通过OpenSSL解析X509证书基本项

       在之前的文章“通过OpenSSL解码X509证书文件”里，讲述了如何使用OpenSSL将证书文件解码，得到证书上下文结构体X509的方法。下面我们接着讲述如何通过证书上下文结构体X509，获得想要的证书项。本文先讲述如何获取证书的基本项，后面还有文章介绍如何获取证书的扩展项。

       下面的代码，都是假定已经通过解码证书文件、得到了证书上下文结构体X509。至于如何使用OpenSSL解码证书文件、得到证书上下文结构体X509，请阅读之前的文章。

        首先，我们看看关于证书结构体X509定义：

struct x509_st
{
	X509_CINF *cert_info;
	X509_ALGOR *sig_alg;
	ASN1_BIT_STRING *signature;
	int valid;
	int references;
	char *name;
	CRYPTO_EX_DATA ex_data;
	/* These contain copies of various extension values */
	long ex_pathlen;
	long ex_pcpathlen;
	unsigned long ex_flags;
	unsigned long ex_kusage;
	unsigned long ex_xkusage;
	unsigned long ex_nscert;
	ASN1_OCTET_STRING *skid;
	AUTHORITY_KEYID *akid;
	X509_POLICY_CACHE *policy_cache;
	STACK_OF(DIST_POINT) *crldp;
	STACK_OF(GENERAL_NAME) *altname;
	NAME_CONSTRAINTS *nc;
#ifndef OPENSSL_NO_RFC3779
	STACK_OF(IPAddressFamily) *rfc3779_addr;
	struct ASIdentifiers_st *rfc3779_asid;
#endif
#ifndef OPENSSL_NO_SHA
	unsigned char sha1_hash[SHA_DIGEST_LENGTH];
#endif
	X509_CERT_AUX *aux;
} /* X509 */;

typedef struct x509_cinf_st
{
	ASN1_INTEGER *version;		/* [ 0 ] default of v1 */
	ASN1_INTEGER *serialNumber;
	X509_ALGOR *signature;
	X509_NAME *issuer;
	X509_VAL *validity;
	X509_NAME *subject;
	X509_PUBKEY *key;
	ASN1_BIT_STRING *issuerUID;		/* [ 1 ] optional in v2 */
	ASN1_BIT_STRING *subjectUID;		/* [ 2 ] optional in v2 */
	STACK_OF(X509_EXTENSION) *extensions;	/* [ 3 ] optional in v3 */
	ASN1_ENCODING enc;
} X509_CINF;

      我们想要获取的证书基本项，有些就直接存在于这两个结构体中。
一、版本号

      通过解码证书文件，得到证书结构体m_pX509之后，可以通过函数X509_get_version()获取证书的版本。具体代码如下：

int ver = X509_get_version(m_pX509);
switch(ver)  	
{
	case 0:		//V1
		//...
	break;
	case 1:		//V2
		//...
	break;
	case 2:		//V3
		//...
	break;
	default:
		//Error!
	break;
}

需要注意的是，0代表V1；1代表V2；2代表V3。目前绝大多数证书都是V3版本。

二、序列号

      同样，有了m_pX509之后，调用函数X509_get_serialNumber()即可获得证书的序列号。只是该函数返回的是ASN1_INTEGER类型，需要转换后才能是我们平常看到的十六进制表示的序列号。具体实现函数如下：

ULONG COpenSSLCertificate::get_SN(LPSTR lptcSN, ULONG *pulLen)
{
	ULONG ulRet = CERT_ERR_OK;
	ASN1_INTEGER *asn1_i = NULL;
	BIGNUM *bignum = NULL;
	char *serial = NULL;

	if (!m_pX509)
	{
	    return CERT_ERR_INVILIDCALL;
	}
	if (!pulLen)
	{
		return CERT_ERR_INVALIDPARAM;
	}
	asn1_i = X509_get_serialNumber(m_pX509);
	bignum = ASN1_INTEGER_to_BN(asn1_i, NULL);
	if (bignum == NULL) 
	{
		ulRet = CERT_ERR_FAILED;
		goto FREE_MEMORY;
	}
	serial = BN_bn2hex(bignum);
    if (serial == NULL) 
	{
		ulRet = CERT_ERR_FAILED;
		goto FREE_MEMORY;
	}
    BN_free(bignum);
	if (!lptcSN)
	{
		*pulLen = strlen(serial) + 1;
		ulRet = CERT_ERR_OK;
		goto FREE_MEMORY;
	}
	if (*pulLen < strlen(serial) + 1)
	{
		ulRet = CERT_ERR_BUFFER_TOO_SMALL;
		goto FREE_MEMORY;
	}
    strcpy_s(lptcSN, *pulLen, serial);
	*pulLen = strlen(serial);
FREE_MEMORY:
    OPENSSL_free(serial);	
	return ulRet;
}

三、公钥算法（证书算法）

      要想获取证书公钥算法，需要先调用函数X509_get_pubkey()得到公钥属性结构体，然后通过type字段来判断公钥的算法类型。具体实现函数如下：

ULONG COpenSSLCertificate::get_KeyType(ULONG* pulType)
{
	EVP_PKEY *pk = NULL;
	stack_st_X509* chain = NULL;
	X509_EXTENSION *pex = NULL;
	
	if (!m_pX509)
	{
		return CERT_ERR_INVILIDCALL;
	}
	if (!pulType)
	{
		return CERT_ERR_INVALIDPARAM;
	}

	pk = X509_get_pubkey(m_pX509);
	if (!pk)
	{
		return CERT_ERR_FAILED;
	}

	if (EVP_PKEY_RSA == pk->type)
	{
		*pulType = CERT_KEY_ALG_RSA;
	}
	else if (EVP_PKEY_EC == pk->type)
	{
		*pulType = CERT_KEY_ALG_ECC;
	}
	else if (EVP_PKEY_DSA == pk->type)
	{
		*pulType = CERT_KEY_ALG_DSA;
	}
	else if (EVP_PKEY_DH == pk->type)
	{
		*pulType = CERT_KEY_ALG_DH;
	}
	else
	{
		return CERT_KEY_ALG_UNKNOWN;
	}		
	
	return CERT_ERR_OK;
}

目前常见的证书算法为RSA和ECC，ECC在国内又成为SM2。SM2是国家密码管理局基于椭圆算法(ECC)制定的国内非对称算法标准。

四、证书用途

      证书从用途来分，分为“签名证书”和“加密证书”两大类。“签名证书”的公钥用来验证签名，而“加密证书”的公钥则用来加密数据。我们可以通过调用X509中的ex_kusage字段来判断证书的用途，具体函数实现如下：

ULONG COpenSSLCertificate::get_KeyUsage(ULONG* lpUsage)
{
	ULONG lKeyUsage = 0;

	if (!m_pX509)
	{
		return CERT_ERR_INVILIDCALL;
	}
	if (!lpUsage)
	{
		return CERT_ERR_INVALIDPARAM;
	}

	*lpUsage = CERT_USAGE_UNKNOWN;
	
	//X509_check_ca() MUST be called!
	X509_check_ca(m_pX509);
	lKeyUsage = m_pX509->ex_kusage;
	if ((lKeyUsage & KU_DATA_ENCIPHERMENT) == KU_DATA_ENCIPHERMENT)
	{
		*lpUsage = CERT_USAGE_EXCH;	//加密证书
	}
	else if ((lKeyUsage & KU_DIGITAL_SIGNATURE) == KU_DIGITAL_SIGNATURE)
	{
		*lpUsage = CERT_USAGE_SIGN;	//签名证书
	}

	return CERT_ERR_OK;
}

五、签名算法

      证书的签名算法，是指证书用来签名时使用的算法(包含HASH算法)。签名算法用结构体X509中sig_alg字段来表示，可以通过sig_alg的子字段algorithm返回签名算法对象，从而得到签名算法的Oid。首先，签名算法的Oid常见得定义如下：

/*	Certificate siganture alg */
#define CERT_SIGNATURE_ALG_RSA_RSA			"1.2.840.113549.1.1.1"
#define CERT_SIGNATURE_ALG_MD2RSA			"1.2.840.113549.1.1.2"
#define CERT_SIGNATURE_ALG_MD4RSA			"1.2.840.113549.1.1.3"
#define CERT_SIGNATURE_ALG_MD5RSA			"1.2.840.113549.1.1.4"
#define CERT_SIGNATURE_ALG_SHA1RSA			"1.2.840.113549.1.1.5"
#define CERT_SIGNATURE_ALG_SM3SM2			"1.2.156.10197.1.501"

      获取签名算法Oid的具体实现函数如下：

ULONG COpenSSLCertificate::get_SignatureAlgOid(LPSTR lpscOid, ULONG *pulLen)
{
	char oid[128] = {0};
	ASN1_OBJECT* salg  = NULL;

	if (!m_pX509)
	{
		return CERT_ERR_INVILIDCALL;
	}
	if (!pulLen)
	{
		return CERT_ERR_INVALIDPARAM;
	}

	salg = m_pX509->sig_alg->algorithm;
	OBJ_obj2txt(oid, 128, salg, 1);
	if (!lpscOid)
	{
		*pulLen = strlen(oid) + 1;
		return CERT_ERR_OK;
	}
	if (*pulLen < strlen(oid) + 1)
	{
		return CERT_ERR_BUFFER_TOO_SMALL;
	}

	strcpy_s(lpscOid, *pulLen, oid);
	*pulLen = strlen(oid) + 1;
	return CERT_ERR_OK;
}

由于Windows对SM2/SM3算法还没有定义，所以对于ECC(SM2)证书，Windows直接显示签名算法的Oid：“1.2.156.10197.1.501”，如下图所示：

六、颁发者

      关于颁发者，我们可以通过调用函数X509_get_issuer_name()获取属性。不过该函数返回的是X509_NAME类型，需要调用函数X509_NAME_get_text_by_NID()将其转化为ASCII字符形式。具体通过下面函数实现：

ULONG COpenSSLCertificate::get_Issuer(LPSTR lpValue, ULONG *pulLen)
{
	int nNameLen = 512;
	CHAR csCommonName[512] = {0};
	X509_NAME *pCommonName = NULL;

	if (!m_pX509)
	{
		return CERT_ERR_INVILIDCALL;
	}
	if (!pulLen)
	{
		return CERT_ERR_INVALIDPARAM;
	}

	pCommonName = X509_get_issuer_name(m_pX509);
	if (!pCommonName)
	{
		return CERT_ERR_FAILED;
	}
	nNameLen = X509_NAME_get_text_by_NID(pCommonName, NID_commonName, csCommonName, nNameLen);
	if (-1 == nNameLen)
	{
		return CERT_ERR_FAILED;
	};	
	if (!lpValue)
	{
		*pulLen = nNameLen + 1;
		return CERT_ERR_OK;
	}
	if (*pulLen < (ULONG)nNameLen + 1)
	{
		return CERT_ERR_BUFFER_TOO_SMALL;
	}

	strcpy_s(lpValue, *pulLen, csCommonName);
	*pulLen = nNameLen;
	return CERT_ERR_OK;
}

七、使用者

      关于证书使用者，我们可以通过调用函数X509_get_subject_name)获取属性。同样，该函数返回的是X509_NAME类型，需要调用函数X509_NAME_get_text_by_NID()将其转化为ASCII字符形式。具体通过下面函数实现：

ULONG COpenSSLCertificate::get_SubjectName(LPSTR lpValue, ULONG *pulLen)
{
	int iLen = 0;
	int iSubNameLen = 0;
	CHAR csSubName[1024] = {0};
	CHAR csBuf[256] = {0};
	X509_NAME *pSubName = NULL;
	
	if (!m_pX509)
	{
		return CERT_ERR_INVILIDCALL;
	}
	if (!pulLen)
	{
		return CERT_ERR_INVALIDPARAM;
	}

	pSubName = X509_get_subject_name(m_pX509);
	if (!pSubName)
	{
		return CERT_ERR_FAILED;
	}
	
	ZeroMemory(csBuf, 256);
	iLen = X509_NAME_get_text_by_NID(pSubName, NID_countryName, csBuf, 256);
	if (iLen > 0)
	{
		strcat_s(csSubName, 1024, "C=");
		strcat_s(csSubName, 1024, csBuf);
		strcat_s(csSubName, 1024, ", ");
	}
	
	ZeroMemory(csBuf, 256);
	iLen = X509_NAME_get_text_by_NID(pSubName, NID_organizationName, csBuf, 256);
	if (iLen > 0)
	{
		strcat_s(csSubName, 1024, "O=");
		strcat_s(csSubName, 1024, csBuf);
		strcat_s(csSubName, 1024, ", ");
	}
	
	ZeroMemory(csBuf, 256);
	iLen = X509_NAME_get_text_by_NID(pSubName, NID_organizationalUnitName, csBuf, 256);
	if (iLen > 0)
	{
		strcat_s(csSubName, 1024, "OU=");
		strcat_s(csSubName, 1024, csBuf);
		strcat_s(csSubName, 1024, ", ");
	}
	
	ZeroMemory(csBuf, 256);
	iLen = X509_NAME_get_text_by_NID(pSubName, NID_commonName, csBuf, 256);
	if (iLen > 0)
	{
		strcat_s(csSubName, 1024, "CN=");
		strcat_s(csSubName, 1024, csBuf);
	}
	
	if (!lpValue)
	{
		*pulLen = strlen(csSubName) + 1;
		return CERT_ERR_OK;
	}
	if (*pulLen < strlen(csSubName) + 1)
	{
		return CERT_ERR_BUFFER_TOO_SMALL;
	}
	
	strcpy_s(lpValue, *pulLen, csSubName);
	*pulLen = strlen(csSubName);
	return CERT_ERR_OK;
}

八、有效期限

      要获取证书的有效期属性，需要通过调用函数X509_get_notBefore()和X509_get_notAfter()来实现。而且这两个函数返回的时间是time_t类型，需要转化为SYSTEMTIME类型。具体实现函数如下：

ULONG COpenSSLCertificate::get_ValidDate(SYSTEMTIME *ptmStart, SYSTEMTIME *ptmEnd)
{
	int err = 0;
	ASN1_TIME *start = NULL;
	ASN1_TIME *end = NULL;
	time_t ttStart = {0};
	time_t ttEnd = {0};
	LONGLONG nLLStart = 0;
	LONGLONG nLLEnd = 0;
	FILETIME ftStart = {0};
	FILETIME ftEnd = {0};

	if (!m_pX509)
	{
		return CERT_ERR_INVALIDPARAM;
	}

	start = X509_get_notBefore(m_pX509);
	end = X509_get_notAfter(m_pX509);
	
	ttStart = ASN1_TIME_get(start, &err);
	ttEnd = ASN1_TIME_get(end, &err);	
    nLLStart = Int32x32To64(ttStart, 10000000) + 116444736000000000;
    nLLEnd = Int32x32To64(ttEnd, 10000000) + 116444736000000000;

    ftStart.dwLowDateTime = (DWORD)nLLStart;
    ftStart.dwHighDateTime = (DWORD)(nLLStart >> 32);
    ftEnd.dwLowDateTime = (DWORD)nLLEnd;
    ftEnd.dwHighDateTime = (DWORD)(nLLEnd >> 32);

    FileTimeToSystemTime(&ftStart, ptmStart);
    FileTimeToSystemTime(&ftEnd, ptmEnd);

	return 0;
}

      至此，X509证书的基本项通过OpenSLL均已解析完毕！如需获取证书的扩展项或者公钥等数据，请关注后续博文。