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package com.sun.crypto.provider;

import java.security.InvalidKeyException;


This class represents ciphers in counter (CTR) mode.

This mode is implemented independently of a particular cipher.
Ciphers to which this mode should apply (e.g., DES) must be
plugged-in using the constructor.

NOTE: This class does not deal with buffering or padding.

Author:Andreas Sterbenz
Since:1.4.2
/**
 * 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

Returns:the name of the feedback mechanism
/**
     * 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).

/**
     * 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.

/**
     * 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.

/**
     * 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.

Params:
  • decrypting – flag indicating encryption or decryption
  • algorithm – the algorithm name
  • key – the key
  • iv – the iv
Throws:
/**
     * 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.

The input plain text plain, starting at
plainOffset and ending at
(plainOffset + len - 1), is encrypted.
The result is stored in cipher, starting at
cipherOffset.

It is the application's responsibility to make sure that
plainLen is a multiple of the embedded cipher's block size,
as any excess bytes are ignored.

It is also the application's responsibility to make sure that
init has been called before this method is called.
(This check is omitted here, to avoid double checking.)

Params:
  • in – the buffer with the input data to be encrypted
  • inOffset – the offset in plain
  • len – the length of the input data
  • out – the buffer for the result
  • outOff – the offset in cipher
/**
     * 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.

The input cipher text cipher, starting at
cipherOffset and ending at
(cipherOffset + len - 1), is decrypted.
The result is stored in plain, starting at
plainOffset.

It is the application's responsibility to make sure that
cipherLen is a multiple of the embedded cipher's block
size, as any excess bytes are ignored.

It is also the application's responsibility to make sure that
init has been called before this method is called.
(This check is omitted here, to avoid double checking.)

Params:
  • in – the buffer with the input data to be decrypted
  • inOff – the offset in cipherOffset
  • len – the length of the input data
  • out – the buffer for the result
  • outOff – the offset in plain
/**
     * 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.

/**
     * 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.

/**
     * 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++]);
        }
    }
}