For multimedia data is often voluminous and requires real time operations

multimedia encryption algorithms should meet extra requirement compared with textual data encryption algorithms. Traditional algorithms such as DES

RSA

can't meet some real time requirement. Some selective encryption algorithms can't completely keep the file format and compression ratio unchanged. In order to meet such requirement

the algorithms should combined encryption process with encoding process. Due to the wide application of quad tree structure in image or video encoding

two types of quadtree permutation algorithms are proposed here. They are used in quadtree based image encoding

wavelet zerotree encoding (EZW

SPIHT)

and quadtree based fractal image or video encoding. The encryption algorithms combine encryption with compression

and have some advantages. Firstly

it is secure

and the encryption result is too confused to be understood. Secondly

it keeps the file format unchanged

and thus it is easy to operate compressed and encrypted data directly. Thirdly

it keeps the compression ratio unchanged

that is to say that the encryption operations do not produce extra data. Lastly

it is fast and can meet real time requirement.