package com.amazonaws.kinesisvideo.http;

import java.security.cert.Certificate;
import java.security.cert.CertificateParsingException;
import java.security.cert.X509Certificate;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Locale;
import java.util.regex.Pattern;
import javax.net.ssl.SSLException;
import javax.net.ssl.SSLSession;
import javax.security.auth.x500.X500Principal;

/**
 * A HostnameVerifier consistent with <a href="http://www.ietf.org/rfc/rfc2818.txt">RFC 2818</a>.
 * Adapted from https://github.com/square/okhttp/blob/okhttp_3.9.x/okhttp/src/main/java/okhttp3/internal/tls/OkHostnameVerifier.java
 */
public final class HostnameVerifier implements javax.net.ssl.HostnameVerifier {
    public static final HostnameVerifier INSTANCE = new HostnameVerifier();

    private static final int ALT_DNS_NAME = 2;
    private static final int ALT_IPA_NAME = 7;

    /**
     * Quick and dirty pattern to differentiate IP addresses from hostnames. This is an approximation
     * of Android's private InetAddress#isNumeric API.
     *
     * <p>This matches IPv6 addresses as a hex string containing at least one colon, and possibly
     * including dots after the first colon. It matches IPv4 addresses as strings containing only
     * decimal digits and dots. This pattern matches strings like "a:.23" and "54" that are neither IP
     * addresses nor hostnames; they will be verified as IP addresses (which is a more strict
     * verification).
     */
    private static final Pattern VERIFY_AS_IP_ADDRESS = Pattern.compile(
            "([0-9a-fA-F]*:[0-9a-fA-F:.]*)|([\\d.]+)");

    private HostnameVerifier() {
    }

    @Override
    public boolean verify(String host, SSLSession session) {
        try {
            Certificate[] certificates = session.getPeerCertificates();
            return verify(host, (X509Certificate) certificates[0]);
        } catch (SSLException e) {
            return false;
        }
    }

    public boolean verify(String host, X509Certificate certificate) {
        return VERIFY_AS_IP_ADDRESS.matcher(host).matches()
                ? verifyIpAddress(host, certificate)
                : verifyHostname(host, certificate);
    }

    /** Returns true if {@code certificate} matches {@code ipAddress}. */
    private boolean verifyIpAddress(String ipAddress, X509Certificate certificate) {
        List<String> altNames = getSubjectAltNames(certificate, ALT_IPA_NAME);
        for (int i = 0, size = altNames.size(); i < size; i++) {
            if (ipAddress.equalsIgnoreCase(altNames.get(i))) {
                return true;
            }
        }
        return false;
    }

    /** Returns true if {@code certificate} matches {@code hostname}. */
    private boolean verifyHostname(String hostname, X509Certificate certificate) {
        hostname = hostname.toLowerCase(Locale.US);
        boolean hasDns = false;
        List<String> altNames = getSubjectAltNames(certificate, ALT_DNS_NAME);
        for (int i = 0, size = altNames.size(); i < size; i++) {
            hasDns = true;
            if (verifyHostname(hostname, altNames.get(i))) {
                return true;
            }
        }

        if (!hasDns) {
            X500Principal principal = certificate.getSubjectX500Principal();
            // RFC 2818 advises using the most specific name for matching.
            String cn = new DistinguishedNameParser(principal).findMostSpecific("cn");
            if (cn != null) {
                return verifyHostname(hostname, cn);
            }
        }

        return false;
    }

    public static List<String> allSubjectAltNames(X509Certificate certificate) {
        List<String> altIpaNames = getSubjectAltNames(certificate, ALT_IPA_NAME);
        List<String> altDnsNames = getSubjectAltNames(certificate, ALT_DNS_NAME);
        List<String> result = new ArrayList<>(altIpaNames.size() + altDnsNames.size());
        result.addAll(altIpaNames);
        result.addAll(altDnsNames);
        return result;
    }

    private static List<String> getSubjectAltNames(X509Certificate certificate, int type) {
        List<String> result = new ArrayList<>();
        try {
            Collection<?> subjectAltNames = certificate.getSubjectAlternativeNames();
            if (subjectAltNames == null) {
                return Collections.emptyList();
            }
            for (Object subjectAltName : subjectAltNames) {
                List<?> entry = (List<?>) subjectAltName;
                if (entry == null || entry.size() < 2) {
                    continue;
                }
                Integer altNameType = (Integer) entry.get(0);
                if (altNameType == null) {
                    continue;
                }
                if (altNameType == type) {
                    String altName = (String) entry.get(1);
                    if (altName != null) {
                        result.add(altName);
                    }
                }
            }
            return result;
        } catch (CertificateParsingException e) {
            return Collections.emptyList();
        }
    }

    /**
     * Returns {@code true} iff {@code hostname} matches the domain name {@code pattern}.
     *
     * @param hostname lower-case host name.
     * @param pattern domain name pattern from certificate. May be a wildcard pattern such as {@code
     * *.android.com}.
     */
    public boolean verifyHostname(String hostname, String pattern) {
        // Basic sanity checks
        // Check length == 0 instead of .isEmpty() to support Java 5.
        if ((hostname == null) || (hostname.length() == 0) || (hostname.startsWith("."))
                || (hostname.endsWith(".."))) {
            // Invalid domain name
            return false;
        }
        if ((pattern == null) || (pattern.length() == 0) || (pattern.startsWith("."))
                || (pattern.endsWith(".."))) {
            // Invalid pattern/domain name
            return false;
        }

        // Normalize hostname and pattern by turning them into absolute domain names if they are not
        // yet absolute. This is needed because server certificates do not normally contain absolute
        // names or patterns, but they should be treated as absolute. At the same time, any hostname
        // presented to this method should also be treated as absolute for the purposes of matching
        // to the server certificate.
        //   www.android.com  matches www.android.com
        //   www.android.com  matches www.android.com.
        //   www.android.com. matches www.android.com.
        //   www.android.com. matches www.android.com
        if (!hostname.endsWith(".")) {
            hostname += '.';
        }
        if (!pattern.endsWith(".")) {
            pattern += '.';
        }
        // hostname and pattern are now absolute domain names.

        pattern = pattern.toLowerCase(Locale.US);
        // hostname and pattern are now in lower case -- domain names are case-insensitive.

        if (!pattern.contains("*")) {
            // Not a wildcard pattern -- hostname and pattern must match exactly.
            return hostname.equals(pattern);
        }
        // Wildcard pattern

        // WILDCARD PATTERN RULES:
        // 1. Asterisk (*) is only permitted in the left-most domain name label and must be the
        //    only character in that label (i.e., must match the whole left-most label).
        //    For example, *.example.com is permitted, while *a.example.com, a*.example.com,
        //    a*b.example.com, a.*.example.com are not permitted.
        // 2. Asterisk (*) cannot match across domain name labels.
        //    For example, *.example.com matches test.example.com but does not match
        //    sub.test.example.com.
        // 3. Wildcard patterns for single-label domain names are not permitted.

        if ((!pattern.startsWith("*.")) || (pattern.indexOf('*', 1) != -1)) {
            // Asterisk (*) is only permitted in the left-most domain name label and must be the only
            // character in that label
            return false;
        }

        // Optimization: check whether hostname is too short to match the pattern. hostName must be at
        // least as long as the pattern because asterisk must match the whole left-most label and
        // hostname starts with a non-empty label. Thus, asterisk has to match one or more characters.
        if (hostname.length() < pattern.length()) {
            // hostname too short to match the pattern.
            return false;
        }

        if ("*.".equals(pattern)) {
            // Wildcard pattern for single-label domain name -- not permitted.
            return false;
        }

        // hostname must end with the region of pattern following the asterisk.
        String suffix = pattern.substring(1);
        if (!hostname.endsWith(suffix)) {
            // hostname does not end with the suffix
            return false;
        }

        // Check that asterisk did not match across domain name labels.
        int suffixStartIndexInHostname = hostname.length() - suffix.length();
        if ((suffixStartIndexInHostname > 0)
                && (hostname.lastIndexOf('.', suffixStartIndexInHostname - 1) != -1)) {
            // Asterisk is matching across domain name labels -- not permitted.
            return false;
        }

        // hostname matches pattern
        return true;
    }
}