/*
 * Copyright (c) 2004, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package sun.security.ssl;

import java.lang.ref.*;
import java.net.Socket;
import java.security.AlgorithmConstraints;
import java.security.KeyStore;
import java.security.KeyStore.Builder;
import java.security.KeyStore.Entry;
import java.security.KeyStore.PrivateKeyEntry;
import java.security.Principal;
import java.security.PrivateKey;
import java.security.cert.CertPathValidatorException;
import java.security.cert.Certificate;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;
import java.util.*;
import java.util.concurrent.atomic.AtomicLong;
import javax.net.ssl.*;
import sun.security.provider.certpath.AlgorithmChecker;
import sun.security.validator.Validator;

The new X509 key manager implementation. The main differences to the
old SunX509 key manager are:
 . it is based around the KeyStore.Builder API. This allows it to use
   other forms of KeyStore protection or password input (e.g. a
   CallbackHandler) or to have keys within one KeyStore protected by
   different keys.
 . it can use multiple KeyStores at the same time.
 . it is explicitly designed to accommodate KeyStores that change over
   the lifetime of the process.
 . it makes an effort to choose the key that matches best, i.e. one that
   is not expired and has the appropriate certificate extensions.
Note that this code is not explicitly performance optimzied yet.
Author:
 Andreas Sterbenz/**
 * The new X509 key manager implementation. The main differences to the
 * old SunX509 key manager are:
 *  . it is based around the KeyStore.Builder API. This allows it to use
 *    other forms of KeyStore protection or password input (e.g. a
 *    CallbackHandler) or to have keys within one KeyStore protected by
 *    different keys.
 *  . it can use multiple KeyStores at the same time.
 *  . it is explicitly designed to accommodate KeyStores that change over
 *    the lifetime of the process.
 *  . it makes an effort to choose the key that matches best, i.e. one that
 *    is not expired and has the appropriate certificate extensions.
 *
 * Note that this code is not explicitly performance optimzied yet.
 *
 * @author  Andreas Sterbenz
 */
final class X509KeyManagerImpl extends X509ExtendedKeyManager
        implements X509KeyManager {

    // for unit testing only, set via privileged reflection
    private static Date verificationDate;

    // list of the builders
    private final List<Builder> builders;

    // counter to generate unique ids for the aliases
    private final AtomicLong uidCounter;

    // cached entries
    private final Map<String,Reference<PrivateKeyEntry>> entryCacheMap;

    X509KeyManagerImpl(Builder builder) {
        this(Collections.singletonList(builder));
    }

    X509KeyManagerImpl(List<Builder> builders) {
        this.builders = builders;
        uidCounter = new AtomicLong();
        entryCacheMap = Collections.synchronizedMap
                        (new SizedMap<String,Reference<PrivateKeyEntry>>());
    }

    // LinkedHashMap with a max size of 10
    // see LinkedHashMap JavaDocs
    private static class SizedMap<K,V> extends LinkedHashMap<K,V> {
        private static final long serialVersionUID = -8211222668790986062L;

        @Override protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
            return size() > 10;
        }
    }

    //
    // public methods
    //

    @Override
    public X509Certificate[] getCertificateChain(String alias) {
        PrivateKeyEntry entry = getEntry(alias);
        return entry == null ? null :
                (X509Certificate[])entry.getCertificateChain();
    }

    @Override
    public PrivateKey getPrivateKey(String alias) {
        PrivateKeyEntry entry = getEntry(alias);
        return entry == null ? null : entry.getPrivateKey();
    }

    @Override
    public String chooseClientAlias(String[] keyTypes, Principal[] issuers,
            Socket socket) {
        return chooseAlias(getKeyTypes(keyTypes), issuers, CheckType.CLIENT,
                        getAlgorithmConstraints(socket));
    }

    @Override
    public String chooseEngineClientAlias(String[] keyTypes,
            Principal[] issuers, SSLEngine engine) {
        return chooseAlias(getKeyTypes(keyTypes), issuers, CheckType.CLIENT,
                        getAlgorithmConstraints(engine));
    }

    @Override
    public String chooseServerAlias(String keyType,
            Principal[] issuers, Socket socket) {
        return chooseAlias(getKeyTypes(keyType), issuers, CheckType.SERVER,
            getAlgorithmConstraints(socket),
            X509TrustManagerImpl.getRequestedServerNames(socket),
            "HTTPS");    // The SNI HostName is a fully qualified domain name.
                         // The certificate selection scheme for SNI HostName
                         // is similar to HTTPS endpoint identification scheme
                         // implemented in this provider.
                         //
                         // Using HTTPS endpoint identification scheme to guide
                         // the selection of an appropriate authentication
                         // certificate according to requested SNI extension.
                         //
                         // It is not a really HTTPS endpoint identification.
    }

    @Override
    public String chooseEngineServerAlias(String keyType,
            Principal[] issuers, SSLEngine engine) {
        return chooseAlias(getKeyTypes(keyType), issuers, CheckType.SERVER,
            getAlgorithmConstraints(engine),
            X509TrustManagerImpl.getRequestedServerNames(engine),
            "HTTPS");    // The SNI HostName is a fully qualified domain name.
                         // The certificate selection scheme for SNI HostName
                         // is similar to HTTPS endpoint identification scheme
                         // implemented in this provider.
                         //
                         // Using HTTPS endpoint identification scheme to guide
                         // the selection of an appropriate authentication
                         // certificate according to requested SNI extension.
                         //
                         // It is not a really HTTPS endpoint identification.
    }

    @Override
    public String[] getClientAliases(String keyType, Principal[] issuers) {
        return getAliases(keyType, issuers, CheckType.CLIENT, null);
    }

    @Override
    public String[] getServerAliases(String keyType, Principal[] issuers) {
        return getAliases(keyType, issuers, CheckType.SERVER, null);
    }

    //
    // implementation private methods
    //

    // Gets algorithm constraints of the socket.
    private AlgorithmConstraints getAlgorithmConstraints(Socket socket) {
        if (socket != null && socket.isConnected() &&
                                        socket instanceof SSLSocket) {

            SSLSocket sslSocket = (SSLSocket)socket;
            SSLSession session = sslSocket.getHandshakeSession();

            if (session != null) {
                if (ProtocolVersion.useTLS12PlusSpec(session.getProtocol())) {
                    String[] peerSupportedSignAlgs = null;

                    if (session instanceof ExtendedSSLSession) {
                        ExtendedSSLSession extSession =
                            (ExtendedSSLSession)session;
                        peerSupportedSignAlgs =
                            extSession.getPeerSupportedSignatureAlgorithms();
                    }

                    return new SSLAlgorithmConstraints(
                        sslSocket, peerSupportedSignAlgs, true);
                }
            }

            return new SSLAlgorithmConstraints(sslSocket, true);
        }

        return new SSLAlgorithmConstraints((SSLSocket)null, true);
    }

    // Gets algorithm constraints of the engine.
    private AlgorithmConstraints getAlgorithmConstraints(SSLEngine engine) {
        if (engine != null) {
            SSLSession session = engine.getHandshakeSession();
            if (session != null) {
                if (ProtocolVersion.useTLS12PlusSpec(session.getProtocol())) {
                    String[] peerSupportedSignAlgs = null;

                    if (session instanceof ExtendedSSLSession) {
                        ExtendedSSLSession extSession =
                            (ExtendedSSLSession)session;
                        peerSupportedSignAlgs =
                            extSession.getPeerSupportedSignatureAlgorithms();
                    }

                    return new SSLAlgorithmConstraints(
                        engine, peerSupportedSignAlgs, true);
                }
            }
        }

        return new SSLAlgorithmConstraints(engine, true);
    }

    // we construct the alias we return to JSSE as seen in the code below
    // a unique id is included to allow us to reliably cache entries
    // between the calls to getCertificateChain() and getPrivateKey()
    // even if tokens are inserted or removed
    private String makeAlias(EntryStatus entry) {
        return uidCounter.incrementAndGet() + "." + entry.builderIndex + "."
                + entry.alias;
    }

    private PrivateKeyEntry getEntry(String alias) {
        // if the alias is null, return immediately
        if (alias == null) {
            return null;
        }

        // try to get the entry from cache
        Reference<PrivateKeyEntry> ref = entryCacheMap.get(alias);
        PrivateKeyEntry entry = (ref != null) ? ref.get() : null;
        if (entry != null) {
            return entry;
        }

        // parse the alias
        int firstDot = alias.indexOf('.');
        int secondDot = alias.indexOf('.', firstDot + 1);
        if ((firstDot == -1) || (secondDot == firstDot)) {
            // invalid alias
            return null;
        }
        try {
            int builderIndex = Integer.parseInt
                                (alias.substring(firstDot + 1, secondDot));
            String keyStoreAlias = alias.substring(secondDot + 1);
            Builder builder = builders.get(builderIndex);
            KeyStore ks = builder.getKeyStore();
            Entry newEntry = ks.getEntry
                    (keyStoreAlias, builder.getProtectionParameter(alias));
            if (newEntry instanceof PrivateKeyEntry == false) {
                // unexpected type of entry
                return null;
            }
            entry = (PrivateKeyEntry)newEntry;
            entryCacheMap.put(alias, new SoftReference<PrivateKeyEntry>(entry));
            return entry;
        } catch (Exception e) {
            // ignore
            return null;
        }
    }

    // Class to help verify that the public key algorithm (and optionally
    // the signature algorithm) of a certificate matches what we need.
    private static class KeyType {

        final String keyAlgorithm;

        // In TLS 1.2, the signature algorithm  has been obsoleted by the
        // supported_signature_algorithms, and the certificate type no longer
        // restricts the algorithm used to sign the certificate.
        //
        // However, because we don't support certificate type checking other
        // than rsa_sign, dss_sign and ecdsa_sign, we don't have to check the
        // protocol version here.
        final String sigKeyAlgorithm;

        KeyType(String algorithm) {
            int k = algorithm.indexOf('_');
            if (k == -1) {
                keyAlgorithm = algorithm;
                sigKeyAlgorithm = null;
            } else {
                keyAlgorithm = algorithm.substring(0, k);
                sigKeyAlgorithm = algorithm.substring(k + 1);
            }
        }

        boolean matches(Certificate[] chain) {
            if (!chain[0].getPublicKey().getAlgorithm().equals(keyAlgorithm)) {
                return false;
            }
            if (sigKeyAlgorithm == null) {
                return true;
            }
            if (chain.length > 1) {
                // if possible, check the public key in the issuer cert
                return sigKeyAlgorithm.equals(
                        chain[1].getPublicKey().getAlgorithm());
            } else {
                // Check the signature algorithm of the certificate itself.
                // Look for the "withRSA" in "SHA1withRSA", etc.
                X509Certificate issuer = (X509Certificate)chain[0];
                String sigAlgName =
                        issuer.getSigAlgName().toUpperCase(Locale.ENGLISH);
                String pattern =
                        "WITH" + sigKeyAlgorithm.toUpperCase(Locale.ENGLISH);
                return sigAlgName.contains(pattern);
            }
        }
    }

    private static List<KeyType> getKeyTypes(String ... keyTypes) {
        if ((keyTypes == null) ||
                (keyTypes.length == 0) || (keyTypes[0] == null)) {
            return null;
        }
        List<KeyType> list = new ArrayList<>(keyTypes.length);
        for (String keyType : keyTypes) {
            list.add(new KeyType(keyType));
        }
        return list;
    }

    /*
     * Return the best alias that fits the given parameters.
     * The algorithm we use is:
     *   . scan through all the aliases in all builders in order
     *   . as soon as we find a perfect match, return
     *     (i.e. a match with a cert that has appropriate key usage,
     *      qualified endpoint identity, and is not expired).
     *   . if we do not find a perfect match, keep looping and remember
     *     the imperfect matches
     *   . at the end, sort the imperfect matches. we prefer expired certs
     *     with appropriate key usage to certs with the wrong key usage.
     *     return the first one of them.
     */
    private String chooseAlias(List<KeyType> keyTypeList, Principal[] issuers,
            CheckType checkType, AlgorithmConstraints constraints) {

        return chooseAlias(keyTypeList, issuers,
                                    checkType, constraints, null, null);
    }

    private String chooseAlias(List<KeyType> keyTypeList, Principal[] issuers,
            CheckType checkType, AlgorithmConstraints constraints,
            List<SNIServerName> requestedServerNames, String idAlgorithm) {

        if (keyTypeList == null || keyTypeList.isEmpty()) {
            return null;
        }

        Set<Principal> issuerSet = getIssuerSet(issuers);
        List<EntryStatus> allResults = null;
        for (int i = 0, n = builders.size(); i < n; i++) {
            try {
                List<EntryStatus> results = getAliases(i, keyTypeList,
                            issuerSet, false, checkType, constraints,
                            requestedServerNames, idAlgorithm);
                if (results != null) {
                    // the results will either be a single perfect match
                    // or 1 or more imperfect matches
                    // if it's a perfect match, return immediately
                    EntryStatus status = results.get(0);
                    if (status.checkResult == CheckResult.OK) {
                        if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
                            SSLLogger.fine("KeyMgr: choosing key: " + status);
                        }
                        return makeAlias(status);
                    }
                    if (allResults == null) {
                        allResults = new ArrayList<EntryStatus>();
                    }
                    allResults.addAll(results);
                }
            } catch (Exception e) {
                // ignore
            }
        }
        if (allResults == null) {
            if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
                SSLLogger.fine("KeyMgr: no matching key found");
            }
            return null;
        }
        Collections.sort(allResults);
        if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
            SSLLogger.fine(
                    "KeyMgr: no good matching key found, "
                    + "returning best match out of", allResults);
        }
        return makeAlias(allResults.get(0));
    }

    /*
     * Return all aliases that (approximately) fit the parameters.
     * These are perfect matches plus imperfect matches (expired certificates
     * and certificates with the wrong extensions).
     * The perfect matches will be first in the array.
     */
    public String[] getAliases(String keyType, Principal[] issuers,
            CheckType checkType, AlgorithmConstraints constraints) {
        if (keyType == null) {
            return null;
        }

        Set<Principal> issuerSet = getIssuerSet(issuers);
        List<KeyType> keyTypeList = getKeyTypes(keyType);
        List<EntryStatus> allResults = null;
        for (int i = 0, n = builders.size(); i < n; i++) {
            try {
                List<EntryStatus> results = getAliases(i, keyTypeList,
                                    issuerSet, true, checkType, constraints,
                                    null, null);
                if (results != null) {
                    if (allResults == null) {
                        allResults = new ArrayList<>();
                    }
                    allResults.addAll(results);
                }
            } catch (Exception e) {
                // ignore
            }
        }
        if (allResults == null || allResults.isEmpty()) {
            if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
                SSLLogger.fine("KeyMgr: no matching alias found");
            }
            return null;
        }
        Collections.sort(allResults);
        if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
            SSLLogger.fine("KeyMgr: getting aliases", allResults);
        }
        return toAliases(allResults);
    }

    // turn candidate entries into unique aliases we can return to JSSE
    private String[] toAliases(List<EntryStatus> results) {
        String[] s = new String[results.size()];
        int i = 0;
        for (EntryStatus result : results) {
            s[i++] = makeAlias(result);
        }
        return s;
    }

    // make a Set out of the array
    private Set<Principal> getIssuerSet(Principal[] issuers) {
        if ((issuers != null) && (issuers.length != 0)) {
            return new HashSet<>(Arrays.asList(issuers));
        } else {
            return null;
        }
    }

    // a candidate match
    // identifies the entry by builder and alias
    // and includes the result of the certificate check
    private static class EntryStatus implements Comparable<EntryStatus> {

        final int builderIndex;
        final int keyIndex;
        final String alias;
        final CheckResult checkResult;

        EntryStatus(int builderIndex, int keyIndex, String alias,
                Certificate[] chain, CheckResult checkResult) {
            this.builderIndex = builderIndex;
            this.keyIndex = keyIndex;
            this.alias = alias;
            this.checkResult = checkResult;
        }

        @Override
        public int compareTo(EntryStatus other) {
            int result = this.checkResult.compareTo(other.checkResult);
            return (result == 0) ? (this.keyIndex - other.keyIndex) : result;
        }

        @Override
        public String toString() {
            String s = alias + " (verified: " + checkResult + ")";
            if (builderIndex == 0) {
                return s;
            } else {
                return "Builder #" + builderIndex + ", alias: " + s;
            }
        }
    }

    // enum for the type of certificate check we want to perform
    // (client or server)
    // also includes the check code itself
    private static enum CheckType {

        // enum constant for "no check" (currently not used)
        NONE(Collections.<String>emptySet()),

        // enum constant for "tls client" check
        // valid EKU for TLS client: any, tls_client
        CLIENT(new HashSet<String>(Arrays.asList(new String[] {
            "2.5.29.37.0", "1.3.6.1.5.5.7.3.2" }))),

        // enum constant for "tls server" check
        // valid EKU for TLS server: any, tls_server, ns_sgc, ms_sgc
        SERVER(new HashSet<String>(Arrays.asList(new String[] {
            "2.5.29.37.0", "1.3.6.1.5.5.7.3.1", "2.16.840.1.113730.4.1",
            "1.3.6.1.4.1.311.10.3.3" })));

        // set of valid EKU values for this type
        final Set<String> validEku;

        CheckType(Set<String> validEku) {
            this.validEku = validEku;
        }

        private static boolean getBit(boolean[] keyUsage, int bit) {
            return (bit < keyUsage.length) && keyUsage[bit];
        }

        // Check if this certificate is appropriate for this type of use
        // first check extensions, if they match, check expiration.
        //
        // Note: we may want to move this code into the sun.security.validator
        // package
        CheckResult check(X509Certificate cert, Date date,
                List<SNIServerName> serverNames, String idAlgorithm) {

            if (this == NONE) {
                return CheckResult.OK;
            }

            // check extensions
            try {
                // check extended key usage
                List<String> certEku = cert.getExtendedKeyUsage();
                if ((certEku != null) &&
                        Collections.disjoint(validEku, certEku)) {
                    // if extension present and it does not contain any of
                    // the valid EKU OIDs, return extension_mismatch
                    return CheckResult.EXTENSION_MISMATCH;
                }

                // check key usage
                boolean[] ku = cert.getKeyUsage();
                if (ku != null) {
                    String algorithm = cert.getPublicKey().getAlgorithm();
                    boolean supportsDigitalSignature = getBit(ku, 0);
                    switch (algorithm) {
                        case "RSA":
                            // require either signature bit
                            // or if server also allow key encipherment bit
                            if (!supportsDigitalSignature) {
                                if (this == CLIENT || getBit(ku, 2) == false) {
                                    return CheckResult.EXTENSION_MISMATCH;
                                }
                            }
                            break;
                        case "RSASSA-PSS":
                            if (!supportsDigitalSignature && (this == SERVER)) {
                                return CheckResult.EXTENSION_MISMATCH;
                            }
                            break;
                        case "DSA":
                            // require signature bit
                            if (!supportsDigitalSignature) {
                                return CheckResult.EXTENSION_MISMATCH;
                            }
                            break;
                        case "DH":
                            // require keyagreement bit
                            if (getBit(ku, 4) == false) {
                                return CheckResult.EXTENSION_MISMATCH;
                            }
                            break;
                        case "EC":
                            // require signature bit
                            if (!supportsDigitalSignature) {
                                return CheckResult.EXTENSION_MISMATCH;
                            }
                            // For servers, also require key agreement.
                            // This is not totally accurate as the keyAgreement
                            // bit is only necessary for static ECDH key
                            // exchange and not ephemeral ECDH. We leave it in
                            // for now until there are signs that this check
                            // causes problems for real world EC certificates.
                            if ((this == SERVER) && (getBit(ku, 4) == false)) {
                                return CheckResult.EXTENSION_MISMATCH;
                            }
                            break;
                    }
                }
            } catch (CertificateException e) {
                // extensions unparseable, return failure
                return CheckResult.EXTENSION_MISMATCH;
            }

            try {
                cert.checkValidity(date);
            } catch (CertificateException e) {
                return CheckResult.EXPIRED;
            }

            if (serverNames != null && !serverNames.isEmpty()) {
                for (SNIServerName serverName : serverNames) {
                    if (serverName.getType() ==
                                StandardConstants.SNI_HOST_NAME) {
                        if (!(serverName instanceof SNIHostName)) {
                            try {
                                serverName =
                                    new SNIHostName(serverName.getEncoded());
                            } catch (IllegalArgumentException iae) {
                                // unlikely to happen, just in case ...
                                if (SSLLogger.isOn &&
                                        SSLLogger.isOn("keymanager")) {
                                    SSLLogger.fine(
                                       "Illegal server name: " + serverName);
                                }

                                return CheckResult.INSENSITIVE;
                            }
                        }
                        String hostname =
                                ((SNIHostName)serverName).getAsciiName();

                        try {
                            X509TrustManagerImpl.checkIdentity(hostname,
                                                        cert, idAlgorithm);
                        } catch (CertificateException e) {
                            if (SSLLogger.isOn &&
                                    SSLLogger.isOn("keymanager")) {
                                SSLLogger.fine(
                                    "Certificate identity does not match " +
                                    "Server Name Inidication (SNI): " +
                                    hostname);
                            }
                            return CheckResult.INSENSITIVE;
                        }

                        break;
                    }
                }
            }

            return CheckResult.OK;
        }

        public String getValidator() {
            if (this == CLIENT) {
                return Validator.VAR_TLS_CLIENT;
            } else if (this == SERVER) {
                return Validator.VAR_TLS_SERVER;
            }
            return Validator.VAR_GENERIC;
        }
    }

    // enum for the result of the extension check
    // NOTE: the order of the constants is important as they are used
    // for sorting, i.e. OK is best, followed by EXPIRED and EXTENSION_MISMATCH
    private static enum CheckResult {
        OK,                     // ok or not checked
        INSENSITIVE,            // server name indication insensitive
        EXPIRED,                // extensions valid but cert expired
        EXTENSION_MISMATCH,     // extensions invalid (expiration not checked)
    }

    /*
     * Return a List of all candidate matches in the specified builder
     * that fit the parameters.
     * We exclude entries in the KeyStore if they are not:
     *  . private key entries
     *  . the certificates are not X509 certificates
     *  . the algorithm of the key in the EE cert doesn't match one of keyTypes
     *  . none of the certs is issued by a Principal in issuerSet
     * Using those entries would not be possible or they would almost
     * certainly be rejected by the peer.
     *
     * In addition to those checks, we also check the extensions in the EE
     * cert and its expiration. Even if there is a mismatch, we include
     * such certificates because they technically work and might be accepted
     * by the peer. This leads to more graceful failure and better error
     * messages if the cert expires from one day to the next.
     *
     * The return values are:
     *   . null, if there are no matching entries at all
     *   . if 'findAll' is 'false' and there is a perfect match, a List
     *     with a single element (early return)
     *   . if 'findAll' is 'false' and there is NO perfect match, a List
     *     with all the imperfect matches (expired, wrong extensions)
     *   . if 'findAll' is 'true', a List with all perfect and imperfect
     *     matches
     */
    private List<EntryStatus> getAliases(int builderIndex,
            List<KeyType> keyTypes, Set<Principal> issuerSet,
            boolean findAll, CheckType checkType,
            AlgorithmConstraints constraints,
            List<SNIServerName> requestedServerNames,
            String idAlgorithm) throws Exception {

        Builder builder = builders.get(builderIndex);
        KeyStore ks = builder.getKeyStore();
        List<EntryStatus> results = null;
        Date date = verificationDate;
        boolean preferred = false;
        for (Enumeration<String> e = ks.aliases(); e.hasMoreElements(); ) {
            String alias = e.nextElement();
            // check if it is a key entry (private key or secret key)
            if (!ks.isKeyEntry(alias)) {
                continue;
            }

            Certificate[] chain = ks.getCertificateChain(alias);
            if ((chain == null) || (chain.length == 0)) {
                // must be secret key entry, ignore
                continue;
            }

            boolean incompatible = false;
            for (Certificate cert : chain) {
                if (cert instanceof X509Certificate == false) {
                    // not an X509Certificate, ignore this alias
                    incompatible = true;
                    break;
                }
            }
            if (incompatible) {
                continue;
            }

            // check keytype
            int keyIndex = -1;
            int j = 0;
            for (KeyType keyType : keyTypes) {
                if (keyType.matches(chain)) {
                    keyIndex = j;
                    break;
                }
                j++;
            }
            if (keyIndex == -1) {
                if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
                    SSLLogger.fine("Ignore alias " + alias
                                + ": key algorithm does not match");
                }
                continue;
            }
            // check issuers
            if (issuerSet != null) {
                boolean found = false;
                for (Certificate cert : chain) {
                    X509Certificate xcert = (X509Certificate)cert;
                    if (issuerSet.contains(xcert.getIssuerX500Principal())) {
                        found = true;
                        break;
                    }
                }
                if (found == false) {
                    if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
                        SSLLogger.fine(
                                "Ignore alias " + alias
                                + ": issuers do not match");
                    }
                    continue;
                }
            }

            // check the algorithm constraints
            if (constraints != null &&
                    !conformsToAlgorithmConstraints(constraints, chain,
                            checkType.getValidator())) {

                if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
                    SSLLogger.fine("Ignore alias " + alias +
                            ": certificate list does not conform to " +
                            "algorithm constraints");
                }
                continue;
            }

            if (date == null) {
                date = new Date();
            }
            CheckResult checkResult =
                    checkType.check((X509Certificate)chain[0], date,
                                    requestedServerNames, idAlgorithm);
            EntryStatus status =
                    new EntryStatus(builderIndex, keyIndex,
                                        alias, chain, checkResult);
            if (!preferred && checkResult == CheckResult.OK && keyIndex == 0) {
                preferred = true;
            }
            if (preferred && (findAll == false)) {
                // if we have a good match and do not need all matches,
                // return immediately
                return Collections.singletonList(status);
            } else {
                if (results == null) {
                    results = new ArrayList<>();
                }
                results.add(status);
            }
        }
        return results;
    }

    private static boolean conformsToAlgorithmConstraints(
            AlgorithmConstraints constraints, Certificate[] chain,
            String variant) {

        AlgorithmChecker checker =
                new AlgorithmChecker(constraints, null, variant);
        try {
            checker.init(false);
        } catch (CertPathValidatorException cpve) {
            // unlikely to happen
            if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
                SSLLogger.fine(
                    "Cannot initialize algorithm constraints checker", cpve);
            }

            return false;
        }

        // It is a forward checker, so we need to check from trust to target.
        for (int i = chain.length - 1; i >= 0; i--) {
            Certificate cert = chain[i];
            try {
                // We don't care about the unresolved critical extensions.
                checker.check(cert, Collections.<String>emptySet());
            } catch (CertPathValidatorException cpve) {
                if (SSLLogger.isOn && SSLLogger.isOn("keymanager")) {
                    SSLLogger.fine("Certificate does not conform to " +
                            "algorithm constraints", cert, cpve);
                }

                return false;
            }
        }

        return true;
    }
}