static String toSignedBase64(SignableSAMLObject signableObj,
                                 Credential signingCredential,
                                 String signatureAlgorithm) {
        sign(signableObj, signingCredential, signatureAlgorithm);
        final String messageStr = nodeToString(serialize(signableObj));
        return Base64.getEncoder().encodeToString(messageStr.getBytes(StandardCharsets.UTF_8));
    }
Signs the specified {@link SignableSAMLObject} with the specified {@link Credential} and
{@code signatureAlgorithm}, and then encodes the object into a base64 string.
saml/src/main/java/com/linecorp/armeria/server/saml/HttpPostBindingUtil.java