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package com.sun.crypto.provider;

import java.security.InvalidKeyException;


This class represents ciphers in output-feedback (OFB) 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:Gigi Ankeny
/**
 * This class represents ciphers in output-feedback (OFB) 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 Gigi Ankeny
 */
final class OutputFeedback extends FeedbackCipher {

    /*
     * output buffer
     */
    private byte[] k = null;

    /*
     * register buffer
     */
    private byte[] register = null;

    /*
     * number of bytes for each stream unit, defaults to the blocksize
     * of the embedded cipher
     */
    private int numBytes;

    // variables for save/restore calls
    private byte[] registerSave = null;

    OutputFeedback(SymmetricCipher embeddedCipher, int numBytes) {
        super(embeddedCipher);
        if (numBytes > blockSize) {
            numBytes = blockSize;
        }
        this.numBytes = numBytes;
        k = new byte[blockSize];
        register = new byte[blockSize];
    }

    
Gets the name of this feedback mode.

Returns:the string OFB
/**
     * Gets the name of this feedback mode.
     *
     * @return the string <code>OFB</code>
     */
    String getFeedback() {
        return "OFB";
    }

    
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);
    }

    
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, register, 0, blockSize);
    }

    
Save the current content of this cipher.

/**
     * Save the current content of this cipher.
     */
    void save() {
        if (registerSave == null) {
            registerSave = new byte[blockSize];
        }
        System.arraycopy(register, 0, registerSave, 0, blockSize);
    }

    
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(registerSave, 0, register, 0, blockSize);
    }

    
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 stream unit size
numBytes, 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:
  • plain – the buffer with the input data to be encrypted
  • plainOffset – the offset in plain
  • plainLen – the length of the input data
  • cipher – the buffer for the result
  • cipherOffset – 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 stream unit size
     * <code>numBytes</code>, 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 plain the buffer with the input data to be encrypted
     * @param plainOffset the offset in <code>plain</code>
     * @param plainLen the length of the input data
     * @param cipher the buffer for the result
     * @param cipherOffset the offset in <code>cipher</code>
     */
    void encrypt(byte[] plain, int plainOffset, int plainLen,
                        byte[] cipher, int cipherOffset)
    {
        int i;
        int len = blockSize - numBytes;
        int loopCount = plainLen / numBytes;
        int oddBytes = plainLen % numBytes;

        if (len == 0) {
            for (; loopCount > 0;
                 plainOffset += numBytes, cipherOffset += numBytes,
                 loopCount--) {
                embeddedCipher.encryptBlock(register, 0, k, 0);
                for (i=0; i<numBytes; i++)
                    cipher[i+cipherOffset] =
                        (byte)(k[i] ^ plain[i+plainOffset]);
                System.arraycopy(k, 0, register, 0, numBytes);
            }
            if (oddBytes > 0) {
                embeddedCipher.encryptBlock(register, 0, k, 0);
                for (i=0; i<oddBytes; i++)
                    cipher[i+cipherOffset] =
                        (byte)(k[i] ^ plain[i+plainOffset]);
                System.arraycopy(k, 0, register, 0, numBytes);
            }
        } else {
            for (; loopCount > 0;
                 plainOffset += numBytes, cipherOffset += numBytes,
                 loopCount--) {
                embeddedCipher.encryptBlock(register, 0, k, 0);
                for (i=0; i<numBytes; i++)
                    cipher[i+cipherOffset] =
                        (byte)(k[i] ^ plain[i+plainOffset]);
                System.arraycopy(register, numBytes, register, 0, len);
                System.arraycopy(k, 0, register, len, numBytes);
            }
            if (oddBytes > 0) {
                embeddedCipher.encryptBlock(register, 0, k, 0);
                for (i=0; i<oddBytes; i++)
                    cipher[i+cipherOffset] =
                        (byte)(k[i] ^ plain[i+plainOffset]);
                System.arraycopy(register, numBytes, register, 0, len);
                System.arraycopy(k, 0, register, len, numBytes);
            }
        }
    }

    
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 stream unit size
numBytes, 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:
  • cipher – the buffer with the input data to be decrypted
  • cipherOffset – the offset in cipherOffset
  • cipherLen – the length of the input data
  • plain – the buffer for the result
  • plainOffset – 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 stream unit size
     * <code>numBytes</code>, 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 cipher the buffer with the input data to be decrypted
     * @param cipherOffset the offset in <code>cipherOffset</code>
     * @param cipherLen the length of the input data
     * @param plain the buffer for the result
     * @param plainOffset the offset in <code>plain</code>
     */
    void decrypt(byte[] cipher, int cipherOffset, int cipherLen,
                        byte[] plain, int plainOffset)
    {
        // OFB encrypt and decrypt are identical
        encrypt(cipher, cipherOffset, cipherLen, plain, plainOffset);
    }
}