使用充气城堡从证书中提取密钥用法扩展

Question

>我正在编写一个小的CA实现。可以从现有证书引导此 CA。执行此操作后，我想验证输入是否具有正确的扩展名：

private static final Set<String> REQUIRED_CA_EXTENSIONS = Set.of(
Extension.keyUsage.getId(),
Extension.subjectKeyIdentifier.getId());
private static void validateExtensions(final X509Certificate certificate) {
if (!CertificateExtensions.getAll(certificate).containsAll(REQUIRED_CA_EXTENSIONS)) {
throw new RuntimeException("Attempted to create a CA from a certificate without required extensions");
}
}
// Util method
public static Set<String> getAll(final X509Certificate certificate) {
final Set<String> extensions = new HashSet<>();
extensions.addAll(certificate.getCriticalExtensionOIDs());
extensions.addAll(certificate.getNonCriticalExtensionOIDs());
return extensions;
}

但是，这仅验证扩展是否存在。我还需要验证keyUsage扩展是否包含keyCertSign和cRLSign，以便能够签署证书。

如何使用充气城堡和/或JCA来做到这一点？

Answer 1

JCAX509Certificate上有一个方法可用于返回 keyUsage 扩展的位，简称为getKeyUsage()。根据 Java 文档，返回的数组中的布尔值表示的密钥用法的各个位。

也可以使用 Bouncycastle 库做更多的工作来完成相同的结果。我展示了这两种方法，因为 Bouncycastle 库为检查 X509TCertificate 提供了更完整的支持，因此，如果您想做更困难的事情，有一个示例来说明一些简单的事情很有用。

请注意，在撰写本文时，这需要两个Bouncycastle库，主提供程序/加密库以及PKIX/CMS库。

import org.bouncycastle.asn1.ASN1Primitive;
import org.bouncycastle.asn1.x509.KeyUsage;
import org.bouncycastle.cert.X509CertificateHolder;
import java.io.FileInputStream;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
public class X509CheckKeyUsage {
public static void main(String[] args) throws Exception {
FileInputStream fis = new FileInputStream("DST_X3_CA.pem"); // for example
CertificateFactory cf = CertificateFactory.getInstance("X.509");
X509Certificate cert = (X509Certificate) cf.generateCertificate(fis);
// check with simple JCA methods
boolean [] keyUsageBools = cert.getKeyUsage();
final int KEY_CERT_SIGN = 5;
final int CRL_SIGN = 6;
boolean usagesVerified = keyUsageBools[KEY_CERT_SIGN] && keyUsageBools[CRL_SIGN];
System.out.println("key usage bits verified? " + usagesVerified);
// Convert the jca x.509 cert to a bouncycastle x.509 cert, in two steps
org.bouncycastle.asn1.x509.Certificate bcCert = org.bouncycastle.asn1.x509.Certificate
.getInstance(ASN1Primitive.fromByteArray(cert.getEncoded())); // step 1
X509CertificateHolder bcX509Cert = new X509CertificateHolder(bcCert); // step 2
// now verify keyUsage bits
final int requiredKeyUsageBits = KeyUsage.keyCertSign | KeyUsage.cRLSign;
usagesVerified = KeyUsage.fromExtensions(bcX509Cert.getExtensions()).hasUsages(requiredKeyUsageBits);
System.out.println("key usage bits verified? " + usagesVerified);
}
}