阅读密钥用法的最简单解决方案似乎是
X509* x509_cert = ...
// without X509_check_ca x509_cert->ex_kusage always returns 0 (no idea why)
int ca = X509_check_ca(x509_cert);
unsigned long usage = x509_cert->ex_kusage;
结果值在 opensc/pkcs15-init.h 中定义
SC_PKCS15INIT_X509_DIGITAL_SIGNATURE 0x0080UL
SC_PKCS15INIT_X509_NON_REPUDIATION 0x0040UL
SC_PKCS15INIT_X509_KEY_ENCIPHERMENT 0x0020UL
SC_PKCS15INIT_X509_DATA_ENCIPHERMENT 0x0010UL
SC_PKCS15INIT_X509_KEY_AGREEMENT 0x0008UL
SC_PKCS15INIT_X509_KEY_CERT_SIGN 0x0004UL
SC_PKCS15INIT_X509_CRL_SIGN 0x0002UL
我通过找到以下openssl源文件的代码来到了那个灵魂
/* Handle key usage */
if((usage=X509_get_ext_d2i(x, NID_key_usage, NULL, NULL))) {
if(usage->length > 0) {
x->ex_kusage = usage->data[0];
if(usage->length > 1)
x->ex_kusage |= usage->data[1] << 8;
} else x->ex_kusage = 0;
x->ex_flags |= EXFLAG_KUSAGE;
ASN1_BIT_STRING_free(usage);
}