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package com.sun.security.sasl;

import java.io.IOException;
import java.security.NoSuchAlgorithmException;
import java.util.logging.Level;
import java.util.Map;
import java.util.Random;
import javax.security.sasl.*;
import javax.security.auth.callback.*;

import static java.nio.charset.StandardCharsets.UTF_8;

/**
 * Implements the CRAM-MD5 SASL server-side mechanism.
 * (<A HREF="http://www.ietf.org/rfc/rfc2195.txt">RFC 2195</A>).
 * CRAM-MD5 has no initial response.
 *
 * client <---- M={random, timestamp, server-fqdn} ------- server
 * client ----- {username HMAC_MD5(pw, M)} --------------> server
 *
 * CallbackHandler must be able to handle the following callbacks:
 * - NameCallback: default name is name of user for whom to get password
 * - PasswordCallback: must fill in password; if empty, no pw
 * - AuthorizeCallback: must setAuthorized() and canonicalized authorization id
 *      - auth id == authzid, but needed to get canonicalized authzid
 *
 * @author Rosanna Lee
 */
final class CramMD5Server extends CramMD5Base implements SaslServer {
    private String fqdn;
    private byte[] challengeData = null;
    private String authzid;
    private CallbackHandler cbh;

    /**
     * Creates a CRAM-MD5 SASL server.
     *
     * @param protocol ignored in CRAM-MD5
     * @param serverFqdn non-null, used in generating a challenge
     * @param props ignored in CRAM-MD5
     * @param cbh find password, authorize user
     */
    CramMD5Server(String protocol, String serverFqdn, Map<String, ?> props,
        CallbackHandler cbh) throws SaslException {
        if (serverFqdn == null) {
            throw new SaslException(
                "CRAM-MD5: fully qualified server name must be specified");
        }

        fqdn = serverFqdn;
        this.cbh = cbh;
    }

    /**
     * Generates challenge based on response sent by client.
     *
     * CRAM-MD5 has no initial response.
     * First call generates challenge.
     * Second call verifies client response. If authentication fails, throws
     * SaslException.
     *
     * @param responseData A non-null byte array containing the response
     *        data from the client.
     * @return A non-null byte array containing the challenge to be sent to
     *        the client for the first call; null when 2nd call is successful.
     * @throws SaslException If authentication fails.
     */
    public byte[] evaluateResponse(byte[] responseData)
        throws SaslException {

        // See if we've been here before
        if (completed) {
            throw new IllegalStateException(
                "CRAM-MD5 authentication already completed");
        }

        if (aborted) {
            throw new IllegalStateException(
                "CRAM-MD5 authentication previously aborted due to error");
        }

        try {
            if (challengeData == null) {
                if (responseData.length != 0) {
                    aborted = true;
                    throw new SaslException(
                        "CRAM-MD5 does not expect any initial response");
                }

                // Generate challenge {random, timestamp, fqdn}
                Random random = new Random();
                long rand = random.nextLong();
                long timestamp = System.currentTimeMillis();

                StringBuilder sb = new StringBuilder();
                sb.append('<');
                sb.append(rand);
                sb.append('.');
                sb.append(timestamp);
                sb.append('@');
                sb.append(fqdn);
                sb.append('>');
                String challengeStr = sb.toString();

                logger.log(Level.FINE,
                    "CRAMSRV01:Generated challenge: {0}", challengeStr);

                challengeData = challengeStr.getBytes(UTF_8);
                return challengeData.clone();

            } else {
                // Examine response to see if correctly encrypted challengeData
                if(logger.isLoggable(Level.FINE)) {
                    logger.log(Level.FINE,
                        "CRAMSRV02:Received response: {0}",
                        new String(responseData, UTF_8));
                }

                // Extract username from response
                int ulen = 0;
                for (int i = 0; i < responseData.length; i++) {
                    if (responseData[i] == ' ') {
                        ulen = i;
                        break;
                    }
                }
                if (ulen == 0) {
                    aborted = true;
                    throw new SaslException(
                        "CRAM-MD5: Invalid response; space missing");
                }
                String username = new String(responseData, 0, ulen, UTF_8);

                logger.log(Level.FINE,
                    "CRAMSRV03:Extracted username: {0}", username);

                // Get user's password
                NameCallback ncb =
                    new NameCallback("CRAM-MD5 authentication ID: ", username);
                PasswordCallback pcb =
                    new PasswordCallback("CRAM-MD5 password: ", false);
                cbh.handle(new Callback[]{ncb,pcb});
                char[] pwChars = pcb.getPassword();
                if (pwChars == null || pwChars.length == 0) {
                    // user has no password; OK to disclose to server
                    aborted = true;
                    throw new SaslException(
                        "CRAM-MD5: username not found: " + username);
                }
                pcb.clearPassword();
                String pwStr = new String(pwChars);
                for (int i = 0; i < pwChars.length; i++) {
                    pwChars[i] = 0;
                }
                pw = pwStr.getBytes(UTF_8);

                // Generate a keyed-MD5 digest from the user's password and
                // original challenge.
                String digest = HMAC_MD5(pw, challengeData);

                logger.log(Level.FINE,
                    "CRAMSRV04:Expecting digest: {0}", digest);

                // clear pw when we no longer need it
                clearPassword();

                // Check whether digest is as expected
                byte[] expectedDigest = digest.getBytes(UTF_8);
                int digestLen = responseData.length - ulen - 1;
                if (expectedDigest.length != digestLen) {
                    aborted = true;
                    throw new SaslException("Invalid response");
                }
                int j = 0;
                for (int i = ulen + 1; i < responseData.length ; i++) {
                    if (expectedDigest[j++] != responseData[i]) {
                        aborted = true;
                        throw new SaslException("Invalid response");
                    }
                }

                // All checks out, use AuthorizeCallback to canonicalize name
                AuthorizeCallback acb = new AuthorizeCallback(username, username);
                cbh.handle(new Callback[]{acb});
                if (acb.isAuthorized()) {
                    authzid = acb.getAuthorizedID();
                } else {
                    // Not authorized
                    aborted = true;
                    throw new SaslException(
                        "CRAM-MD5: user not authorized: " + username);
                }

                logger.log(Level.FINE,
                    "CRAMSRV05:Authorization id: {0}", authzid);

                completed = true;
                return null;
            }
        } catch (NoSuchAlgorithmException e) {
            aborted = true;
            throw new SaslException("MD5 algorithm not available on platform", e);
        } catch (SaslException e) {
            throw e; // rethrow
        } catch (UnsupportedCallbackException | IOException e) {
            aborted = true;
            throw new SaslException("CRAM-MD5 authentication failed", e);
        }
    }

    public String getAuthorizationID() {
        if (completed) {
            return authzid;
        } else {
            throw new IllegalStateException(
                "CRAM-MD5 authentication not completed");
        }
    }
}
