OpenSSL源代码分析

OpenSSL的源代码包括三部分：加密算法库、SSL库和应用程序。

加密算法库的源代码主要在crypto文件夹里，包括ASN.1编码与解码接口（crypto/asn1/asn1.h），伪随机数产生器（crypto/rand/rand.h），ENGINE机制（crypto/engine），统一密码算法的EVP密码算法接口（crypto/evp/evp.h），大数运算接口（crypto/bn/bn.h），私钥信息语法（crypto/x509/x509.h），非对称密码算法（crypto/rsa/ras.h）等。

SSL库的源代码主要在ssl文件夹里，我们重点分析。

SSL相关的几个重要的数据结构包括：

1、SSL连接结构：核心结构，应用程序通过该结构获取所有其他结构。

2、上下文结构SSL_CTX：是全局的上下文结构，含有结构SSL的主要默认值，包含SSL会话结构。

3、会话结构SSL_SESSION：含有链接的当前TLS/SSL会话细节。

4、密码结构SSL_CIPHER：含有制定的加密算法信息。

5、方法结构SSL_METHOD：功能函数接口，统一了各种SSL协议版本（SSLv1, SSLv2, SSLv3, TLSv1）。

SSL连接结构列出如下：

struct ssl_st
    {
    /* protocol version
     * (one of SSL2_VERSION, SSL3_VERSION, TLS1_VERSION, DTLS1_VERSION)
     */
    int version;
    int type; /* SSL_ST_CONNECT or SSL_ST_ACCEPT */

    const SSL_METHOD *method; /* SSLv3 */

    /* There are 2 BIO's even though they are normally both the
     * same.  This is so data can be read and written to different
     * handlers */

#ifndef OPENSSL_NO_BIO
    BIO *rbio; /* used by SSL_read */
    BIO *wbio; /* used by SSL_write */
    BIO *bbio; /* used during session-id reuse to concatenate
            * messages */
#else
    char *rbio; /* used by SSL_read */
    char *wbio; /* used by SSL_write */
    char *bbio;
#endif
    /* This holds a variable that indicates what we were doing
     * when a 0 or -1 is returned.  This is needed for
     * non-blocking IO so we know what request needs re-doing when
     * in SSL_accept or SSL_connect */
    int rwstate;

    /* true when we are actually in SSL_accept() or SSL_connect() */
    int in_handshake;
    int (*handshake_func)(SSL *);

    /* Imagine that here's a boolean member "init" that is
     * switched as soon as SSL_set_{accept/connect}_state
     * is called for the first time, so that "state" and
     * "handshake_func" are properly initialized.  But as
     * handshake_func is == 0 until then, we use this
     * test instead of an "init" member.
     */

    int server; /* are we the server side? - mostly used by SSL_clear*/

    int new_session;/* 1 if we are to use a new session.
                     * 2 if we are a server and are inside a handshake
                     *   (i.e. not just sending a HelloRequest)
                     * NB: For servers, the 'new' session may actually be a previously
                     * cached session or even the previous session unless
                     * SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION is set */
    int quiet_shutdown;/* don't send shutdown packets */
    int shutdown;   /* we have shut things down, 0x01 sent, 0x02
             * for received */
    int state;  /* where we are */
    int rstate; /* where we are when reading */

    BUF_MEM *init_buf;  /* buffer used during init */
    void *init_msg;     /* pointer to handshake message body, set by ssl3_get_message() */
    int init_num;       /* amount read/written */
    int init_off;       /* amount read/written */

    /* used internally to point at a raw packet */
    unsigned char *packet;
    unsigned int packet_length;

    struct ssl2_state_st *s2; /* SSLv2 variables */
    struct ssl3_state_st *s3; /* SSLv3 variables */
    struct dtls1_state_st *d1; /* DTLSv1 variables */

    int read_ahead;     /* Read as many input bytes as possible
                         * (for non-blocking reads) */

    /* 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 hit;        /* reusing a previous session */

    X509_VERIFY_PARAM *param;

#if 0
    int purpose;        /* Purpose setting */
    int trust;      /* Trust setting */
#endif

    /* crypto */
    STACK_OF(SSL_CIPHER) *cipher_list;
    STACK_OF(SSL_CIPHER) *cipher_list_by_id;

    /* These are the ones being used, the ones in SSL_SESSION are
     * the ones to be 'copied' into these ones */
    int mac_flags;
    EVP_CIPHER_CTX *enc_read_ctx;       /* cryptographic state */
    EVP_MD_CTX *read_hash;      /* used for mac generation */
#ifndef OPENSSL_NO_COMP
    COMP_CTX *expand;           /* uncompress */
#else
    char *expand;
#endif

    EVP_CIPHER_CTX *enc_write_ctx;      /* cryptographic state */
    EVP_MD_CTX *write_hash;     /* used for mac generation */
#ifndef OPENSSL_NO_COMP
    COMP_CTX *compress;         /* compression */
#else
    char *compress;
#endif

    /* session info */

    /* client cert? */
    /* This is used to hold the server certificate used */
    struct cert_st /* CERT */ *cert;

    /* the session_id_context is used to ensure sessions are only reused
     * in the appropriate context */
    unsigned int sid_ctx_length;
    unsigned char sid_ctx[SSL_MAX_SID_CTX_LENGTH];

    /* This can also be in the session once a session is established */
    SSL_SESSION *session;

    /* Default generate session ID callback. */
    GEN_SESSION_CB generate_session_id;

    /* Used in SSL2 and SSL3 */
    int verify_mode;    /* 0 don't care about verify failure.
                 * 1 fail if verify fails */
    int (*verify_callback)(int ok,X509_STORE_CTX *ctx); /* fail if callback returns 0 */

    void (*info_callback)(const SSL *ssl,int type,int val); /* optional informational callback */

    int error;      /* error bytes to be written */
    int error_code;     /* actual code */

#ifndef OPENSSL_NO_KRB5
    KSSL_CTX *kssl_ctx;     /* Kerberos 5 context */
#endif  /* OPENSSL_NO_KRB5 */

#ifndef OPENSSL_NO_PSK
    unsigned int (*psk_client_callback)(SSL *ssl, const char *hint, char *identity,
        unsigned int max_identity_len, unsigned char *psk,
        unsigned int max_psk_len);
    unsigned int (*psk_server_callback)(SSL *ssl, const char *identity,
        unsigned char *psk, unsigned int max_psk_len);
#endif

    SSL_CTX *ctx;
    /* set this flag to 1 and a sleep(1) is put into all SSL_read()
     * and SSL_write() calls, good for nbio debuging :-) */
    int debug;

    /* extra application data */
    long verify_result;
    CRYPTO_EX_DATA ex_data;

    /* for server side, keep the list of CA_dn we can use */
    STACK_OF(X509_NAME) *client_CA;

    int references;
    unsigned long options; /* protocol behaviour */
    unsigned long mode; /* API behaviour */
    long max_cert_list;
    int first_packet;
    int client_version; /* what was passed, used for
                 * SSLv3/TLS rollback check */
    unsigned int max_send_fragment;
#ifndef OPENSSL_NO_TLSEXT
    /* TLS extension debug callback */
    void (*tlsext_debug_cb)(SSL *s, int client_server, int type,
                    unsigned char *data, int len,
                    void *arg);
    void *tlsext_debug_arg;
    char *tlsext_hostname;
    int servername_done;   /* no further mod of servername
                              0 : call the servername extension callback.
                              1 : prepare 2, allow last ack just after in server callback.
                              2 : don't call servername callback, no ack in server hello
                           */
    /* certificate status request info */
    /* Status type or -1 if no status type */
    int tlsext_status_type;
    /* Expect OCSP CertificateStatus message */
    int tlsext_status_expected;
    /* OCSP status request only */
    STACK_OF(OCSP_RESPID) *tlsext_ocsp_ids;
    X509_EXTENSIONS *tlsext_ocsp_exts;
    /* OCSP response received or to be sent */
    unsigned char *tlsext_ocsp_resp;
    int tlsext_ocsp_resplen;

    /* RFC4507 session ticket expected to be received or sent */
    int tlsext_ticket_expected;
#ifndef OPENSSL_NO_EC
    size_t tlsext_ecpointformatlist_length;
    unsigned char *tlsext_ecpointformatlist; /* our list */
    size_t tlsext_ellipticcurvelist_length;
    unsigned char *tlsext_ellipticcurvelist; /* our list */
#endif /* OPENSSL_NO_EC */

    /* draft-rescorla-tls-opaque-prf-input-00.txt information to be used for handshakes */
    void *tlsext_opaque_prf_input;
    size_t tlsext_opaque_prf_input_len;

    /* TLS Session Ticket extension override */
    TLS_SESSION_TICKET_EXT *tlsext_session_ticket;

    /* TLS Session Ticket extension callback */
    tls_session_ticket_ext_cb_fn tls_session_ticket_ext_cb;
    void *tls_session_ticket_ext_cb_arg;

    /* TLS pre-shared secret session resumption */
    tls_session_secret_cb_fn tls_session_secret_cb;
    void *tls_session_secret_cb_arg;

    SSL_CTX * initial_ctx; /* initial ctx, used to store sessions */
#define session_ctx initial_ctx
#else
#define session_ctx ctx
#endif /* OPENSSL_NO_TLSEXT */
    };

列出SSL方法结构SSL_METHOD如下：

/* Used to hold functions for SSLv2 or SSLv3/TLSv1 functions */
typedef struct ssl_method_st
    {
    int version;
    int (*ssl_new)(SSL *s);
    void (*ssl_clear)(SSL *s);
    void (*ssl_free)(SSL *s);
    int (*ssl_accept)(SSL *s);
    int (*ssl_connect)(SSL *s);
    int (*ssl_read)(SSL *s,void *buf,int len);
    int (*ssl_peek)(SSL *s,void *buf,int len);
    int (*ssl_write)(SSL *s,const void *buf,int len);
    int (*ssl_shutdown)(SSL *s);
    int (*ssl_renegotiate)(SSL *s);
    int (*ssl_renegotiate_check)(SSL *s);
    long (*ssl_get_message)(SSL *s, int st1, int stn, int mt, long
        max, int *ok);
    int (*ssl_read_bytes)(SSL *s, int type, unsigned char *buf, int len,
        int peek);
    int (*ssl_write_bytes)(SSL *s, int type, const void *buf_, int len);
    int (*ssl_dispatch_alert)(SSL *s);
    long (*ssl_ctrl)(SSL *s,int cmd,long larg,void *parg);
    long (*ssl_ctx_ctrl)(SSL_CTX *ctx,int cmd,long larg,void *parg);
    const SSL_CIPHER *(*get_cipher_by_char)(const unsigned char *ptr);
    int (*put_cipher_by_char)(const SSL_CIPHER *cipher,unsigned char *ptr);
    int (*ssl_pending)(const SSL *s);
    int (*num_ciphers)(void);
    const SSL_CIPHER *(*get_cipher)(unsigned ncipher);
    const struct ssl_method_st *(*get_ssl_method)(int version);
    long (*get_timeout)(void);
    struct ssl3_enc_method *ssl3_enc; /* Extra SSLv3/TLS stuff */
    int (*ssl_version)(void);
    long (*ssl_callback_ctrl)(SSL *s, int cb_id, void (*fp)(void));
    long (*ssl_ctx_callback_ctrl)(SSL_CTX *s, int cb_id, void (*fp)(void));
    } SSL_METHOD;