A method and apparatus for encrypting and decrypting digital data employing multiple Huffman tables and at least one encryption key to enhance security of the digital data. At least one image parameter for characterizing the digital data, such as a motion vector table or DC-luminance, is selected as an image parameter. All possible Huffman tables according to the image parameter are then generated by Huffman tree mutation. A predetermined number of active Huffman tables from all possible Huffman tables are selected using a first encryption key and a hash function. Afterward, a coding sequence for the active Huffman tables is generated using a second encryption key and the hash function. Finally, the digital data is encrypted into an encrypted bit stream by the active Huffman tables with the coding sequence. Encrypted symbols of the image parameter can be reduced by symbol statistic analysis, thus reducing computation effort.