Ensure intended key is selected in SamlAuthenticatorTests (elastic#30993)

* Ensure that a purposefully wrong key is used

Uses a specific keypair for tests that require a purposefully wrong
keypair instead of selecting one randomly from the same pull from
which the correct one is selected. Entropy is low because of the
small space and the same key can be randomly selected as both the
correct one and the wrong one, causing the tests to fail.
The purposefully wrong key is also used in 
testSigningKeyIsReloadedForEachRequest and needs to be cleaned
up afterwards so the rest of the tests don't use that for signing.

Resolves elastic#30970

Author: jkakavas

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/authc/saml/SamlAuthenticatorTests.java

@@ -374,7 +374,7 @@ public void testFailWhenAssertionsCannotBeDecrypted() throws Exception {
         final String xml = getSimpleResponse(now);
 
         // Encrypting with different cert instead of sp cert will mean that the SP cannot decrypt
-        final String encrypted = encryptAssertions(xml, readKeyPair("RSA_1024"));
+        final String encrypted = encryptAssertions(xml, readKeyPair("RSA_4096_updated"));
         assertThat(encrypted, not(equalTo(xml)));
 
         final String signed = signDoc(encrypted);
@@ -937,7 +937,7 @@ public void testIncorrectSigningKeyIsRejected() throws Exception {
         assertThat(authenticator.authenticate(token(signer.transform(xml, idpSigningCertificatePair))), notNullValue());
 
         // check is rejected when signed by a different key-pair
-        final Tuple<X509Certificate, PrivateKey> wrongKey = readRandomKeyPair(randomSigningAlgorithm());
+        final Tuple<X509Certificate, PrivateKey> wrongKey = readKeyPair("RSA_4096_updated");
         final ElasticsearchSecurityException exception = expectThrows(ElasticsearchSecurityException.class,
                 () -> authenticator.authenticate(token(signer.transform(xml, wrongKey))));
         assertThat(exception.getMessage(), containsString("SAML Signature"));
@@ -953,10 +953,12 @@ public void testSigningKeyIsReloadedForEachRequest() throws Exception {
         assertThat(authenticator.authenticate(token(signer.transform(xml, idpSigningCertificatePair))), notNullValue());
 
         final Tuple<X509Certificate, PrivateKey> oldKeyPair = idpSigningCertificatePair;
-        //Ensure we won't read any of the ones we could have picked randomly before
+        // Ensure we won't read any of the ones we could have picked randomly before
         idpSigningCertificatePair = readKeyPair("RSA_4096_updated");
         assertThat(idpSigningCertificatePair.v2(), not(equalTo(oldKeyPair.v2())));
         assertThat(authenticator.authenticate(token(signer.transform(xml, idpSigningCertificatePair))), notNullValue());
+        // Restore the keypair to one from the keypair pool of all algorithms and keys
+        idpSigningCertificatePair = readRandomKeyPair(randomSigningAlgorithm());
     }
 
     public void testParsingRejectsTamperedContent() throws Exception {

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/authc/saml/SamlTestCase.java

@@ -71,7 +71,7 @@ protected static Tuple<X509Certificate, PrivateKey> readRandomKeyPair() throws E
     }
 
     /**
-     * Generates key pair for given algorithm and then associates with a certificate.
+     * Reads a key pair and associated certificate for given algorithm and key length
      * For testing, for "EC" algorithm 256 key size is used, others use 2048 as default.
      * @param algorithm
      * @return X509Certificate a signed certificate, it's PrivateKey {@link Tuple}