使用 AndroidKeyStoreProvider 生成证书签名请求的最佳方法是什么？

Question

I read this article .

It says how to generate a KeyPair , however it doesn't specify how to generate a Certificate Signing Request based on the generated keys.

From my research, to generate a CSR in Java, the samples from the web usually use the package sun.* or the BouncyCastle library. It seems like there isn't a way to generate a CSR with the standard java.security API. I read this and it seems to say the same thing.

Do I have no choice but to use BouncyCastle? It is hard to imagine that Android Developers don't consider this kind of usage.

By the way, the article also mentions that:

Generating a new PrivateKey requires that you also specify the initial X.509 attributes that the self-signed certificate will have. You can replace the certificate at a later time with a certificate signed by a Certificate Authority

Suppose I finally get a certificate signed by a Certificate Authority. What exactly should I do to "replace the certificate at a later time"?

Answer 1

Regarding generating a CSR (certificate sign request) on the android phone, I think it is rather straightforward to use Spongycastle instead. It is an android port of Bouncycastle.

Suppose I finally get certificate signed by a Certificate Authority. What exactly should I do to "replace the certificate at a later time"?

Once you have the actual signed certificate which you are supposed to get from the CA (Certificate Authority), you no longer need your CSR; you should just store the signed certificate on the phone. Where to save them - I guess you can get help here .

Answer 2

The best way to create a CSR on Android is to use SpongyCastle , which is an implementation of BouncyCastle for Android. SpongyCastle already does a lot of the heavy-lifting for you so it will make your life much easier.

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My implementation is heavily based on the answer found here , but uses the Android KeyStore for security and SpongyCastle's JcaContentSignerBuilder() instead of the custom ContentSigner .

Add SpongyCastle to your build.gradle file:

compile 'com.madgag.spongycastle:core:1.51.0.0'
compile 'com.madgag.spongycastle:pkix:1.51.0.0'

Create the KeyPair in the Android KeyStore :

KeyPairGenerator keyGen = KeyPairGenerator.getInstance(KeyProperties.KEY_ALGORITHM_RSA, "AndroidKeyStore"); // store the key in the Android KeyStore for security purposes
keyGen.initialize(new KeyGenParameterSpec.Builder(
                  "key1",
                  KeyProperties.PURPOSE_SIGN)
                  .setSignaturePaddings(KeyProperties.SIGNATURE_PADDING_RSA_PKCS1)
                  .setDigests(KeyProperties.DIGEST_SHA256,
                                KeyProperties.DIGEST_SHA384,
                                KeyProperties.DIGEST_SHA512)
                  .build()); // defaults to RSA 2048
KeyPair keyPair = keyGen.generateKeyPair();

Create the CSR using said KeyPair:

private final static String CN_PATTERN = "CN=%s, O=Aralink, OU=OrgUnit";

//Create the certificate signing request (CSR) from private and public keys
public static PKCS10CertificationRequest generateCSR(KeyPair keyPair, String cn) throws IOException, OperatorCreationException {
        String principal = String.format(CN_PATTERN, cn);

        ContentSigner signer = new JcaContentSignerBuilder(DEFAULT_RSA_SIGNATURE_ALGORITHM).build(keyPair.getPrivate());

        PKCS10CertificationRequestBuilder csrBuilder = new JcaPKCS10CertificationRequestBuilder(
                new X500Name(principal), keyPair.getPublic());
        ExtensionsGenerator extensionsGenerator = new ExtensionsGenerator();
        extensionsGenerator.addExtension(Extension.basicConstraints, true, new BasicConstraints(
                true));
        csrBuilder.addAttribute(PKCSObjectIdentifiers.pkcs_9_at_extensionRequest,
                extensionsGenerator.generate());
        PKCS10CertificationRequest csr = csrBuilder.build(signer);

        return csr;
    }
}

And that's it, now you have a PKCS10CertificationRequest that you can send to your server.

Answer 3

You can use the Bouncy Castle to work with the keys from Android KeyStore. This doesn't mean that you have to set the Bouncy Castle as a security provider. It is enough to include the library:

implementation 'org.bouncycastle:bcpkix-jdk18on:1.72' 

Note: Spongy Castle is obsolete. Standard Bouncy Castle library has to be included in your Android application. For details on why see: https://github.com/rtyley/spongycastle/issues/34

val keyPairGenerator: KeyPairGenerator = KeyPairGenerator.getInstance(
    KeyProperties.KEY_ALGORITHM_EC,
    "AndroidKeyStore"
)
val keySpecBuilder = KeyGenParameterSpec.Builder(
    Constants.clientCertificateKeyAlias,
    KeyProperties.PURPOSE_SIGN or KeyProperties.PURPOSE_VERIFY
).setDigests(KeyProperties.DIGEST_NONE, KeyProperties.DIGEST_SHA256, KeyProperties.DIGEST_SHA512)
keyPairGenerator.initialize(keySpecBuilder.build())
val pair = keyPairGenerator.generateKeyPair()

val p10Builder: PKCS10CertificationRequestBuilder = JcaPKCS10CertificationRequestBuilder(
    X500Principal("CN=My Client Certificate"), pair.public
)
val csBuilder = JcaContentSignerBuilder("SHA256withECDSA")
val signer: ContentSigner = csBuilder.build(pair.private)
val csr: PKCS10CertificationRequest = p10Builder.build(signer)

val pemObject = PemObject("CERTIFICATE REQUEST", csr.encoded)
val csrAsString = csrAsStringingWriter()
val pemWriter = PEMWriter(csrAsString)
pemWriter.writeObject(pemObject)
pemWriter.close()
csrAsString.close()
Log.v("CSR", "CSR: $csrAsString")