fd (ssl, sd);
// SSL连接
err = SSL_connect (ssl);

server的SSL_accept()和client的SSL_connect()函数共同完毕SSL的握手协商过程。

1.3 SSL通信
和普通的read()/write()调用一样，用以下的函数完毕数据的SSL发送和接收，函数输入数据是明文，SSL自己主动将数据封装进SSL中：
读/接收：SSL_read()
写/发送：SSL_write()
1.4 SSL释放
SSL释放非常easy：
SSL_free (ssl);

2. SSL实现分析
下面SSL源码取自openssl-0.9.7b。

2.1 SSL_load_error_strings
该函数载入错误字符串信息：
void SSL_load_error_strings(void)
{
#ifndef OPENSSL_NO_ERR
ERR_load_crypto_strings();
ERR_load_SSL_strings();
#endif
}
最后将会进入函数：
static void err_load_strings(int lib, ERR_STRING_DATA *str)
{
while (str->error)
  {
  str->error|=ERR_PACK(lib,0,0);
  ERRFN(err_set_item)(str);
  str++;
  }
}
当中：
#define ERR_PACK(l,f,r)  (((((unsigned long)l)&0xffL)*0x1000000)| /
    ((((unsigned long)f)&0xfffL)*0x1000)| /
    ((((unsigned long)r)&0xfffL)))
#define ERRFN(a) err_fns->cb_##a
ERRFN(err_set_item)(str)的实际函数实现为：
static ERR_STRING_DATA *int_err_set_item(ERR_STRING_DATA *d)
{
ERR_STRING_DATA *p;
LHASH *hash;
err_fns_check();
hash = ERRFN(err_get)(1);
if (!hash)
  return NULL;
CRYPTO_w_lock(CRYPTO_LOCK_ERR);
p = (ERR_STRING_DATA *)lh_insert(hash, d);
CRYPTO_w_unlock(CRYPTO_LOCK_ERR);
return p;
}
Lh_insert()将错误信息插入到一个链表中
如关于加密算法的错误信息：
/* crypto/err/err.c */
static ERR_STRING_DATA ERR_str_functs[]=
……
static ERR_STRING_DATA ERR_str_libraries[]=
……
static ERR_STRING_DATA ERR_str_reasons[]=
……

2.2 SSLeay_add_ssl_algorithms()
这实际是个宏：
#define OpenSSL_add_ssl_algorithms()    SSL_library_init()
#define SSLeay_add_ssl_algorithms() SSL_library_init()
实际函数为SSL_library_init()，函数比較简单，就是载入各种加密和HASH算法：
/* ssl/ssl_algs.c */
int SSL_library_init(void)
{
#ifndef OPENSSL_NO_DES
EVP_add_cipher(EVP_des_cbc());
EVP_add_cipher(EVP_des_ede3_cbc());
#endif
#ifndef OPENSSL_NO_IDEA
EVP_add_cipher(EVP_idea_cbc());
#endif
#ifndef OPENSSL_NO_RC4
EVP_add_cipher(EVP_rc4());
#endif
#ifndef OPENSSL_NO_RC2
EVP_add_cipher(EVP_rc2_cbc());
#endif
#ifndef OPENSSL_NO_AES
EVP_add_cipher(EVP_aes_128_cbc());
EVP_add_cipher(EVP_aes_192_cbc());
EVP_add_cipher(EVP_aes_256_cbc());
#endif
#ifndef OPENSSL_NO_MD2
EVP_add_digest(EVP_md2());
#endif
#ifndef OPENSSL_NO_MD5
EVP_add_digest(EVP_md5());
EVP_add_digest_alias(SN_md5,”ssl2-md5″);
EVP_add_digest_alias(SN_md5,”ssl3-md5″);
#endif
#ifndef OPENSSL_NO_SHA
EVP_add_digest(EVP_sha1()); /* RSA with sha1 */
EVP_add_digest_alias(SN_sha1,”ssl3-sha1″);
EVP_add_digest_alias(SN_sha1WithRSAEncryption,SN_sha1WithRSA);
#endif
#if !defined(OPENSSL_NO_SHA) && !defined(OPENSSL_NO_DSA)
EVP_add_digest(EVP_dss1()); /* DSA with sha1 */
EVP_add_digest_alias(SN_dsaWithSHA1,SN_dsaWithSHA1_2);
EVP_add_digest_alias(SN_dsaWithSHA1,”DSS1″);
EVP_add_digest_alias(SN_dsaWithSHA1,”dss1″);
#endif
/* If you want support for phased out ciphers, add the following */
#if 0
EVP_add_digest(EVP_sha());
EVP_add_digest(EVP_dss());
#endif
return(1);
}

2.3 SSL23_server_method()
建立服务器端的方法库，这是个通用函数，可动态选择SSL协议。假设想固定协议，能够仅仅用SSLv2_server_method(), SSLv3_server_method() 等函数来初始化，该函数返回一个SSL_METHOD结构：
/* ssl/ssl.h */
/* Used to hold functions for SSLv2 or SSLv3/TLSv1 functions */
typedef struct ssl_method_st
{
int version; // 版本号号
int (*ssl_new)(SSL *s); // 建立新SSL
void (*ssl_clear)(SSL *s); // 清除SSL
void (*ssl_free)(SSL *s); // 释放SSL
int (*ssl_accept)(SSL *s); // server接受SSL连接
int (*ssl_connect)(SSL *s); // client的SSL连接
int (*ssl_read)(SSL *s,void *buf,int len); // SSL读
int (*ssl_peek)(SSL *s,void *buf,int len); // SSL查看数据
int (*ssl_write)(SSL *s,const void *buf,int len); // SSL写
int (*ssl_shutdown)(SSL *s); // SSL半关闭
int (*ssl_renegotiate)(SSL *s); // SSL重协商
int (*ssl_renegotiate_check)(SSL *s); // SSL重协商检查
long (*ssl_ctrl)(SSL *s,int cmd,long larg,void *parg); // SSL控制
long (*ssl_ctx_ctrl)(SSL_CTX *ctx,int cmd,long larg,void *parg); //SSL上下文控制
SSL_CIPHER *(*get_cipher_by_char)(const unsigned char *ptr); // 通过名称获取SSL的算法
int (*put_cipher_by_char)(const SSL_CIPHER *cipher,unsigned char *ptr);
int (*ssl_pending)(SSL *s);
int (*num_ciphers)(void); // 算法数
SSL_CIPHER *(*get_cipher)(unsigned ncipher); // 获取算法
struct ssl_method_st *(*get_ssl_method)(int version);
long (*get_timeout)(void); // 超时
struct ssl3_enc_method *ssl3_enc; /* Extra SSLv3/TLS stuff */ // SSL3加密
int (*ssl_version)(); // SSL版本号
long (*ssl_callback_ctrl)(SSL *s, int cb_id, void (*fp)()); // SSL控制回调函数
long (*ssl_ctx_callback_ctrl)(SSL_CTX *s, int cb_id, void (*fp)()); //SSL上下文控制回调函数
} SSL_METHOD;

/* ssl/s23_srvr.c */
SSL_METHOD *SSLv23_server_method(void)
{
static int init=1;
// 静态量，每一个进程仅仅初始化一次
static SSL_METHOD SSLv23_server_data;
if (init)
  {
  CRYPTO_w_lock(CRYPTO_LOCK_SSL_METHOD);
  if (init)
   {
// ssl23的基本方法结构
   memcpy((char *)&SSLv23_server_data,
    (char *)sslv23_base_method(),sizeof(SSL_METHOD));
// 服务器,所以要定义accept方法
   SSLv23_server_data.ssl_accept=ssl23_accept;
// 依据SSL的版本号设置SSL的详细方法函数
   SSLv23_server_data.get_ssl_method=ssl23_get_server_method;
   init=0;
   }
  CRYPTO_w_unlock(CRYPTO_LOCK_SSL_METHOD);
  }
return(&SSLv23_server_data);
}

static SSL_METHOD *ssl23_get_server_method(int ver)
{
#ifndef OPENSSL_NO_SSL2
if (ver == SSL2_VERSION)
  return(SSLv2_server_method());
#endif
if (ver == SSL3_VERSION)
  return(SSLv3_server_method());
else if (ver == TLS1_VERSION)
  return(TLSv1_server_method());
// 随着TLS1.1(RFC4346)的推出，预计不久将出现TLSv1_1_server_method()
else
  return(NULL);
}
// SSL23的方法基本数据定义
/* ssl/s23_lib.c */
SSL_METHOD *sslv23_base_method(void)
{
return(&SSLv23_data);
}
static SSL_METHOD SSLv23_data= {
TLS1_VERSION,
tls1_new,
tls1_clear,
tls1_free,
ssl_undefined_function,
ssl_undefined_function,
ssl23_read,
ssl23_peek,
ssl23_write,
ssl_undefined_function,
ssl_undefined_function,
ssl_ok,
ssl3_ctrl,
ssl3_ctx_ctrl,
ssl23_get_cipher_by_char,
ssl23_put_cipher_by_char,
ssl_undefined_function,
ssl23_num_ciphers,
ssl23_get_cipher,
ssl_bad_method,
ssl23_default_timeout,
&ssl3_undef_enc_method,
ssl_undefined_function,
ssl3_callback_ctrl,
ssl3_ctx_callback_ctrl,
};
以SSL3的server方法函数为例，其它方法相似：
/* ssl/s3_srvr.c */
SSL_METHOD *SSLv3_server_method(void)
{
static int init=1;
static SSL_METHOD SSLv3_server_data;
// 仅仅初始化一次
if (init)
  {
  CRYPTO_w_lock(CRYPTO_LOCK_SSL_METHOD);
  if (init)
   {
// ssl3的基本方法结构
   memcpy((char *)&SSLv3_server_data,(char *)sslv3_base_method(),
    sizeof(SSL_METHOD));
// ssl3的接受方法
   SSLv3_server_data.ssl_accept=ssl3_accept;
// ssl3获取服务器的方法函数
   SSLv3_server_data.get_ssl_method=ssl3_get_server_method;
   init=0;
   }

  CRYPTO_w_unlock(CRYPTO_LOCK_SSL_METHOD);
  }
return(&SSLv3_server_data);
}
// SSL3的方法基本数据定义
/* ssl/s3_lib.c */
static SSL_METHOD SSLv3_data= {
SSL3_VERSION,
ssl3_new,
ssl3_clear,
ssl3_free,
ssl_undefined_function,
ssl_undefined_function,
ssl3_read,
ssl3_peek,
ssl3_write,
ssl3_shutdown,
ssl3_renegotiate,
ssl3_renegotiate_check,
ssl3_ctrl,
ssl3_ctx_ctrl,
ssl3_get_cipher_by_char,
ssl3_put_cipher_by_char,
ssl3_pending,
ssl3_num_ciphers,
ssl3_get_cipher,
ssl_bad_method,
ssl3_default_timeout,
&SSLv3_enc_data,
ssl_undefined_function,
ssl3_callback_ctrl,
ssl3_ctx_callback_ctrl,
};

2.4 SSL23_client_method()

和server端的事实上是同样的，仅仅是不定义结构中的ssl_accept而是定义ssl_connnect：
SSL_METHOD *SSLv23_client_method(void)
{
static int init=1;
static SSL_METHOD SSLv23_client_data;
if (init)
  {
  CRYPTO_w_lock(CRYPTO_LOCK_SSL_METHOD);
  if (init)
   {
   memcpy((char *)&SSLv23_client_data,
    (char *)sslv23_base_method(),sizeof(SSL_METHOD));
   SSLv23_client_data.ssl_connect=ssl23_connect;
   SSLv23_client_data.get_ssl_method=ssl23_get_client_method;
   init=0;
   }
  CRYPTO_w_unlock(CRYPTO_LOCK_SSL_METHOD);
  }
return(&SSLv23_client_data);
}

2.5 SSL_CTX_new ()
该函数依据SSL方法获取一个SSL上下文结构，该结构定义为：
/* ssl/ssl.h */
struct ssl_ctx_st
{
SSL_METHOD *method;
STACK_OF(SSL_CIPHER) *cipher_list;
/* same as above but sorted for lookup */
STACK_OF(SSL_CIPHER) *cipher_list_by_id;
struct x509_store_st /* X509_STORE */ *cert_store;
struct lhash_st /* LHASH */ *sessions; /* a set of SSL_SESSIONs */
/* Most session-ids that will be cached, default is
* SSL_SESSION_CACHE_MAX_SIZE_DEFAULT. 0 is unlimited. */
unsigned long session_cache_size;
struct ssl_session_st *session_cache_head;
struct ssl_session_st *session_cache_tail;
/* This can have one of 2 values, ored together,
* SSL_SESS_CACHE_CLIENT,
* SSL_SESS_CACHE_SERVER,
* Default is SSL_SESSION_CACHE_SERVER, which means only
* SSL_accept which cache SSL_SESSIONS. */
int session_cache_mode;
/* If timeout is not 0, it is the default timeout value set
* when SSL_new() is called. This has been put in to make
* life easier to set things up */
long session_timeout;
/* If this callback is not null, it will be called each
* time a session id is added to the cache. If this function
* returns 1, it means that the callback will do a
* SSL_SESSION_free() when it has finished using it. Otherwise,
* on 0, it means the callback has finished with it.
* If remove_session_cb is not null, it will be called when
* a session-id is removed from the cache. After the call,
* OpenSSL will SSL_SESSION_free() it. */
int (*new_session_cb)(struct ssl_st *ssl,SSL_SESSION *sess);
void (*remove_session_cb)(struct ssl_ctx_st *ctx,SSL_SESSION *sess);
SSL_SESSION *(*get_session_cb)(struct ssl_st *ssl,
  unsigned char *data,int len,int *copy);
struct
  {
  int sess_connect; /* SSL new conn – started */
  int sess_connect_renegotiate;/* SSL reneg – requested */
  int sess_connect_good; /* SSL new conne/reneg – finished */
  int sess_accept; /* SSL new accept – started */
  int sess_accept_renegotiate;/* SSL reneg – requested */
  int sess_accept_good; /* SSL accept/reneg – finished */
  int sess_miss;  /* session lookup misses */
  int sess_timeout; /* reuse attempt on timeouted session */
  int sess_cache_full; /* session removed due to full cache */
  int sess_hit;  /* session reuse actually done */
  int sess_cb_hit; /* session-id that was not
      * in the cache was
      * passed back via the callback. This
      * indicates that the application is
      * supplying session-id’s from other
      * processes – spooky 🙂 */
  } stats;
int references;
/* if defined, these override the X509_verify_cert() calls */
int (*app_verify_callback)(X509_STORE_CTX *, void *);
void *app_verify_arg;
/* before OpenSSL 0.9.7, ‘app_verify_arg’ was ignored
* (‘app_verify_callback’ was called with just one argument) */
/* Default password callback. */
pem_password_cb *default_passwd_callback;
/* Default password callback user data. */
void *default_passwd_callback_userdata;
/* get client cert callback */
int (*client_cert_cb)(SSL *ssl, X509 **x509, EVP_PKEY **pkey);
CRYPTO_EX_DATA ex_data;
const EVP_MD *rsa_md5;/* For SSLv2 – name is ‘ssl2-md5’ */
const EVP_MD *md5; /* For SSLv3/TLSv1 ‘ssl3-md5’ */
const EVP_MD *sha1;   /* For SSLv3/TLSv1 ‘ssl3->sha1’ */
STACK_OF(X509) *extra_certs;
STACK_OF(SSL_COMP) *comp_methods; /* stack of SSL_COMP, SSLv3/TLSv1 */

/* Default values used when no per-SSL value is defined follow */
void (*info_callback)(const SSL *ssl,int type,int val); /* used if SSL’s info_callback is NULL */
/* what we put in client cert requests */
STACK_OF(X509_NAME) *client_CA;

/* Default values to use in SSL structures follow (these are copied by SSL_new) */
unsigned long options;
unsigned long mode;
long max_cert_list;
struct cert_st /* CERT */ *cert;
int read_ahead;
/* callback that allows applications to peek at protocol messages */
void (*msg_callback)(int write_p, int version, int content_type, const void *buf, size_t len, SSL *ssl, void *arg);
void *msg_callback_arg;
int verify_mode;
int verify_depth;
unsigned int sid_ctx_length;
unsigned char sid_ctx[SSL_MAX_SID_CTX_LENGTH];
int (*default_verify_callback)(int ok,X509_STORE_CTX *ctx); /* called ‘verify_callback’ in the SSL */
/* Default generate session ID callback. */
GEN_SESSION_CB generate_session_id;
int purpose;  /* Purpose setting */
int trust;  /* Trust setting */
int quiet_shutdown;
};

typedef struct ssl_ctx_st SSL_CTX;
/* ssl/ssl_lib.h */
SSL_CTX *SSL_CTX_new(SSL_METHOD *meth)
{
SSL_CTX *ret=NULL;

if (meth == NULL)
  {
  SSLerr(SSL_F_SSL_CTX_NEW,SSL_R_NULL_SSL_METHOD_PASSED);
  return(NULL);
  }
if (SSL_get_ex_data_X509_STORE_CTX_idx() < 0)
  {
  SSLerr(SSL_F_SSL_CTX_NEW,SSL_R_X509_VERIFICATION_SETUP_PROBLEMS);
  goto err;
  }
// 分配上下文的内存空间
ret=(SSL_CTX *)OPENSSL_malloc(sizeof(SSL_CTX));
if (ret == NULL)
  goto err;
memset(ret,0,sizeof(SSL_CTX));

// 初始化上下文的结构參数
ret->method=meth;
ret->cert_store=NULL;
ret->session_cache_mode=SSL_SESS_CACHE_SERVER;
ret->session_cache_size=SSL_SESSION_CACHE_MAX_SIZE_DEFAULT;
ret->session_cache_head=NULL;
ret->session_cache_tail=NULL;
/* We take the system default */
ret->session_timeout=meth->get_timeout();
ret->new_session_cb=0;
ret->remove_session_cb=0;
ret->get_session_cb=0;
ret->generate_session_id=0;
memset((char *)&ret->stats,0,sizeof(ret->stats));
ret->references=1;
ret->quiet_shutdown=0;
/* ret->cipher=NULL;*/
/* ret->s2->challenge=NULL;
ret->master_key=NULL;
ret->key_arg=NULL;
ret->s2->conn_id=NULL; */
ret->info_callback=NULL;
ret->app_verify_callback=0;
ret->app_verify_arg=NULL;
ret->max_cert_list=SSL_MAX_CERT_LIST_DEFAULT;
ret->read_ahead=0;
ret->msg_callback=0;
ret->msg_callback_arg=NULL;
ret->verify_mode=SSL_VERIFY_NONE;
ret->verify_depth=-1; /* Don’t impose a limit (but x509_lu.c does) */
ret->sid_ctx_length=0;
ret->default_verify_callback=NULL;
if ((ret->cert=ssl_cert_new()) == NULL)
  goto err;
ret->default_passwd_callback=0;
ret->default_passwd_callback_userdata=NULL;
ret->client_cert_cb=0;
ret->sessions=lh_new(LHASH_HASH_FN(SSL_SESSION_hash),
   LHASH_COMP_FN(SSL_SESSION_cmp));
if (ret->sessions == NULL) goto err;
ret->cert_store=X509_STORE_new();
if (ret->cert_store == NULL) goto err;

// 建立加密算法链表
ssl_create_cipher_list(ret->method,
  &ret->cipher_list,&ret->cipher_list_by_id,
  SSL_DEFAULT_CIPHER_LIST);
if (ret->cipher_list == NULL
     || sk_SSL_CIPHER_num(ret->cipher_list) <= 0)
  {
  SSLerr(SSL_F_SSL_CTX_NEW,SSL_R_LIBRARY_HAS_NO_CIPHERS);
  goto err2;
  }

// 定义上下文结构中HASH算法
if ((ret->rsa_md5=EVP_get_digestbyname(“ssl2-md5”)) == NULL)
  {
  SSLerr(SSL_F_SSL_CTX_NEW,SSL_R_UNABLE_TO_LOAD_SSL2_MD5_ROUTINES);
  goto err2;
  }
if ((ret->md5=EVP_get_digestbyname(“ssl3-md5”)) == NULL)
  {
  SSLerr(SSL_F_SSL_CTX_NEW,SSL_R_UNABLE_TO_LOAD_SSL3_MD5_ROUTINES);
  goto err2;
  }
if ((ret->sha1=EVP_get_digestbyname(“ssl3-sha1”)) == NULL)
  {
  SSLerr(SSL_F_SSL_CTX_NEW,SSL_R_UNABLE_TO_LOAD_SSL3_SHA1_ROUTINES);
  goto err2;
  }
if ((ret->client_CA=sk_X509_NAME_new_null()) == NULL)
  goto err;

CRYPTO_new_ex_data(CRYPTO_EX_INDEX_SSL_CTX, ret, &ret->ex_data);
ret->extra_certs=NULL;

// 压缩算法
ret->comp_methods=SSL_COMP_get_compression_methods();
return(ret);

err:
SSLerr(SSL_F_SSL_CTX_NEW,ERR_R_MALLOC_FAILURE);
err2:
if (ret != NULL) SSL_CTX_free(ret);
return(NULL);
}

…待续…