TBC (GNU cryptographic primitives and tools)
Keywords: gnu.crypto.pad.TBC,TBC class

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## gnu.crypto.pad
Class TBC

    java.lang.Object
      |
      +--gnu.crypto.pad.BasePad
            |
            +--gnu.crypto.pad.TBC

All Implemented Interfaces:
  IPad

public final class TBC
extends BasePad

The implementation of the Trailing Bit Complement (TBC) padding algorithm.

In this mode, "...the data string is padded at the trailing end with the complement of the trailing bit of the unpadded message: if the trailing bit is `1`, then `0` bits are appended, and if the trailing bit is `0`, then `1` bits are appended. As few bits are added as are necessary to meet the formatting size requirement."

References:
Recommendation for Block Cipher Modes of Operation Methods and Techniques, Morris Dworkin.

Field Summary

Fields inherited from class gnu.crypto.pad.BasePad
`blockSize, name`

Method Summary
`byte[]` | `pad(byte[] in, int offset, int length)` Returns the byte sequence that should be appended to the designated input.
`void` | `setup()` If any additional checks or resource setup must be done by the subclass, then this is the hook for it.
`int` | `unpad(byte[] in, int offset, int length)` Returns the number of bytes to discard from a designated input buffer.

Methods inherited from class gnu.crypto.pad.BasePad
`init, name, reset, selfTest`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Method Detail

### setup

    public void setup()

  Description copied from class: `BasePad`

  If any additional checks or resource setup must be done by the subclass, then this is the hook for it. This method will be called before the `BasePad.init(int)` method returns.

  Specified by:
    `setup` in class `BasePad`

### pad

    public byte[] pad(byte[] in,
                      int offset,
                      int length)

  Description copied from interface: `IPad`
  Returns the byte sequence that should be appended to the designated input.

  Specified by:
    `pad` in interface `IPad`
  Specified by:
    `pad` in class `BasePad`
  Parameters:
    `in` - the input buffer containing the bytes to pad.
    `offset` - the starting index of meaningful data in in.
    `length` - the number of meaningful bytes in in.
  Returns:
    the possibly 0-byte long sequence to be appended to the designated input.

### unpad

    public int unpad(byte[] in,
                     int offset,
                     int length)
              throws WrongPaddingException

  Description copied from interface: `IPad`
  Returns the number of bytes to discard from a designated input buffer.

  Specified by:
    `unpad` in interface `IPad`
  Specified by:
    `unpad` in class `BasePad`
  Parameters:
    `in` - the input buffer containing the bytes to unpad.
    `offset` - the starting index of meaningful data in in.
    `length` - the number of meaningful bytes in in.
  Returns:
    the number of bytes to discard, to the left of index position `offset + length` in in. In other words, if the return value of a successful invocation of this method is `result`, then the unpadded byte sequence will be `offset + length - result` bytes in in, starting from index position `offset`.
  Throws:
    `WrongPaddingException` - if the data is not terminated with the expected padding bytes.

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