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Kiwi is a utility library. We really like Google's Guava, and also use Apache Commons. But if they don't have something we need, and we think it is useful, this is where we put it.

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package org.kiwiproject.jsch;

import static com.google.common.base.Preconditions.checkState;
import static org.kiwiproject.base.KiwiPreconditions.checkArgumentNotBlank;
import static org.kiwiproject.base.KiwiPreconditions.checkArgumentNotNull;

import com.google.common.net.InetAddresses;
import com.jcraft.jsch.HostKey;
import com.jcraft.jsch.HostKeyRepository;
import com.jcraft.jsch.Session;
import lombok.experimental.UtilityClass;

import java.util.Arrays;
import java.util.Optional;

/**
 * Static utilities for working with JSch.
 */
@UtilityClass
public class KiwiJSchHelpers {

    /**
     * Detect the key exchange for a given host (or IP address) and {@link HostKeyRepository}, which is
     * JSch's encapsulation of the {@code known hosts} file.
     *
     * @param host       the host name or IP to match
     * @param knownHosts the known hosts
     * @return an Optional containing the detected key exchange type, or an empty Optional
     * @implNote Finds the first matching known host entry
     */
    public static Optional detectKeyExchangeTypeForHost(String host, HostKeyRepository knownHosts) {
        checkArgumentNotBlank(host);
        checkArgumentNotNull(knownHosts);

        return Arrays.stream(knownHosts.getHostKey())
                .filter(knownHost -> hostMatchesKnownHost(host, knownHost))
                .map(HostKey::getType)
                .findFirst();
    }

    /**
     * Sets the given key exchange type (e.g. {@code ssh-rsa} or {@code ecdsa-sha2-nistp256} on the specified session.
     *
     * @param session         the JSch session
     * @param keyExchangeType key exchange type
     * @implNote JSch does not appear to have any constants to use, so it's just raw strings
     */
    public static void setSessionKeyExchangeType(Session session, String keyExchangeType) {
        checkArgumentNotNull(session);
        checkArgumentNotBlank(keyExchangeType);
        session.setConfig("server_host_key", keyExchangeType);
    }

    @SuppressWarnings("UnstableApiUsage")
    private static boolean hostMatchesKnownHost(String hostOrIpToFind, HostKey hostKey) {
        var knownHost = new KnownHost(hostKey);
        if (InetAddresses.isInetAddress(hostOrIpToFind)) {
            return hostOrIpToFind.equals(knownHost.ipAddress);
        }
        return hostOrIpToFind.equals(knownHost.hostName);
    }

    private static class KnownHost {
        final String hostName;
        final String ipAddress;

        KnownHost(HostKey knownHost) {
            String host = knownHost.getHost();
            if (containsCommaSeparatedHostAndIpAddress(host)) {
                String[] splat = host.split(",");
                checkState(splat.length > 1, "Expecting host key to be in format: hostName,IP");
                hostName = splat[0];
                ipAddress = splat[1];
            } else {
                hostName = host;
                ipAddress = null;
            }
        }

        private static boolean containsCommaSeparatedHostAndIpAddress(String value) {
            return value.contains(",");
        }
    }
}