# OCSPVerifier.java

    /*
     *  Licensed to the Apache Software Foundation (ASF) under one
     *  or more contributor license agreements.  See the NOTICE file
     *  distributed with this work for additional information
     *  regarding copyright ownership.  The ASF licenses this file
     *  to you 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.apache.synapse.transport.certificatevalidation.ocsp;

    import org.apache.commons.logging.Log;
    import org.apache.commons.logging.LogFactory;
    import org.apache.http.HttpResponse;
    import org.apache.http.client.config.RequestConfig;
    import org.apache.http.client.methods.HttpPost;
    import org.apache.http.entity.ByteArrayEntity;
    import org.apache.http.entity.ContentType;
    import org.apache.http.impl.client.CloseableHttpClient;
    import org.apache.http.impl.client.HttpClientBuilder;
    import org.apache.synapse.transport.certificatevalidation.CertificateVerificationException;
    import org.apache.synapse.transport.certificatevalidation.Constants;
    import org.apache.synapse.transport.certificatevalidation.RevocationStatus;
    import org.apache.synapse.transport.certificatevalidation.RevocationVerifier;
    import org.bouncycastle.asn1.ASN1IA5String;
    import org.bouncycastle.asn1.ASN1InputStream;
    import org.bouncycastle.asn1.DEROctetString;
    import org.bouncycastle.asn1.ocsp.OCSPObjectIdentifiers;
    import org.bouncycastle.asn1.ocsp.OCSPResponseStatus;
    import org.bouncycastle.asn1.x509.AccessDescription;
    import org.bouncycastle.asn1.x509.AuthorityInformationAccess;
    import org.bouncycastle.asn1.x509.Extension;
    import org.bouncycastle.asn1.x509.Extensions;
    import org.bouncycastle.asn1.x509.GeneralName;
    import org.bouncycastle.cert.X509CertificateHolder;
    import org.bouncycastle.cert.ocsp.BasicOCSPResp;
    import org.bouncycastle.cert.ocsp.CertificateID;
    import org.bouncycastle.cert.ocsp.CertificateStatus;
    import org.bouncycastle.cert.ocsp.OCSPReq;
    import org.bouncycastle.cert.ocsp.OCSPReqBuilder;
    import org.bouncycastle.cert.ocsp.OCSPResp;
    import org.bouncycastle.cert.ocsp.SingleResp;
    import org.bouncycastle.operator.DigestCalculatorProvider;
    import org.bouncycastle.operator.jcajce.JcaDigestCalculatorProviderBuilder;

    import java.io.ByteArrayInputStream;
    import java.io.IOException;
    import java.io.InputStream;
    import java.math.BigInteger;
    import java.security.Provider;
    import java.security.Security;
    import java.security.cert.X509Certificate;
    import java.util.ArrayList;
    import java.util.List;

    /**
     * Used to check if a Certificate is revoked or not by its CA using Online Certificate Status Protocol (OCSP).
     */
    public class OCSPVerifier implements RevocationVerifier {

        private OCSPCache cache;
        private static final Log log = LogFactory.getLog(OCSPVerifier.class);

        public OCSPVerifier(OCSPCache cache) {
            this.cache = cache;
        }

        public static final String CONTENT_TYPE = "Content-Type";
        public static final String JSON_TYPE ="application/json";
        public static final String ACCEPT_TYPE = "Accept";
        public static final String OCSP_REQUEST_TYPE = "application/ocsp-request";
        public static final String OCSP_RESPONSE_TYPE = "application/ocsp-response";

        /**
         * Gets the revocation status (Good, Revoked or Unknown) of the given peer certificate.
         *
         * @param peerCert   The certificate we want to check if revoked.
         * @param issuerCert Needed to create OCSP request.
         * @return revocation status of the peer certificate.
         * @throws CertificateVerificationException
         *
         */
        public RevocationStatus checkRevocationStatus(X509Certificate peerCert, X509Certificate issuerCert)
                throws CertificateVerificationException {

            //check cache
            if (cache != null) {
                SingleResp resp = cache.getCacheValue(peerCert.getSerialNumber());
                if (resp != null) {
                    //If cant be casted, we have used the wrong cache.
                    RevocationStatus status = getRevocationStatus(resp);
                    log.info("OCSP response taken from cache....");
                    return status;
                }
            }

            OCSPReq request = generateOCSPRequest(issuerCert, peerCert.getSerialNumber());
            //This list will sometimes have non ocsp urls as well.
            List<String> locations = getAIALocations(peerCert);

            for (String serviceUrl : locations) {

                SingleResp[] responses;
                try {
                    OCSPResp ocspResponse = getOCSPResponce(serviceUrl, request);
                    if (OCSPResponseStatus.SUCCESSFUL != ocspResponse.getStatus()) {
                        continue; // Server didn't give the response right.
                    }

                    BasicOCSPResp basicResponse = (BasicOCSPResp) ocspResponse.getResponseObject();
                    responses = (basicResponse == null) ? null : basicResponse.getResponses();
                    //todo use the super exception
                } catch (Exception e) {
                    continue;
                }

                if (responses != null && responses.length == 1) {
                    SingleResp resp = responses[0];
                    RevocationStatus status = getRevocationStatus(resp);
                    if (cache != null)
                        cache.setCacheValue(peerCert.getSerialNumber(), resp, request, serviceUrl);
                    return status;
                }
            }
            throw new CertificateVerificationException("Cant get Revocation Status from OCSP.");
        }

        private RevocationStatus getRevocationStatus(SingleResp resp) throws CertificateVerificationException {
            Object status = resp.getCertStatus();
            if (status == CertificateStatus.GOOD) {
                return RevocationStatus.GOOD;
            } else if (status instanceof org.bouncycastle.cert.ocsp.RevokedStatus) {
                return RevocationStatus.REVOKED;
            } else if (status instanceof org.bouncycastle.cert.ocsp.UnknownStatus) {
                return RevocationStatus.UNKNOWN;
            }
            throw new CertificateVerificationException("Cant recognize Certificate Status");
        }

        /**
         * Gets an ASN.1 encoded OCSP response (as defined in RFC 2560) from the given service URL. Currently supports
         * only HTTP.
         *
         * @param serviceUrl URL of the OCSP endpoint.
         * @param request    an OCSP request object.
         * @return OCSP response encoded in ASN.1 structure.
         * @throws CertificateVerificationException
         *
         */
        protected OCSPResp getOCSPResponce(String serviceUrl, OCSPReq request) throws CertificateVerificationException {

            if (log.isDebugEnabled()) {
                log.debug("Initiating HTTP request to URL: " + serviceUrl + " to get the OCSP response");
            }

            try (CloseableHttpClient client = HttpClientBuilder.create().build()) {
                HttpPost httpPost = new HttpPost(serviceUrl);

                // adding request timeout configurations
                if (httpPost.getConfig() == null) {
                    httpPost.setConfig(RequestConfig.custom().build());
                }
                httpPost.addHeader(CONTENT_TYPE, OCSP_REQUEST_TYPE);
                httpPost.addHeader(ACCEPT_TYPE, OCSP_RESPONSE_TYPE);
                httpPost.setEntity(new ByteArrayEntity(request.getEncoded(), ContentType.create(JSON_TYPE)));
                HttpResponse httpResponse = client.execute(httpPost);

                //Check errors in response, if response status code is not 200 (success) range, throws exception
                // eg: if response code is 200 (success) or 201 (accepted) return true,
                //     if response code is 404 (not found) or 500 throw exception
                if (httpResponse.getStatusLine().getStatusCode() / 100 != 2) {
                    throw new CertificateVerificationException("Error getting ocsp response." +
                            "Response code is " + httpResponse.getStatusLine().getStatusCode());
                }
                InputStream in = httpResponse.getEntity().getContent();
                return new OCSPResp(in);
            } catch (IOException e) {
                throw new CertificateVerificationException("Cannot get ocspResponse from url: " + serviceUrl, e);
            }
        }

        /**
         * This method generates an OCSP Request to be sent to an OCSP endpoint.
         *
         * @param issuerCert   is the Certificate of the Issuer of the peer certificate we are interested in.
         * @param serialNumber of the peer certificate.
         * @return generated OCSP request.
         * @throws CertificateVerificationException
         *
         */
        private OCSPReq generateOCSPRequest(X509Certificate issuerCert, BigInteger serialNumber)
                throws CertificateVerificationException {
            String jceProvider = getPreferredJceProvider();
            String providerClass;
            if (jceProvider.equals(Constants.BOUNCY_CASTLE_PROVIDER)) {
                providerClass = "org.bouncycastle.jce.provider.BouncyCastleProvider";
            } else if (jceProvider.equals(Constants.BOUNCY_CASTLE_FIPS_PROVIDER)) {
                providerClass = "org.bouncycastle.jcajce.provider.BouncyCastleFipsProvider";
            } else {
                throw new CertificateVerificationException("Unsupported JCE provider: " + jceProvider);
            }
            try {
                Security.addProvider((Provider) Class.forName(providerClass).getDeclaredConstructor().newInstance());
            } catch (Exception e) {
                throw new CertificateVerificationException("Error while initializing the JCE provider: "
                        + providerClass, e);
            }

            try {

                byte[] issuerCertEnc = issuerCert.getEncoded();
                X509CertificateHolder certificateHolder = new X509CertificateHolder(issuerCertEnc);
                DigestCalculatorProvider digCalcProv = new JcaDigestCalculatorProviderBuilder().setProvider(jceProvider)
                        .build();

                //  CertID structure is used to uniquely identify certificates that are the subject of
                // an OCSP request or response and has an ASN.1 definition. CertID structure is defined in RFC 2560
                CertificateID id = new CertificateID(digCalcProv.get(CertificateID.HASH_SHA1), certificateHolder, serialNumber);

                // basic request generation with nonce
                OCSPReqBuilder builder = new OCSPReqBuilder();
                builder.addRequest(id);

                // create details for nonce extension. The nonce extension is used to bind
                // a request to a response to prevent replay attacks. As the name implies,
                // the nonce value is something that the client should only use once within a reasonably small period.
                BigInteger nonce = BigInteger.valueOf(System.currentTimeMillis());

                //to create the request Extension
                builder.setRequestExtensions(new Extensions(new Extension(OCSPObjectIdentifiers.id_pkix_ocsp_nonce,false, new DEROctetString(nonce.toByteArray()))));

                return builder.build();
            } catch (Exception e) {
                throw new CertificateVerificationException("Cannot generate OSCP Request with the given certificate", e);
            }
        }

        /**
         * Authority Information Access (AIA) is a non-critical extension in an X509 Certificate. This contains the
         * URL of the OCSP endpoint if one is available.
         * TODO: This might contain non OCSP urls as well. Handle this.
         *
         * @param cert is the certificate
         * @return a lit of URLs in AIA extension of the certificate which will hopefully contain an OCSP endpoint.
         * @throws CertificateVerificationException
         *
         */
        private List<String> getAIALocations(X509Certificate cert) throws CertificateVerificationException {

            //Gets the DER-encoded OCTET string for the extension value for Authority information access Points
            byte[] aiaExtensionValue = cert.getExtensionValue(Extension.authorityInfoAccess.getId());
            if (aiaExtensionValue == null)
                throw new CertificateVerificationException("Certificate Doesn't have Authority Information Access points");
            AuthorityInformationAccess authorityInformationAccess;

            try {
                DEROctetString oct = (DEROctetString) (new ASN1InputStream(new ByteArrayInputStream(aiaExtensionValue)).readObject());
                authorityInformationAccess = AuthorityInformationAccess.getInstance(new ASN1InputStream(oct.getOctets()).readObject());
            } catch (IOException e) {
                throw new CertificateVerificationException("Cannot read certificate to get OSCP urls", e);
            }

            List<String> ocspUrlList = new ArrayList<String>();
            AccessDescription[] accessDescriptions = authorityInformationAccess.getAccessDescriptions();
            for (AccessDescription accessDescription : accessDescriptions) {

                GeneralName gn = accessDescription.getAccessLocation();
                if (gn.getTagNo() == GeneralName.uniformResourceIdentifier) {
                    ASN1IA5String str = ASN1IA5String.getInstance(gn.getName());
                    String accessLocation = str.getString();
                    ocspUrlList.add(accessLocation);
                }
            }
            if(ocspUrlList.isEmpty())
                throw new CertificateVerificationException("Cant get OCSP urls from certificate");

            return ocspUrlList;
        }

        /**
         * This method returns the preferred JCE provider to be used.
         *modules/transports/core/nhttp/src/main/java/org/apache/synapse/transport/certificatevalidation/ocsp/OCSPVerifier.java
         * @return
         */
        private static String getPreferredJceProvider() {
            String provider = System.getProperty("security.jce.provider");
            if (provider != null && provider.equalsIgnoreCase(Constants.BOUNCY_CASTLE_FIPS_PROVIDER)) {
                return Constants.BOUNCY_CASTLE_FIPS_PROVIDER;
            }
            return Constants.BOUNCY_CASTLE_PROVIDER;
        }
    }