the illustration in the drawing is schematic . it is noted that in different figures , similar or identical elements are provided with reference signs , which are different from the corresponding reference signs only within the first digit . the subsequent described embodiments of the invention essentially provide a new form of stream cipher . therefore , in order to facilitate the understanding of these embodiments , there will be given first some background information about known stream cipher encryptions respectively stream cipher decryption . stream ciphers are usually based on linear feedback shift registers and non - linear boolean components . they are designed to be simple and easy to implement in hardware . given a key k and an initialization vector iv , a stream of bits is generated from the internal state s of the internal cipher registers . in the following f is a stream cipher update function and s i is the internal register configuration of the stream cipher after a bit i of a plaintext has been encrypted . the cipher of the internal state is updated as with each iteration for current stream ciphers s is of the order or 150 - 300 bits . the key k and the initialization vector iv are used to seed the initial register configuration respectively the internal register state s 0 . a bit k i of the keystream is entirely determined from a given internal cipher configuration s i and is generated from the following equation : thereby , g is a function representing the keystream generation , which extracts at any one time a bit from internal state register configuration . the actual encryption of a bit of plaintext x , is performed by thereby , “+” represents a bitwise xor operation . the corresponding decryption is performed by generation of the same keystream k i and simply performing the xor operation x i = c i + k i . if an internal state s i of a stream cipher f k , which comprises the function f for state update , the function g for generating the keystream and the cipher key k , is known , then the entire keystream from that point can be generated . this is performed by a one bit per iteration , which can be described by the following equation : f ( s i )={ k i , k i + 1 , k i + 2 , . . . } an important property is that a stream cipher is very efficient in encryption respectively in decryption of a linear stream of data such as for instance a large file that is being downloaded . the encryption respectively decryption procedure does depend , however , on getting the data bit by bit in the exact right order . this is because the way a particular bit is encrypted depends on the states of the cipher , which states have been adopted in all previous encryptions . therefore , to skip a part of the data stream , the cipher needs to know all the previous internal configurations respectively all the cipher states from the point where the data stream is supposed to the point where the decryption starts again . in this respect a stream cipher decryption significantly differs from a block cipher , which depending on the exact usage is capable of decrypting each block of data independently . as will be apparent from the description below , the multi - dimensional encryption respectively decryption procedure according to the described embodiment of the invention has the advantage that it can support complex memory access patterns , which are increasingly encountered in particular in multimedia applications . by contrast thereto , neither an encryption respectively decryption based on known block ciphers nor an encryption respectively decryption based on known stream cipher can provide this advantage . in order to illustrate this and in order to improve the understanding of the invention , with reference to fig3 there will be briefly described as an example a block cipher based decryption of a block cipher based encrypted video content stored in a two - dimensional ( 2d ) memory such as a flash memory . with reference to fig4 there will be briefly described as a further example a stream cipher based decryption of a stream cipher based encrypted video content stored in a two - dimensional ( 2d ) memory . these examples illustrate how a 2d access of data of random size and position is typically performed for instance during a video decompression or graphic generation . fig3 shows the decryption of an irregular portion 355 of a 2d memory 350 , which data content has been encrypted by using a block cipher . even though the portion 355 of the memory 350 required for decryption may be the same as a single block 380 , the portion 355 will be typically not aligned with block boundaries . this implies that the decryption of in the depicted case six memory blocks 380 is required . this illustrates a general problem with block ciphers and memory accesses to this type . although in principle memory access using block ciphers is indeed random access , unless the portion 355 of the memory 350 for decryption is exactly aligned along block boundaries , an amount of extra redundant decryption work must be performed . this means that data have to be decrypted , which data are not needed . fig4 shows the decryption of an irregular portion 455 of a 2d memory 450 , which data content has been encrypted by using a stream cipher 490 . in order to decrypt the irregular portion 455 of the memory 450 four entry points 491 in the cipher stream are required . since stream ciphers typically need to decrypt the data in the exact order as it was encrypted , a data block , which is not needed , cannot be skipped during the decryption procedure . therefore , also when using stream cipher encryption is used an amount of extra redundant decryption work must be performed . in the following a two - dimensional ( 2d ) encryption respectively decryption procedure employing a horizontal stream cipher f kx and a vertical stream cipher f ky will be described in more detail . even though this two - dimensional ( 2d ) encryption respectively decryption procedure represents a preferred embodiment of the invention , it is pointed out that the invention described with this application is not limited to a 2d cryptographic procedure . the invention can rather also be realized by a corresponding a cryptographic procedure , which is based on three or even more different stream ciphers each representing a dimension of a memory and / or a dataset . the 2d encryption procedure for effectively encrypting a 2d portion of memory such as for instance a frame buffer comprises the following important steps : in a first step two separate stream ciphers , a horizontal stream cipher f kx and a vertical stream cipher f ky are generated . thereby , these stream ciphers can be the same in case different keys are used . in a second step a horizontal keystream f kx ( s i ) is formed . this horizontal keystream f kx ( s i ) is generated by internal states s i of an internal register configuration . in a third step a vertical keystream f ky ( s j ) is formed . this vertical keystream f ky ( s j ) is generated by internal states s j of an internal register configuration . in a fourth step for a 2d memory each bit x ij representing a bit of a plaintext is encrypted by the following logical operation : thereby k ij = f kx ( s i )+ f ky ( s j ) and “+” is the logical xor operation . this embodiment is characterized by the fact that compared to a one - dimensional stream cipher encryption using a single keystream only ( see description above ), in the 2d stream cipher encryption the entire 2d memory space is covered by the xor of a horizontal and a vertical keystream . a corresponding decryption of an arbitrary rectangular portion of such an encrypted dataset being stored in a 2d memory can be performed with using the knowledge of the internal states s i of the horizontal keystream and the internal states s j of the vertical keystream . the decryption can be performed by applying the following logical operation : although only a single repeated horizontal keystream is used in the horizontal and vertical directions across the 2d memory , the memory space is uniquely covered by the xor operation of both streams at each memory location . the corresponding logical equation is the following : k ij = f kx ( s i )+ f ky ( s j ) the described 2d encryption respectively decryption method may provide the advantage that the whole cryptography requires only internal storage of the internal states s i and s j of ciphers for the horizontal keystreams f kx ( s i ) and for the vertical keystream f ky ( s j ). in case the corresponding key comprises repeated key codes , the required internal storage capacity is a constant regardless of size of the 2d memory . the described 2d cryptographic procedure further provides the advantage that an effective area and time efficient solution for the so far unsolved problem of an arbitrary access of a portion of a 2d memory within in a larger 2d memory is provided . as compared to previous approaches the provided solution is faster , cheaper and requires less storage capacity . fig1 shows a diagram depicting the decryption of an irregular portion 155 of a 2d memory 250 . the memory is for instance a flash memory 250 . the data stored in the 2d memory 250 have been encrypted by performing a sequential xor operation by means of a horizontal stream cipher f kx 160 and a vertical stream cipher f ky 170 . the horizontal keystreams f kx for internal cipher state s i are denoted with reference sign 165 . the vertical keystreams f ky for internal cipher state s j are denoted with reference sign 175 . as can be seen from fig1 , the memory 150 comprises an 11 × 7 2d memory grid . with a 7 bit horizontal keystream f kx 160 each bit in the horizontal stream is described by a 4 bit internal cipher state s i , wherein i = 0 . . . 6 . with an 11 bit vertical keystream f ky 170 each bit in the vertical stream is described by a 4 bit internal cipher state s j , wherein j = 0 . . . 10 . therefore , in this case only 7 × 4 + 11 × 4 = 336 bits are required to describe the entire keystream over the 2d memory 150 and only two sections of state information ( s i , s j ) are necessary in order to describe any rectangular portion 155 of the memory 150 . as can be seen from fig1 , the horizontal keystreams f kx 160 and the vertical keystreams f ky 170 are the same across the larger 2d memory 150 . an attacker could recover points of the keystreams if the plaintext is known . in this case of using the same keystreams each point in the typically insecure 2d memory 150 is described by the following equation : f kx ( s i )+ f ky ( s j )= k ij this represents in fact i × j linear equations for i + j variables . this means that these portions of the keystream can be recovered by solving these linear equations . therefore , in order to increase the security of the described 2d cryptographic procedure one should frequently re - key the stream ciphers . when the key is changed often and the plaintext will in general not be known , this scheme provides a high degree of security . fig2 shows a multimedia device 200 representing an embodiment of the invention . the multimedia device may be for instance a dvd video player 200 . the multimedia device 200 comprises a processing device 210 and a working memory 250 . the processing device 210 is considered to represent a secure component of the multimedia device 200 . the external 2d memory 250 is considered to represent a comparatively unsecure component of the multimedia device 200 . the processing device 210 comprises a data input 230 , which allows for a loading of video data into the processing device 210 . according to the embodiment described here the video data are raw data from a data carrier such as a dvd . the raw data are plaintext data , which are compressed by a known compression procedure such as mpeg coding . the processing device 210 further comprises a central processing unit 215 , which is adapted for performing an mpeg decoding of data being transferred from the data input 230 to the central processing unit 215 . further , the processing device 210 comprises a first encrypting unit 221 and a second encrypting unit 222 . in combination these encrypting units 221 , 222 are adapted to perform a two - dimensional decrypting of the decompressed data being provided by the central processing unit . further , the processing device 210 comprises a memory subsystem control unit 225 and a masking unit 228 . the memory subsystem control unit 225 is used for controlling the 2d encryption procedure . in particular , the memory subsystem control unit 225 decides , which data portions have to remain within the secure processing device 210 and which data portions are allowed to leave the secure processing device 210 . the masking unit 228 is used for encrypting the horizontal and the vertical stream ciphers . this ensures that these stream ciphers can be stored externally in a memory portion 252 of the 2d working memory 250 only in an encrypted form . the encrypted video data are stored in a 2d video frame buffer 251 also representing a part of the external and comparatively unsecure 2d working memory 250 . the multimedia device 200 further comprises an input / output unit 240 and a monitor 242 . the input / output unit 240 , which adapted for instance for providing information about the operating condition of the multimedia device 200 to a user and / or for receiving control commands from the user . the monitor 242 , which is coupled to the input / output unit 240 , is used for displaying the desired video content . in the following the operation of the multimedia device 200 will be described : a ) initially the digital rights management ( drm ) protected content is loaded from a data carrier and the drm protection is removed . this step is not shown in diagram . b ) the 2d video frame buffer 251 will then be filled with a video content . however , before the video content is stored in the buffer 251 the video content is encrypted using the 2d encrypting method described in detail in the previous section of this application . c ) at the same time the state data for the horizontal and vertical stream ciphers ( s i , s j ) are also generated . prior to storage of the video content in the external memory 251 the video content is masked by means of an xor operation with a random value . d ) when a particular 2d portion of the video data stored in the memory 251 is required for media decrypting , this portion is retrieved from the frame buffer 251 . at the same time the appropriate s i and s j data are also obtained and the mask is removed . e ) this s i and s j data is then decrypted using the stream ciphers f kx and f ky as previously discussed and used before by the media decoder . f ) any update information is then written to the frame buffer 251 going through the same encryption process . g ) occasionally the frame buffer 251 will be completely refilled . before this is performed both the horizontal and vertical stream ciphers and the masking value are re - keyed . this may improve the security significantly . the above described invention is for instance applicable to processors , which require a fast and an efficient encryption and decryption of 2d portions of memory . this holds for instance for many processors , which process encoded video content such as mpeg4 compressed video data . compared to known encrypting respectively decrypting procedures the described 2d dimensional cryptography method may provide the following advantages : faster and more efficient encryption and decryption than block cipher cryptography . less storage requirements and more efficient than using a single stream cipher . with only low hardware requirements and providing frequent rekeying an acceptable security level can be provided . the so far unsolved problem of effective 2d random memory access and decryption is solved . it should be noted that the term “ comprising ” does not exclude other elements or steps and the “ a ” or “ an ” does not exclude a plurality . also elements described in association with different embodiments may be combined . it should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims .