/*
 * Copyright 2002-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.springframework.security.config.annotation.web.configurers;

import java.net.URI;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import javax.servlet.http.HttpServletRequest;

import org.springframework.security.config.annotation.web.HttpSecurityBuilder;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
import org.springframework.security.web.header.HeaderWriter;
import org.springframework.security.web.header.HeaderWriterFilter;
import org.springframework.security.web.header.writers.*;
import org.springframework.security.web.header.writers.ReferrerPolicyHeaderWriter.ReferrerPolicy;
import org.springframework.security.web.header.writers.frameoptions.XFrameOptionsHeaderWriter;
import org.springframework.security.web.header.writers.frameoptions.XFrameOptionsHeaderWriter.XFrameOptionsMode;
import org.springframework.security.web.util.matcher.RequestMatcher;
import org.springframework.util.Assert;

/**
 * <p>
 * Adds the Security HTTP headers to the response. Security HTTP headers is activated by
 * default when using {@link WebSecurityConfigurerAdapter}'s default constructor.
 * </p>
 *
 * <p>
 * The default headers include are:
 * </p>
 *
 * <pre>
 * Cache-Control: no-cache, no-store, max-age=0, must-revalidate
 * Pragma: no-cache
 * Expires: 0
 * X-Content-Type-Options: nosniff
 * Strict-Transport-Security: max-age=31536000 ; includeSubDomains
 * X-Frame-Options: DENY
 * X-XSS-Protection: 1; mode=block
 * </pre>
 *
 * @author Rob Winch
 * @author Tim Ysewyn
 * @author Joe Grandja
 * @author Eddú Meléndez
 * @since 3.2
 */
public class HeadersConfigurer<H extends HttpSecurityBuilder<H>> extends
		AbstractHttpConfigurer<HeadersConfigurer<H>, H> {
	private List<HeaderWriter> headerWriters = new ArrayList<HeaderWriter>();

	// --- default header writers ---

	private final ContentTypeOptionsConfig contentTypeOptions = new ContentTypeOptionsConfig();

	private final XXssConfig xssProtection = new XXssConfig();

	private final CacheControlConfig cacheControl = new CacheControlConfig();

	private final HstsConfig hsts = new HstsConfig();

	private final FrameOptionsConfig frameOptions = new FrameOptionsConfig();

	private final HpkpConfig hpkp = new HpkpConfig();

	private final ContentSecurityPolicyConfig contentSecurityPolicy = new ContentSecurityPolicyConfig();

	private final ReferrerPolicyConfig referrerPolicy = new ReferrerPolicyConfig();

	/**
	 * Creates a new instance
	 *
	 * @see HttpSecurity#headers()
	 */
	public HeadersConfigurer() {
	}

	/**
	 * Adds a {@link HeaderWriter} instance
	 *
	 * @param headerWriter the {@link HeaderWriter} instance to add
	 * @return the {@link HeadersConfigurer} for additional customizations
	 */
	public HeadersConfigurer<H> addHeaderWriter(HeaderWriter headerWriter) {
		Assert.notNull(headerWriter, "headerWriter cannot be null");
		this.headerWriters.add(headerWriter);
		return this;
	}

	/**
	 * Configures the {@link XContentTypeOptionsHeaderWriter} which inserts the <a href=
	 * "http://msdn.microsoft.com/en-us/library/ie/gg622941(v=vs.85).aspx"
	 * >X-Content-Type-Options</a>:
	 *
	 * <pre>
	 * X-Content-Type-Options: nosniff
	 * </pre>
	 *
	 * @return the ContentTypeOptionsConfig for additional customizations
	 */
	public ContentTypeOptionsConfig contentTypeOptions() {
		return contentTypeOptions.enable();
	}

	public final class ContentTypeOptionsConfig {
		private XContentTypeOptionsHeaderWriter writer;

		private ContentTypeOptionsConfig() {
			enable();
		}

		/**
		 * Removes the X-XSS-Protection header.
		 *
		 * @return {@link HeadersConfigurer} for additional customization.
		 */
		public HeadersConfigurer<H> disable() {
			writer = null;
			return and();
		}

		/**
		 * Allows customizing the {@link HeadersConfigurer}
		 * @return the {@link HeadersConfigurer} for additional customization
		 */
		public HeadersConfigurer<H> and() {
			return HeadersConfigurer.this;
		}

		/**
		 * Ensures that Content Type Options is enabled
		 *
		 * @return the {@link ContentTypeOptionsConfig} for additional customization
		 */
		private ContentTypeOptionsConfig enable() {
			if (writer == null) {
				writer = new XContentTypeOptionsHeaderWriter();
			}
			return this;
		}
	}

	/**
	 * <strong>Note this is not comprehensive XSS protection!</strong>
	 *
	 * <p>
	 * Allows customizing the {@link XXssProtectionHeaderWriter} which adds the <a href=
	 * "http://blogs.msdn.com/b/ieinternals/archive/2011/01/31/controlling-the-internet-explorer-xss-filter-with-the-x-xss-protection-http-header.aspx"
	 * >X-XSS-Protection header</a>
	 * </p>
	 *
	 * @return the {@link HeadersConfigurer} for additional customizations
	 */
	public XXssConfig xssProtection() {
		return xssProtection.enable();
	}

	public final class XXssConfig {
		private XXssProtectionHeaderWriter writer;

		private XXssConfig() {
			enable();
		}

		/**
		 * If false, will not specify the mode as blocked. In this instance, any content
		 * will be attempted to be fixed. If true, the content will be replaced with "#".
		 *
		 * @param enabled the new value
		 */
		public XXssConfig block(boolean enabled) {
			writer.setBlock(enabled);
			return this;
		}

		/**
		 * If true, the header value will contain a value of 1. For example:
		 *
		 * <pre>
		 * X-XSS-Protection: 1
		 * </pre>
		 *
		 * or if {@link XXssProtectionHeaderWriter#setBlock(boolean)} of the given {@link XXssProtectionHeaderWriter} is true
		 *
		 *
		 * <pre>
		 * X-XSS-Protection: 1; mode=block
		 * </pre>
		 *
		 * If false, will explicitly disable specify that X-XSS-Protection is disabled.
		 * For example:
		 *
		 * <pre>
		 * X-XSS-Protection: 0
		 * </pre>
		 *
		 * @param enabled the new value
		 */
		public XXssConfig xssProtectionEnabled(boolean enabled) {
			writer.setEnabled(enabled);
			return this;
		}

		/**
		 * Disables X-XSS-Protection header (does not include it)
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> disable() {
			writer = null;
			return and();
		}

		/**
		 * Allows completing configuration of Strict Transport Security and continuing
		 * configuration of headers.
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> and() {
			return HeadersConfigurer.this;
		}

		/**
		 * Ensures the X-XSS-Protection header is enabled if it is not already.
		 *
		 * @return the {@link XXssConfig} for additional customization
		 */
		private XXssConfig enable() {
			if (writer == null) {
				writer = new XXssProtectionHeaderWriter();
			}
			return this;
		}
	}

	/**
	 * Allows customizing the {@link CacheControlHeadersWriter}. Specifically it adds the
	 * following headers:
	 * <ul>
	 * <li>Cache-Control: no-cache, no-store, max-age=0, must-revalidate</li>
	 * <li>Pragma: no-cache</li>
	 * <li>Expires: 0</li>
	 * </ul>
	 *
	 * @return the {@link HeadersConfigurer} for additional customizations
	 */
	public CacheControlConfig cacheControl() {
		return cacheControl.enable();
	}

	public final class CacheControlConfig {
		private CacheControlHeadersWriter writer;

		private CacheControlConfig() {
			enable();
		}

		/**
		 * Disables Cache Control
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> disable() {
			writer = null;
			return HeadersConfigurer.this;
		}

		/**
		 * Allows completing configuration of Strict Transport Security and continuing
		 * configuration of headers.
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> and() {
			return HeadersConfigurer.this;
		}

		/**
		 * Ensures the Cache Control headers are enabled if they are not already.
		 *
		 * @return the {@link CacheControlConfig} for additional customization
		 */
		private CacheControlConfig enable() {
			if (writer == null) {
				writer = new CacheControlHeadersWriter();
			}
			return this;
		}
	}

	/**
	 * Allows customizing the {@link HstsHeaderWriter} which provides support for <a
	 * href="http://tools.ietf.org/html/rfc6797">HTTP Strict Transport Security
	 * (HSTS)</a>.
	 *
	 * @return the {@link HeadersConfigurer} for additional customizations
	 */
	public HstsConfig httpStrictTransportSecurity() {
		return hsts.enable();
	}

	public final class HstsConfig {
		private HstsHeaderWriter writer;

		private HstsConfig() {
			enable();
		}

		/**
		 * <p>
		 * Sets the value (in seconds) for the max-age directive of the
		 * Strict-Transport-Security header. The default is one year.
		 * </p>
		 *
		 * <p>
		 * This instructs browsers how long to remember to keep this domain as a known
		 * HSTS Host. See <a
		 * href="http://tools.ietf.org/html/rfc6797#section-6.1.1">Section 6.1.1</a> for
		 * additional details.
		 * </p>
		 *
		 * @param maxAgeInSeconds the maximum amount of time (in seconds) to consider this
		 * domain as a known HSTS Host.
		 * @throws IllegalArgumentException if maxAgeInSeconds is negative
		 */
		public HstsConfig maxAgeInSeconds(long maxAgeInSeconds) {
			writer.setMaxAgeInSeconds(maxAgeInSeconds);
			return this;
		}

		/**
		 * Sets the {@link RequestMatcher} used to determine if the
		 * "Strict-Transport-Security" should be added. If true the header is added, else
		 * the header is not added. By default the header is added when
		 * {@link HttpServletRequest#isSecure()} returns true.
		 *
		 * @param requestMatcher the {@link RequestMatcher} to use.
		 * @throws IllegalArgumentException if {@link RequestMatcher} is null
		 */
		public HstsConfig requestMatcher(RequestMatcher requestMatcher) {
			writer.setRequestMatcher(requestMatcher);
			return this;
		}

		/**
		 * <p>
		 * If true, subdomains should be considered HSTS Hosts too. The default is true.
		 * </p>
		 *
		 * <p>
		 * See <a href="http://tools.ietf.org/html/rfc6797#section-6.1.2">Section
		 * 6.1.2</a> for additional details.
		 * </p>
		 *
		 * @param includeSubDomains true to include subdomains, else false
		 */
		public HstsConfig includeSubDomains(boolean includeSubDomains) {
			writer.setIncludeSubDomains(includeSubDomains);
			return this;
		}

		/**
		 * Disables Strict Transport Security
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> disable() {
			writer = null;
			return HeadersConfigurer.this;
		}

		/**
		 * Allows completing configuration of Strict Transport Security and continuing
		 * configuration of headers.
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> and() {
			return HeadersConfigurer.this;
		}

		/**
		 * Ensures that Strict-Transport-Security is enabled if it is not already
		 *
		 * @return the {@link HstsConfig} for additional customization
		 */
		private HstsConfig enable() {
			if (writer == null) {
				writer = new HstsHeaderWriter();
			}
			return this;
		}
	}

	/**
	 * Allows customizing the {@link XFrameOptionsHeaderWriter}.
	 *
	 * @return the {@link HeadersConfigurer} for additional customizations
	 */
	public FrameOptionsConfig frameOptions() {
		return frameOptions.enable();
	}

	public final class FrameOptionsConfig {
		private XFrameOptionsHeaderWriter writer;

		private FrameOptionsConfig() {
			enable();
		}

		/**
		 * Specify to DENY framing any content from this application.
		 *
		 * @return the {@link HeadersConfigurer} for additional customization.
		 */
		public HeadersConfigurer<H> deny() {
			writer = new XFrameOptionsHeaderWriter(XFrameOptionsMode.DENY);
			return and();
		}

		/**
		 * <p>
		 * Specify to allow any request that comes from the same origin to frame this
		 * application. For example, if the application was hosted on example.com, then
		 * example.com could frame the application, but evil.com could not frame the
		 * application.
		 * </p>
		 *
		 * @return
		 */
		public HeadersConfigurer<H> sameOrigin() {
			writer = new XFrameOptionsHeaderWriter(XFrameOptionsMode.SAMEORIGIN);
			return and();
		}

		/**
		 * Prevents the header from being added to the response.
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration.
		 */
		public HeadersConfigurer<H> disable() {
			writer = null;
			return and();
		}

		/**
		 * Allows continuing customizing the headers configuration.
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> and() {
			return HeadersConfigurer.this;
		}

		/**
		 * Enables FrameOptionsConfig if it is not already enabled.
		 *
		 * @return the FrameOptionsConfig for additional customization.
		 */
		private FrameOptionsConfig enable() {
			if (writer == null) {
				writer = new XFrameOptionsHeaderWriter(XFrameOptionsMode.DENY);
			}
			return this;
		}
	}

	/**
	 * Allows customizing the {@link HpkpHeaderWriter} which provides support for <a
	 * href="http://tools.ietf.org/html/rfc7469">HTTP Public Key Pinning (HPKP)</a>.
	 *
	 * @return the {@link HeadersConfigurer} for additional customizations
	 *
	 * @since 4.1
	 */
	public HpkpConfig httpPublicKeyPinning() {
		return hpkp.enable();
	}

	public final class HpkpConfig {
		private HpkpHeaderWriter writer;

		private HpkpConfig() {}

		/**
		 * <p>
		 * Sets the value for the pin- directive of the Public-Key-Pins header.
		 * </p>
		 *
		 * <p>
		 * The pin directive specifies a way for web host operators to indicate
		 * a cryptographic identity that should be bound to a given web host.
		 * See <a href="http://tools.ietf.org/html/rfc7469#section-2.1.1">Section 2.1.1</a> for additional details.
		 * </p>
		 *
		 * @param pins the map of base64-encoded SPKI fingerprint &amp; cryptographic hash algorithm pairs.
		 * @throws IllegalArgumentException if pins is null
		 */
		public HpkpConfig withPins(Map<String, String> pins) {
			writer.setPins(pins);
			return this;
		}

		/**
		 * <p>
		 * Adds a list of SHA256 hashed pins for the pin- directive of the Public-Key-Pins header.
		 * </p>
		 *
		 * <p>
		 * The pin directive specifies a way for web host operators to indicate
		 * a cryptographic identity that should be bound to a given web host.
		 * See <a href="http://tools.ietf.org/html/rfc7469#section-2.1.1">Section 2.1.1</a> for additional details.
		 * </p>
		 *
		 * @param pins a list of base64-encoded SPKI fingerprints.
		 * @throws IllegalArgumentException if a pin is null
		 */
		public HpkpConfig addSha256Pins(String ... pins) {
			writer.addSha256Pins(pins);
			return this;
		}

		/**
		 * <p>
		 * Sets the value (in seconds) for the max-age directive of the Public-Key-Pins header.
		 * The default is 60 days.
		 * </p>
		 *
		 * <p>
		 * This instructs browsers how long they should regard the host (from whom the message was received)
		 * as a known pinned host. See <a href="http://tools.ietf.org/html/rfc7469#section-2.1.2">Section
		 * 2.1.2</a> for additional details.
		 * </p>
		 *
		 * @param maxAgeInSeconds the maximum amount of time (in seconds) to regard the host
		 * as a known pinned host.
		 * @throws IllegalArgumentException if maxAgeInSeconds is negative
		 */
		public HpkpConfig maxAgeInSeconds(long maxAgeInSeconds) {
			writer.setMaxAgeInSeconds(maxAgeInSeconds);
			return this;
		}

		/**
		 * <p>
		 * If true, the pinning policy applies to this pinned host as well as any subdomains
		 * of the host's domain name. The default is false.
		 * </p>
		 *
		 * <p>
		 * See <a href="http://tools.ietf.org/html/rfc7469#section-2.1.3">Section 2.1.3</a>
		 * for additional details.
		 * </p>
		 *
		 * @param includeSubDomains true to include subdomains, else false
		 */
		public HpkpConfig includeSubDomains(boolean includeSubDomains) {
			writer.setIncludeSubDomains(includeSubDomains);
			return this;
		}

		/**
		 * <p>
		 * If true, the browser should not terminate the connection with the server. The default is true.
		 * </p>
		 *
		 * <p>
		 * See <a href="http://tools.ietf.org/html/rfc7469#section-2.1">Section 2.1</a>
		 * for additional details.
		 * </p>
		 *
		 * @param reportOnly true to report only, else false
		 */
		public HpkpConfig reportOnly(boolean reportOnly) {
			writer.setReportOnly(reportOnly);
			return this;
		}

		/**
		 * <p>
		 * Sets the URI to which the browser should report pin validation failures.
		 * </p>
		 *
		 * <p>
		 * See <a href="http://tools.ietf.org/html/rfc7469#section-2.1.4">Section 2.1.4</a>
		 * for additional details.
		 * </p>
		 *
		 * @param reportUri the URI where the browser should send the report to.
		 */
		public HpkpConfig reportUri(URI reportUri) {
			writer.setReportUri(reportUri);
			return this;
		}

		/**
		 * <p>
		 * Sets the URI to which the browser should report pin validation failures.
		 * </p>
		 *
		 * <p>
		 * See <a href="http://tools.ietf.org/html/rfc7469#section-2.1.4">Section 2.1.4</a>
		 * for additional details.
		 * </p>
		 *
		 * @param reportUri the URI where the browser should send the report to.
		 * @throws IllegalArgumentException if the reportUri is not a valid URI
		 */
		public HpkpConfig reportUri(String reportUri) {
			writer.setReportUri(reportUri);
			return this;
		}

		/**
		 * Prevents the header from being added to the response.
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration.
		 */
		public HeadersConfigurer<H> disable() {
			writer = null;
			return and();
		}

		/**
		 * Allows completing configuration of Public Key Pinning and continuing
		 * configuration of headers.
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> and() {
			return HeadersConfigurer.this;
		}

		/**
		 * Ensures that Public-Key-Pins or Public-Key-Pins-Report-Only is enabled if it is not already
		 *
		 * @return the {@link HstsConfig} for additional customization
		 */
		private HpkpConfig enable() {
			if (writer == null) {
				writer = new HpkpHeaderWriter();
			}
			return this;
		}
	}

	/**
	 * <p>
	 * Allows configuration for <a href="https://www.w3.org/TR/CSP2/">Content Security Policy (CSP) Level 2</a>.
	 * </p>
	 *
	 * <p>
	 * Calling this method automatically enables (includes) the Content-Security-Policy header in the response
	 * using the supplied security policy directive(s).
	 * </p>
	 *
	 * <p>
	 * Configuration is provided to the {@link ContentSecurityPolicyHeaderWriter} which supports the writing
	 * of the two headers as detailed in the W3C Candidate Recommendation:
	 * </p>
	 * <ul>
	 * 	<li>Content-Security-Policy</li>
	 * 	<li>Content-Security-Policy-Report-Only</li>
	 * </ul>
	 *
	 * @see ContentSecurityPolicyHeaderWriter
	 * @since 4.1
	 * @return the ContentSecurityPolicyConfig for additional configuration
	 * @throws IllegalArgumentException if policyDirectives is null or empty
	 */
	public ContentSecurityPolicyConfig contentSecurityPolicy(String policyDirectives) {
		this.contentSecurityPolicy.writer =
				new ContentSecurityPolicyHeaderWriter(policyDirectives);
		return contentSecurityPolicy;
	}

	public final class ContentSecurityPolicyConfig {
		private ContentSecurityPolicyHeaderWriter writer;

		private ContentSecurityPolicyConfig() {
		}

		/**
		 * Enables (includes) the Content-Security-Policy-Report-Only header in the response.
		 *
		 * @return the {@link ContentSecurityPolicyConfig} for additional configuration
		 */
		public ContentSecurityPolicyConfig reportOnly() {
			this.writer.setReportOnly(true);
			return this;
		}

		/**
		 * Allows completing configuration of Content Security Policy and continuing
		 * configuration of headers.
		 *
		 * @return the {@link HeadersConfigurer} for additional configuration
		 */
		public HeadersConfigurer<H> and() {
			return HeadersConfigurer.this;
		}

	}

	/**
	 * Clears all of the default headers from the response. After doing so, one can add
	 * headers back. For example, if you only want to use Spring Security's cache control
	 * you can use the following:
	 *
	 * <pre>
	 * http.headers().defaultsDisabled().cacheControl();
	 * </pre>
	 *
	 * @return the {@link HeadersConfigurer} for additional customization
	 */
	public HeadersConfigurer<H> defaultsDisabled() {
		contentTypeOptions.disable();
		xssProtection.disable();
		cacheControl.disable();
		hsts.disable();
		frameOptions.disable();
		return this;
	}

	@Override
	public void configure(H http) throws Exception {
		HeaderWriterFilter headersFilter = createHeaderWriterFilter();
		http.addFilter(headersFilter);
	}

	/**
	 * Creates the {@link HeaderWriter}
	 *
	 * @return the {@link HeaderWriter}
	 */
	private HeaderWriterFilter createHeaderWriterFilter() {
		List<HeaderWriter> writers = getHeaderWriters();
		if (writers.isEmpty()) {
			throw new IllegalStateException(
					"Headers security is enabled, but no headers will be added. Either add headers or disable headers security");
		}
		HeaderWriterFilter headersFilter = new HeaderWriterFilter(writers);
		headersFilter = postProcess(headersFilter);
		return headersFilter;
	}

	/**
	 * Gets the {@link HeaderWriter} instances and possibly initializes with the defaults.
	 *
	 * @return
	 */
	private List<HeaderWriter> getHeaderWriters() {
		List<HeaderWriter> writers = new ArrayList<HeaderWriter>();
		addIfNotNull(writers, contentTypeOptions.writer);
		addIfNotNull(writers, xssProtection.writer);
		addIfNotNull(writers, cacheControl.writer);
		addIfNotNull(writers, hsts.writer);
		addIfNotNull(writers, frameOptions.writer);
		addIfNotNull(writers, hpkp.writer);
		addIfNotNull(writers, contentSecurityPolicy.writer);
		addIfNotNull(writers, referrerPolicy.writer);
		writers.addAll(headerWriters);
		return writers;
	}

	private <T> void addIfNotNull(List<T> values, T value) {
		if (value != null) {
			values.add(value);
		}
	}

	/**
	 * <p>
	 * Allows configuration for <a href="https://www.w3.org/TR/referrer-policy/">Referrer Policy</a>.
	 * </p>
	 *
	 * <p>
	 * Configuration is provided to the {@link ReferrerPolicyHeaderWriter} which support the writing
	 * of the header as detailed in the W3C Technical Report:
	 * </p>
	 * <ul>
	 *  <li>Referrer-Policy</li>
	 * </ul>
	 *
	 * <p>Default value is:</p>
	 *
	 * <pre>
	 * Referrer-Policy: no-referrer
	 * </pre>
	 *
	 * @see ReferrerPolicyHeaderWriter
	 * @since 4.2
	 * @return the ReferrerPolicyConfig for additional configuration
	 */
	public ReferrerPolicyConfig referrerPolicy() {
		this.referrerPolicy.writer = new ReferrerPolicyHeaderWriter();
		return this.referrerPolicy;
	}

	/**
	 * <p>
	 * Allows configuration for <a href="https://www.w3.org/TR/referrer-policy/">Referrer Policy</a>.
	 * </p>
	 *
	 * <p>
	 * Configuration is provided to the {@link ReferrerPolicyHeaderWriter} which support the writing
	 * of the header as detailed in the W3C Technical Report:
	 * </p>
	 * <ul>
	 *  <li>Referrer-Policy</li>
	 * </ul>
	 *
	 * @see ReferrerPolicyHeaderWriter
	 * @since 4.2
	 * @return the ReferrerPolicyConfig for additional configuration
	 * @throws IllegalArgumentException if policy is null or empty
	 */
	public ReferrerPolicyConfig referrerPolicy(ReferrerPolicy policy) {
		this.referrerPolicy.writer = new ReferrerPolicyHeaderWriter(policy);
		return this.referrerPolicy;
	}

	public final class ReferrerPolicyConfig {

		private ReferrerPolicyHeaderWriter writer;

		private ReferrerPolicyConfig() {
		}

		public HeadersConfigurer<H> and() {
			return HeadersConfigurer.this;
		}

	}
}