sshd-core/src/main/java/org/apache/sshd/common/io/PacketWriter.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.sshd.common.io;

import java.io.IOException;
import java.nio.channels.Channel;

import org.apache.sshd.common.util.buffer.Buffer;

/**
 * @author <a href="mailto:dev@mina.apache.org">Apache MINA SSHD Project</a>
 */
public interface PacketWriter extends Channel {
    /**
     * Encode and send the given buffer. <B>Note:</B> for session packets the buffer has to have
     * 5 bytes free at the beginning to allow the encoding to take place. Also, the write position
     * of the buffer has to be set to the position of the last byte to write.
     *
     * @param buffer the buffer to encode and send. <B>NOTE:</B> the buffer must not be touched
     * until the returned write future is completed.
     * @return An {@code IoWriteFuture} that can be used to check when the packet has actually been sent
     * @throws IOException if an error occurred when encoding sending the packet
     */
    IoWriteFuture writePacket(Buffer buffer) throws IOException;

    /**
     * @param len The packet payload size
     * @param blockSize The cipher block size
     * @param etmMode Whether using &quot;encrypt-then-MAC&quot; mode
     * @return The required padding length
     */
    static int calculatePadLength(int len, int blockSize, boolean etmMode) {
        /*
         * Note: according to RFC-4253 section 6:
         *
         *    The minimum size of a packet is 16 (or the cipher block size,
         *     whichever is larger) bytes (plus 'mac').
         *
         * Since all out ciphers, MAC(s), etc. have a block size > 8 then
         * the minimum size of the packet will be at least 16 due to the
         * padding at the very least - so even packets that contain an opcode
         * with no arguments will be above this value. This avoids an un-necessary
         * call to Math.max(len, 16) for each and every packet
         */

        len++;  // the pad length
        if (!etmMode) {
            len += Integer.BYTES;
        }

        /*
         * Note: according to RFC-4253 section 6:
         *
         *      Note that the length of the concatenation of 'packet_length',
         *      'padding_length', 'payload', and 'random padding' MUST be a multiple
         *      of the cipher block size or 8, whichever is larger.
         *
         * However, we currently do not have ciphers with a block size of less than
         * 8 so we do not take this into account in order to accelerate the calculation
         * and avoiding an un-necessary call to Math.max(blockSize, 8) for each and every
         * packet.
         */
        int pad = (-len) & (blockSize - 1);
        if (pad < blockSize) {
            pad += blockSize;
        }

        return pad;
    }
}