Abstract:
A method and system for creating authentication signatures for digital images and video frames is provided. The method and system involves partitioning the image into multiple blocks, comparing characteristics from each block and generating data bits based on the comparison. Each block is then broken up into additional blocks and those blocks are compared to create additional signature bits which are combined with the signature sets from the first set of blocks. Each of these new smaller blocks can be further broken up and the procedure can be repeated to provide an authentication signature of desired length.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to authentication systems and methods for digital images and more particularly to improved methods, systems and signals for authenticating digital images. 
     The use of digital images and videos by both consumers and professionals is pervasive. Accordingly, it has become important to provide a system and method for authenticating digital images and videos to insure that they have not been tampered with. As an example, an authentication system could insure that someone has not replaced a person&#39;s face with that of another on a digital picture or series of video frames. 
     Authentication systems are known which extract a short signature from images (or video frames) which can be either inserted into the image signal or stored separately. The owner of the original content can use the signature to verify whether the content has been modified or users can confirm that they are receiving authentic digital images. 
     Conventional content-based image authentication systems typically define an image into many blocks and extract characteristics about the blocks. For example, the image can be broken up into 16×16 blocks as in FIG. 1 or some other number of blocks, and some characteristic about the block, such as average luminance or chrominance values with respect to R, G, B or gray values. The characteristics of adjacent pairs of blocks are commonly compared and a signature is extracted based on this comparison. For example, if the average luminance value for the red component of a first block  110  of an image  100  is greater than or equal to that of a second block  120 , a one bit will be generated. Otherwise, a zero bit will be generated. The process is repeated with successive blocks until a binary signature of ones and zeros is compiled. 
     A disadvantage to this method is that because pairs of blocks each contribute a bit to the signature, it is possible to change the pair of blocks without affecting the signature by maintaining the difference or similarity of the compared characteristic of each block. It can be possible to reverse engineer the signature and then alter the image in such a manner to generate an identical signature and thus frustrate the authentication mechanism. 
     The following references discuss processing video signals, coding image blocks and authentication algorithms for digital images, the contents of which are incorporated herein by reference: WO 93/11502, U.S. Pat. No. 4,254,400, U.S. Pat. No. 5,351,095, U.S. Pat. No. 5,520,290 and U.S. Pat. No. 5,870,471. 
     Techniques for performing pair-wise block comparisons in a non-hierarchical technique are discussed in “Generating robust digital signature for image/video authentication”, C. Y. Lin and S. F. Chang, in Proceedings of Multimedia and Security Workshop at A. C. M. Multimedia, September 1998. Inserting and/or hiding a signature in an image signal is discussed in “Secure spread spectrum water marking for images, audio and video,” I. Cox, et al., in IEEE Int&#39;l. Conf. on Image Processing, Vol. 3, pp. 243-246 (1996). The contents of these references are incorporated herein by reference. 
     Accordingly, it is desirable to provide an improved method and system for authenticating digital images which overcomes drawbacks of conventional methods and systems. 
     SUMMARY OF THE INVENTION 
     Generally speaking, in accordance with the invention, a method and system for creating authentication signatures for digital images is provided. The method and system involves partitioning the image into multiple blocks, comparing characteristics from each block and generating signature data bits based on the comparison. Each block can then be broken up into additional blocks and those blocks can be compared to create additional sets of signature bits which can be combined with the signature bits from the first set of blocks. Each or a portion of these new smaller blocks can be further broken up and the procedure can be repeated to provide an authentication signature of desired length by combining all or parts of the signature segments. 
     Accordingly, it is an object of the invention to provide an improved method and system for authenticating digital images and video. 
     Another object of the invention is to provide a method of creating an authentication signature for a digital image which is difficult to duplicate if the image is altered. 
     Another object of the invention is to provide an improved system for creating, storing and using authentication signatures for digital images which are difficult to duplicate if the original image is altered. 
     The invention accordingly comprises the several steps and relation of one or more of such steps with respect to each of the others and the product, system, signal and media adapted to effect or resulting from such steps, or as is exemplified in the following detailed description and drawings and the scope of the invention will be indicated in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference is had to the following description, taken in connection with the accompanying drawings, in which: 
     FIG. 1 represents a digital image divided into sixteen blocks; 
     FIG. 2A represents the partition of an image into four blocks at scale zero; 
     FIG. 2B shows each of the blocks of FIG. 2A broken down into four sub-blocks at scale one; 
     FIG. 3 is a flow chart of an authentication method in accordance with an embodiment of the invention; 
     FIG. 4 is a flow chart of an authentication method in accordance with another embodiment of the invention; and 
     FIG. 5 is a flow chart of an authentication method in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Authentication algorithms for digital data should be difficult to reverse engineer and should insure that it is difficult, if not impossible to alter the image without changing the correspondence between the authentication signature and the image itself. The authentication is preferably performed in a content-based fashion and small, non-malicious changes in the image such as brightening or subjecting the image to various coding or compression algorithms, such as a lossy JPEG-like compression should be allowed. As used herein, the term image will also be used to refer to video frames. 
     Authentication methods and systems in accordance with the invention use a hierarchical technique, where the image is partitioned into blocks of a selected number of pixels at a first-level or scale (scale 0) and then each or a portion of the blocks are further partitioned into sub-blocks in successive scales. At each scale, the properties or characteristics of blocks (or sub-blocks) are compared to obtain a signature for that scale, and all or part of the signatures for each scale are combined. As the individual blocks are broken down into successive scales of greater detail, the authentication signature generated at each scale can be added onto or otherwise combined with the signature generated from previous scale levels to provide a more robust signature which is more difficult to reverse engineer and evade, compared to conventional mono-scale techniques. 
     Referring to FIG. 2A, as a non-limiting illustration of a technique in accordance with preferred embodiments of the invention, the image (not shown) has been partitioned into four non-overlapping blocks at scale zero. These blocks are identified as A, B, C and D. The blocks together completely cover the entire image. It is also possible, in accordance with preferred embodiments of the invention, to use overlapping blocks or blocks which cover only a portion of the image, such as when authentication is only desired for a particular section of an image. Partitioning only a portion of the image can simplify the procedure if characteristic information will only be located at a certain location of the image. 
     A characteristic value is extracted from each block. This characteristic can be a luminance or chrominance characteristics, and the value can be the average luminance of the R (red) value (for example) over the entire block. Other values for the characteristics, such as the standard deviation of the characteristic of the block or some other characteristic value such as DCT coefficient and the like can be used. The computed characteristic values can be identified as ƒ(A), ƒ(B), ƒ(C) and ƒ(D) where ƒ is the function used to compute the desired characteristic value. 
     The computed characteristic values of the four blocks can then be compared as follows. The ƒ( ) values can be arranged in an ascending, descending or other predefined order. Because there are four blocks, there are twenty-four possible combinations of the ordering. (e.g., ABCD, ABDC, ADBC. . . ). A five-bit binary number can be used to represent each combination in the ordering, i.e., ABCD could be assigned 00001 and CBDA might be assigned 10010. A five-bit binary number can represent 32 different combinations. The remaining eight combinations can be used for the instances where the values of some of the ƒ( ) characteristics are equal. For example, if the four values are all equal, e.g., if the average green level of each block is identical, then a particular 5-bit number such as 11111 can be used to specify that particular combination. Thus, a 5-bit number is obtained at scale 0 (level 1) to form a part of the authentication signature. 
     Although four blocks are shown in this embodiment of the invention, different numbers of blocks and sub-blocks can be used in alternate embodiments of the invention. Also, the number of sub-blocks, to which a block (or sub-block) is divided need not be identical to the number of blocks or sub-blocks of a higher scale. If the number of blocks is greater than four, then the number of bits used to represent all of the combination will be greater than five. 
     Referring to FIG. 2B, each of the four blocks from scale 0 (FIG. 2A) are partitioned into four non-overlapping sub-blocks designated AA, AB and so forth. (As noted above, each of the blocks of FIG. 2A could have been divided into two sub-blocks, nine sub-blocks and so forth.) The partitioning of the image as shown in FIG. 2B represents scale 1 in the hierarchical decomposition. The characteristic values of each of blocks AA, AB, AC and AD are computed as discussed above and a 5-bit binary number is obtained for the four groups of sub-blocks of the scale 0 blocks. In alternate embodiments of the invention, different characteristics from those used a scale 0 can be used in each of the additional scales. 
     After the characteristic values of sub-blocks AA, AB, AC and AD are computed, a 5-bit binary number is obtained. The same process is repeated for the three other sets of sub-blocks at scale 1, resulting in four 5-bit numbers or 20 bits. These 20 bits are combined with the first 5-bit number and the process can be repeated successively for additional sets of sub-blocks at higher scales. The bits obtained from successive scales are concatenated to obtain a signature. For example, if four scales are used, then 5-bits are obtained from scale 0, 20-bits from 1, 80-bits from scale 2, and 320-bits from scale 3. All of these bits are concatenated to form a 425-bit level 4 authentication signature. 
     In alternate embodiments of the invention, the authentication signature can be obtained for a first color band and then similar signatures can be obtained for the additional color bands. The number of scales used would depend on the size of the image and the desired length of the signature. 
     The signature can be stored separately or sent with the image signal and transmitted with the signal or stored on a floppy disk, CD, DVD, video tape and the like. 
     A flow chart  500  corresponding to an authentication method in accordance with preferred embodiments of the invention is shown generally in FIG.  5 . In step  510 , image data  501  is divided into blocks. In step  520 , values corresponding to characteristics of each block are calculated. In step  530 , the blocks are ordered based on the values and in step  540 , a first-level binary code corresponding to the order of the blocks is assigned. In step  550 , each block from the first-level (scale 0) is subdivided into sub-blocks. In step  560 , values for each sub-block are calculated and in step  570 , the sub-blocks are ordered based on these values. In step  580 , sets of binary codes corresponding to the ordered sets of sub-blocks are generated and in step  590 , the binary code is combined with the first-level binary code. In step  600 , the process can be repeated and additional levels (scales) of authentication signature binary code can be developed. 
     Non-limiting uses of the signature obtained in accordance with preferred embodiments of the invention are illustrated in FIGS. 3 and 4. 
     Referring to FIG. 3, a data processor can be used to extract the authentication signature from image data using hierarchical algorithms discussed above in step  310 . In step  320  the signature of the image or video frame can be inserted into or added to the signal representing the image. In step  330  the image together with the inserted signature can be transmitted to an image receiver. 
     In step  340 , the authentication signature can be extracted from the image data using the hierarchical algorithm. In step  350 , the inserted (hidden) signature from the image is extracted. 
     In step  360 , the signature generated from the transmitted signal is compared to the signature inserted with the image. If they match, authentication is acknowledged in step  370 . If they do not match, authentication failure is indicated in step  380 . 
     Referring to FIG. 4, a method of authentication is illustrated where the signature is not inserted into the image signal. In step  410  the authentication signature of an image or video frame is extracted using a hierarchical algorithm in accordance with the invention. In step  420 , this signature is stored at a secure location. When verification is desired, in step  430 , the authentication signature is extracted from the image or video frame using the hierarchical algorithm. In step  440 , the signature is compared to that stored during step  420  and if they match, authentication is acknowledged in step  450 . Otherwise, authentication failure is noted in step  460 .