title: RSA signing keys must be PrivateKey instances
body: <p>I Have a problem in making an Jwt for the firebaseCustomToken
  RSA signing keys must be PrivateKey instances.
  How Can I solve this problem?</p>
  <pre><code> Map&lt;String, Object&gt; headers = new HashMap&lt;&gt;();
  header.put(&quot;typ&quot;, &quot;JWT&quot;);
  header.put(&quot;alg&quot;, RS256);
  
  String temp = Jwts.builder()
          .setHeader(headers)
          .setIssuer(&quot;client_email_firebaseAdminSDK&quot;)
          .setSubject(&quot;client_email_firebaseAdminSDK&quot;)
          .setAudience(&quot;https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit&quot;)
          .setIssuedAt(cur_time)
          .setExpiration(exp_time)
          .setId(Long.toString(kakaoResult.getUserId()))
          .signWith(Keys.hmacShaKeyFor(&quot;private_key_of_firebaseAdminSDK&quot;.getBytes()), RS256)
          .compact();
  </code></pre>