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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * 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,

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package com.sun.crypto.provider;

import java.security.InvalidKeyException;

/**

 * This class represents ciphers in counter (CTR) mode.

 *

 * <p>This mode is implemented independently of a particular cipher.

 * Ciphers to which this mode should apply (e.g., DES) must be

 * <i>plugged-in</i> using the constructor.

 *

 * <p>NOTE: This class does not deal with buffering or padding.

 *

 * @author Andreas Sterbenz

 * @since 1.4.2

 */

final class CounterMode extends FeedbackCipher {

    // current counter value

    private final byte[] counter;

    // encrypted bytes of the previous counter value

    private final byte[] encryptedCounter;

    // number of bytes in encryptedCounter already used up

    private int used;

    // variables for save/restore calls

    private byte[] counterSave = null;

    private byte[] encryptedCounterSave = null;

    private int usedSave = 0;

    CounterMode(SymmetricCipher embeddedCipher) {

        super(embeddedCipher);

        counter = new byte[blockSize];

        encryptedCounter = new byte[blockSize];

    }

    /**

     * Gets the name of the feedback mechanism

     *

     * @return the name of the feedback mechanism

     */

    String getFeedback() {

        return "CTR";

    }

    /**

     * Resets the iv to its original value.

     * This is used when doFinal is called in the Cipher class, so that the

     * cipher can be reused (with its original iv).

     */

    void reset() {

        System.arraycopy(iv, 0, counter, 0, blockSize);

        used = blockSize;

    }

    /**

     * Save the current content of this cipher.

     */

    void save() {

        if (counterSave == null) {

            counterSave = new byte[blockSize];

            encryptedCounterSave = new byte[blockSize];

        }

        System.arraycopy(counter, 0, counterSave, 0, blockSize);

        System.arraycopy(encryptedCounter, 0, encryptedCounterSave, 0,

            blockSize);

        usedSave = used;

    }

    /**

     * Restores the content of this cipher to the previous saved one.

     */

    void restore() {

        System.arraycopy(counterSave, 0, counter, 0, blockSize);

        System.arraycopy(encryptedCounterSave, 0, encryptedCounter, 0,

            blockSize);

        used = usedSave;

    }

    /**

     * Initializes the cipher in the specified mode with the given key

     * and iv.

     *

     * @param decrypting flag indicating encryption or decryption

     * @param algorithm the algorithm name

     * @param key the key

     * @param iv the iv

     *

     * @exception InvalidKeyException if the given key is inappropriate for

     * initializing this cipher

     */

    void init(boolean decrypting, String algorithm, byte[] key, byte[] iv)

            throws InvalidKeyException {

        if ((key == null) || (iv == null) || (iv.length != blockSize)) {

            throw new InvalidKeyException("Internal error");

        }

        this.iv = iv;

        reset();

        // always encrypt mode for embedded cipher

        embeddedCipher.init(false, algorithm, key);

    }

    /**

     * Performs encryption operation.

     *

     * <p>The input plain text <code>plain</code>, starting at

     * <code>plainOffset</code> and ending at

     * <code>(plainOffset + len - 1)</code>, is encrypted.

     * The result is stored in <code>cipher</code>, starting at

     * <code>cipherOffset</code>.

     *

     * <p>It is the application's responsibility to make sure that

     * <code>plainLen</code> is a multiple of the embedded cipher's block size,

     * as any excess bytes are ignored.

     *

     * <p>It is also the application's responsibility to make sure that

     * <code>init</code> has been called before this method is called.

     * (This check is omitted here, to avoid double checking.)

     *

     * @param in the buffer with the input data to be encrypted

     * @param inOffset the offset in <code>plain</code>

     * @param len the length of the input data

     * @param out the buffer for the result

     * @param outOff the offset in <code>cipher</code>

     */

    void encrypt(byte[] in, int inOff, int len, byte[] out, int outOff) {

        crypt(in, inOff, len, out, outOff);

    }

    /**

     * Performs decryption operation.

     *

     * <p>The input cipher text <code>cipher</code>, starting at

     * <code>cipherOffset</code> and ending at

     * <code>(cipherOffset + len - 1)</code>, is decrypted.

     * The result is stored in <code>plain</code>, starting at

     * <code>plainOffset</code>.

     *

     * <p>It is the application's responsibility to make sure that

     * <code>cipherLen</code> is a multiple of the embedded cipher's block

     * size, as any excess bytes are ignored.

     *

     * <p>It is also the application's responsibility to make sure that

     * <code>init</code> has been called before this method is called.

     * (This check is omitted here, to avoid double checking.)

     *

     * @param in the buffer with the input data to be decrypted

     * @param inOff the offset in <code>cipherOffset</code>

     * @param len the length of the input data

     * @param out the buffer for the result

     * @param outOff the offset in <code>plain</code>

     */

    void decrypt(byte[] in, int inOff, int len, byte[] out, int outOff) {

        crypt(in, inOff, len, out, outOff);

    }

    /**

     * Increment the counter value.

     */

    private static void increment(byte[] b) {

        int n = b.length - 1;

        while ((n >= 0) && (++b[n] == 0)) {

            n--;

        }

    }

    /**

     * Do the actual encryption/decryption operation.

     * Essentially we XOR the input plaintext/ciphertext stream with a

     * keystream generated by encrypting the counter values. Counter values

     * are encrypted on demand.

     */

    private void crypt(byte[] in, int inOff, int len, byte[] out, int outOff) {

        while (len-- > 0) {

            if (used >= blockSize) {

                embeddedCipher.encryptBlock(counter, 0, encryptedCounter, 0);

                increment(counter);

                used = 0;

            }

            out[outOff++] = (byte)(in[inOff++] ^ encryptedCounter[used++]);

        }

    }

}