switch to the exepcted SecretTextArea

Now that formvalidation has been manually tested swap back to
SecretTextArea

The formValidation won't work pending JENKINS-73404 (and to some extent
JENKINS-65616) but once they are fixed will magically start working

src/main/java/com/cloudbees/plugins/credentials/impl/CertificateCredentialsImpl.java

@@ -134,6 +134,13 @@ public CertificateCredentialsImpl(@CheckForNull CredentialsScope scope,
         Objects.requireNonNull(keyStoreSource);
         this.password = Secret.fromString(password);
         this.keyStoreSource = keyStoreSource;
+        // ensure the keySore is valid
+        // we check here as otherwise it will lead to hard to diagnose errors when used
+        try {
+            keyStoreSource.toKeyStore(toCharArray(this.password));
+        } catch (GeneralSecurityException | IOException e) {
+            throw new IllegalArgumentException("KeyStore is not valid.", e);
+        }
     }
 
     /**

src/main/resources/com/cloudbees/plugins/credentials/impl/CertificateCredentialsImpl/PEMEntryKeyStoreSource/config.jelly

@@ -26,9 +26,9 @@
 
 <j:jelly xmlns:j="jelly:core" xmlns:f="/lib/form">
    <f:entry title="Certificates" field="certChain">
-        <f:textarea />
+        <f:secretTextarea />
     </f:entry>
     <f:entry title="Private key" field="privateKey">
-        <f:textarea />
+        <f:secretTextarea />
     </f:entry>
 </j:jelly>