function xorBlock(words, offset, blockSize) {
	            var block;

	            // Shortcut
	            var iv = this._iv;

	            // Choose mixing block
	            if (iv) {
	                block = iv;

	                // Remove IV for subsequent blocks
	                this._iv = undefined;
	            } else {
	                block = this._prevBlock;
	            }

	            // XOR blocks
	            for (var i = 0; i < blockSize; i++) {
	                words[offset + i] ^= block[i];
	            }
	        }
Processes the data block at offset.

@param {Array} words The data words to operate on.
@param {number} offset The offset where the block starts.

@example

    mode.processBlock(data.words, offset);