如何在 Java 中将 X509Certificate 转换为 PEM 字符串？

Question

安全对象有很多格式。有时您需要 X509Certificate，有时您需要它作为 PEM 编码的字符串。您如何从 X509Certificate 格式转换为 PEM？

制作这个（X509Certificate）：

  [0]         Version: 3
     SerialNumber: 95573
         IssuerDN: C=US,ST=California,OU=PDX,O=Example Inc.,CN=Example Cust Issuing CA 1
       Start Date: Wed Jan 13 14:21:12 PST 2016
       Final Date: Sat Jan 14 14:21:12 PST 2017
        SubjectDN: C=US,ST=California,OU=TEST,O=Example,CN=vm1452810069963
       Public Key: RSA Public Key
          modulus: 9c2b98b154cbd2bdaed82271e2324e73589356cab9a762b8ba7248fab236347eb44d19322696109e
                   [...]
                   c0868c88e5e7bc09baadb48cf85c631d
  public exponent: 10001
  Signature Algorithm: SHA256WITHRSA
        Signature: 2197491b50f69c317c7b930634d487744f4502cc
                   [...]
                   dfcb0a75ba67f94b958d2edc2c6cea9a
       Extensions: 
                   critical(false) 2.5.29.35 value = Sequence
Tagged [0] IMPLICIT 
    DER Octet String[32] 
Tagged [1]
    Tagged [4]
        DER Sequence
            DER Set
                DER Sequence
                    ObjectIdentifier(2.5.4.6)
                    PrintableString(US) 
            DER Set
                DER Sequence
                    ObjectIdentifier(2.5.4.8)
                    PrintableString(California) 
            DER Set
                DER Sequence
                    ObjectIdentifier(2.5.4.11)
                    PrintableString(PDX) 
            DER Set
                DER Sequence
                    ObjectIdentifier(2.5.4.10)
                    PrintableString(Example Inc.) 
            DER Set
                DER Sequence
                    ObjectIdentifier(2.5.4.3)
                    PrintableString(Example Cust Policy CA 1) 
Tagged [2] IMPLICIT 
    DER Octet String[3] 

                   critical(false) 2.5.29.14 value = DER Octet String[32] 

                   critical(true) BasicConstraints: isCa(false)
                   critical(true) KeyUsage: 0x80
                   critical(false) 1.3.6.1.5.5.7.1.1 value = Sequence
Sequence
    ObjectIdentifier(1.3.6.1.5.5.7.48.1)
    Tagged [6] IMPLICIT 
        DER Octet String[26] 

进入这个（PEM格式）：

-----BEGIN CERTIFICATE-----
MIIEcDCCA1igAwIBAgIDAXVVMA0GCSqGSIb3DQEBCwUAMGsxCzAJBgNVBAYTAlVT
[...]
Ksl1vpZ3T96C6UnU3I9c4arhsSbfywp1umf5S5WNLtwsbOqa
-----END CERTIFICATE-----

Answer 1

import org.bouncycastle.openssl.jcajce.JcaPEMWriter;

public static String x509CertificateToPem(final X509Certificate cert) throws IOException {
    final StringWriter writer = new StringWriter();
    final JcaPEMWriter pemWriter = new JcaPEMWriter(writer);
    pemWriter.writeObject(cert);
    pemWriter.flush();
    pemWriter.close();
    return writer.toString();
}

据我所见， JcaPEMWriter 可以接受不同的格式并将这些格式写入 PEM 字符串。例如：

public static String convertCertToPem(final PKCS10CertificationRequest certRequest) throws IOException {
    final StringWriter writer = new StringWriter();
    final JcaPEMWriter pemWriter = new JcaPEMWriter(writer);
    pemWriter.writeObject(certRequest);
    pemWriter.flush();
    pemWriter.close();
    return writer.toString();
}

除了输入是 PCKS10CertificateRequest 而不是 X509Certificate 之外，与上面相同的代码。