Make CertificateUtils.createTlsSniCertificate more useful for TLS-SNI challenge

acme4j-utils/src/main/java/org/shredzone/acme4j/util/CertificateUtils.java

@@ -95,19 +95,18 @@ public final class CertificateUtils {
      * Creates a self-signed {@link X509Certificate} that can be used for
      * {@link TlsSniChallenge}. The certificate is valid for 7 days.
      *
+     * @param keypair
+     *            A domain {@link KeyPair} to be used for the challenge
      * @param subject
      *            Subject to create a certificate for
      * @return Created certificate
      */
-    public static X509Certificate createTlsSniCertificate(String subject) throws IOException {
+    public static X509Certificate createTlsSniCertificate(KeyPair keypair, String subject) throws IOException {
-        final int certSize = 2048;
         final long now = System.currentTimeMillis();
         final long validSpanMs = 7 * 24 * 60 * 60 * 1000L;
         final String signatureAlg = "SHA256withRSA";
 
         try {
-            KeyPair keypair = KeyPairUtils.createKeyPair(certSize);
-
             X500Name issuer = new X500Name("CN=acme.invalid");
             BigInteger serial = BigInteger.valueOf(now);
             Date notBefore = new Date(now);

acme4j-utils/src/test/java/org/shredzone/acme4j/util/CertificateUtilsTest.java

@@ -20,6 +20,7 @@ import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.security.KeyPair;
 import java.security.cert.CertificateException;
 import java.security.cert.CertificateFactory;
 import java.security.cert.CertificateParsingException;
@@ -84,17 +85,20 @@ public class CertificateUtilsTest {
     }
 
     /**
-     * Test if {@link CertificateUtils#createTlsSniCertificate(String)} creates a
+     * Test if {@link CertificateUtils#createTlsSniCertificate(KeyPair, String)} creates a
      * good certificate.
      */
     @Test
     public void testCreateTlsSniCertificate() throws IOException, CertificateParsingException {
         String subject = "30c452b9bd088cdbc2c4094947025d7c.7364ea602ac325a1b55ceaae024fbe29.acme.invalid";
+
+        KeyPair keypair = KeyPairUtils.createKeyPair(2048);
+
+        X509Certificate cert = CertificateUtils.createTlsSniCertificate(keypair, subject);
+
         Date now = new Date();
         Date end = new Date(now.getTime() + (8 * 24 * 60 * 60 * 1000L));
 
-        X509Certificate cert = CertificateUtils.createTlsSniCertificate(subject);
-
         assertThat(cert, not(nullValue()));
         assertThat(cert.getNotAfter(), is(greaterThan(now)));
         assertThat(cert.getNotAfter(), is(lessThan(end)));

src/site/markdown/challenge/tls-sni.md

@@ -1,6 +1,6 @@
 # TLS-SNI
 
-With the TLS-SNI challenge, you prove to the CA that you are able to control the web server of the domain to be authorized, by letting it respond to a SNI request with a self-signed cert.
+With the TLS-SNI challenge, you prove to the CA that you are able to control the web server of the domain to be authorized, by letting it respond to a SNI request with a specific self-signed cert.
 
 After authorizing the challenge, `TlsSniChallenge` provides a subject:
 
@@ -22,7 +22,20 @@ You need to create a self-signed certificate with the subject set as _Subject Al
 The `TlsSniChallenge` class does not generate a self-signed certificate, as it would require _Bouncy Castle_. However, there is a utility method in the _acme4j-utils_ module for this use case:
 
 ```java
-X509Certificate cert = CertificateUtils.createTlsSniCertificate(String subject);
+KeyPair sniKeyPair = KeyPairUtils.createKeyPair(2048);
+X509Certificate cert = CertificateUtils.createTlsSniCertificate(sniKeyPair, subject);
 ```
 
+Now use `cert` and `sniKeyPair` to let your web server respond to a SNI request to `domain`.
+
 The challenge is completed when the CA was able to send the SNI request and get the correct certificate in return.
+
+This shell command line may be helpful to test your web server configuration:
+
+```shell
+echo QUIT | \
+  openssl s_client -servername $domain -connect $server_ip:443 | \
+  openssl x509 -text -noout
+```
+
+It should return a certificate with `domain` set as `X509v3 Subject Alternative Name`.