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OFBBlockCipher.javaAPI DocAndroid 1.5 API5288Wed May 06 22:41:06 BST 2009org.bouncycastle.crypto.modes

OFBBlockCipher

public class OFBBlockCipher extends Object implements org.bouncycastle.crypto.BlockCipher
implements a Output-FeedBack (OFB) mode on top of a simple cipher.

Fields Summary
private byte[]
IV
private byte[]
ofbV
private byte[]
ofbOutV
private final int
blockSize
private final org.bouncycastle.crypto.BlockCipher
cipher
Constructors Summary
public OFBBlockCipher(org.bouncycastle.crypto.BlockCipher cipher, int blockSize)
Basic constructor.

param
cipher the block cipher to be used as the basis of the feedback mode.
param
blockSize the block size in bits (note: a multiple of 8)

        this.cipher = cipher;
        this.blockSize = blockSize / 8;

        this.IV = new byte[cipher.getBlockSize()];
        this.ofbV = new byte[cipher.getBlockSize()];
        this.ofbOutV = new byte[cipher.getBlockSize()];
Methods Summary
public java.lang.StringgetAlgorithmName()
return the algorithm name and mode.

return
the name of the underlying algorithm followed by "/OFB" and the block size in bits

        return cipher.getAlgorithmName() + "/OFB" + (blockSize * 8);
public intgetBlockSize()
return the block size we are operating at (in bytes).

return
the block size we are operating at (in bytes).

        return blockSize;
public org.bouncycastle.crypto.BlockCiphergetUnderlyingCipher()
return the underlying block cipher that we are wrapping.

return
the underlying block cipher that we are wrapping.

        return cipher;
public voidinit(boolean encrypting, org.bouncycastle.crypto.CipherParameters params)
Initialise the cipher and, possibly, the initialisation vector (IV). If an IV isn't passed as part of the parameter, the IV will be all zeros. An IV which is too short is handled in FIPS compliant fashion.

param
encrypting if true the cipher is initialised for encryption, if false for decryption.
param
params the key and other data required by the cipher.
exception
IllegalArgumentException if the params argument is inappropriate.

        if (params instanceof ParametersWithIV)
        {
                ParametersWithIV ivParam = (ParametersWithIV)params;
                byte[]      iv = ivParam.getIV();

                if (iv.length < IV.length)
                {
                    // prepend the supplied IV with zeros (per FIPS PUB 81)
                    System.arraycopy(iv, 0, IV, IV.length - iv.length, iv.length); 
                    for (int i = 0; i < IV.length - iv.length; i++)
                    {
                        IV[i] = 0;
                    }
                }
                else
                {
                    System.arraycopy(iv, 0, IV, 0, IV.length);
                }

                reset();

                cipher.init(true, ivParam.getParameters());
        }
        else
        {
                reset();

                cipher.init(true, params);
        }
public intprocessBlock(byte[] in, int inOff, byte[] out, int outOff)
Process one block of input from the array in and write it to the out array.

param
in the array containing the input data.
param
inOff offset into the in array the data starts at.
param
out the array the output data will be copied into.
param
outOff the offset into the out array the output will start at.
exception
DataLengthException if there isn't enough data in in, or space in out.
exception
IllegalStateException if the cipher isn't initialised.
return
the number of bytes processed and produced.

        if ((inOff + blockSize) > in.length)
        {
            throw new DataLengthException("input buffer too short");
        }

        if ((outOff + blockSize) > out.length)
        {
            throw new DataLengthException("output buffer too short");
        }

        cipher.processBlock(ofbV, 0, ofbOutV, 0);

        //
        // XOR the ofbV with the plaintext producing the cipher text (and
        // the next input block).
        //
        for (int i = 0; i < blockSize; i++)
        {
            out[outOff + i] = (byte)(ofbOutV[i] ^ in[inOff + i]);
        }

        //
        // change over the input block.
        //
        System.arraycopy(ofbV, blockSize, ofbV, 0, ofbV.length - blockSize);
        System.arraycopy(ofbOutV, 0, ofbV, ofbV.length - blockSize, blockSize);

        return blockSize;
public voidreset()
reset the feedback vector back to the IV and reset the underlying cipher.

        System.arraycopy(IV, 0, ofbV, 0, IV.length);

        cipher.reset();