Abstract:
A system and apparatus for inserting a watermark into a compiled computer program selectively replaces specified optimizations by non-optimized code to encode bit values of the watermark. The watermark is read by decoding the executable code and assigning the decoded bit values, determined by the presence or absence of optimized code, to bit positions in a signature.

Description:
BACKGROUND OF THE INVENTION 
   It can be useful to be able to identify the code produced by different compilers to identify non-licensed uses of the compilers, and to track errors. Accordingly, compiler manufacturers require a method of including a serial number or other identifying mark in code produced by a compiler. Additionally, a method of analyzing a copy of the compiled code to determine the serial number or identifying mark is also required. 
   A private watermark, which is data hidden via steganography, is one method for tracking the outputs of licensed programs. However traditional steganography requires the presence of “low order” bits in the data stream. The low order bits can be changed without the data changing so much that a human can notice the difference. The changed bits, detected when the modified field is compared to the original, can hold the steganographic data. Since traditional stenography changes non-significant low-order bits, steganography is normally applied to digital pictures and sounds. 
   Steganography in computer code can&#39;t be done with the normal methods because computer code does not contain low-order bits. Every bit in the code is important, and flipping even one bit can prevent the code from operating correctly. 
   Accordingly, improved techniques for inserting identifying watermarks in compiled programs is needed. 
   BRIEF SUMMARY OF THE INVENTION 
   In one embodiment of the invention, a method for generating and auditing a watermark for a compiled computer program is provided. The watermark is an integral part of the program and does not appear as an external data item. 
   In another embodiment, a watermarking module selectively replaces n-optimized code segments with non-optimized code segments. For a current signature digit, the optimized code segment is replaced by a non-optimized code segment only if the signature digit has a first binary value. The presence of the optimized encode segment encodes the second binary value. 
   In another embodiment of the invention, a watermarking module searches the executable code for the presence of optimized code for unrolling a loop. If the current signature digit has a first binary value then the optimized code is replaced by non-optimized code to encode the first binary value in the watermark. 
   In another embodiment of the invention, watermarked executable code is searched for the presence of optimized and non-optimized code segments. If a non-optimized code segment is detected then a current signature digit is assigned the first binary value. If an optimized code segment is detected the current signature digit is assigned the second binary value. 
   Other features and advantages of the invention will be apparent in view of the following detailed description and appended drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a computer system  10  configured to implement an embodiment of the invention; 
       FIG. 2  is a block diagram depicting the operation of a first embodiment that encodes the watermark as a loop-unrolling non-optimization; 
       FIG. 3  is flowchart of the watermark encoding process of a an embodiment of the invention; and 
       FIG. 4  is flowchart of the watermark decoding process of a an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention will now be described, by way of example not limitation, with reference to various embodiments.  FIG. 1  is a block diagram of a computer system  10  configured to implement an embodiment of the invention. The computer system  10  includes a computer  12 , an input device  14  such as a keyboard, and output device  16  such as a display screen. The computer  12  includes a main memory  18 , which may include RAM and NVRAM, central processing unit (“CPU”)  20 , and a secondary memory  22 . A compiler  24 , a source code module  26 , a compiled program  30 , and a watermarking module  32  for inserting and retrieving the watermark from the compiled code  30  are stored in secondary memory  22 . 
   The operation of the first embodiment will now be described in more detail with reference to  FIGS. 2-4 . The general concept is to encode first and second bit values, e.g., 0 and 1, as either optimized or non-optimized code in the compiled program. For example, the presence of optimized code could encode the value “1” and the presence of non-optimized code could encode the value “0”. 
   Generally, compilers optimize code by using techniques such as constant propagation (replacing expressions that evaluate to a constant with a constant value), copy propagation (replacing assignment by the assigned value) strength reduction (replacing operations by more efficient operations), loop unrolling (replace loop with code), and so on. 
   Modern compliers make many choices of methods to optimize code as they are compiling it. A method of watermarking code can be executed by changing the choice of optimizations that the compiler makes. 
   For example, take the following C code: for (i=1; i&lt;3; i++) {x+=x*i;}. Most compilers would “unroll” this code, producing an optimized object code segment as though the C code had been: x+=x*1; x+=x*2; x+=x*3; thus saving the cost of incrementing i. If instead, the compiler chose to not unroll the loop, the non-optimized code segment would represent one bit of watermarked information. 
   The process of watermarking will now be described in more detail with reference to  FIGS. 2-4 .  FIG. 2  includes a first block  30  depicting the program code, a second block  32  depicting the signature data to be encoded as a watermark, a third block  34  depicting the optimized compiled code output by the compiler, and a fourth block  36  depicting the modified compiled code having a the value of the first bit of the signature data encoded therein. 
     FIG. 3  is a flowchart depicting the acts performed to encode the signature as a watermark in the compiled code. The code is compiled to generate the compiled code  34  and the first bit of signature data  32 , in this example having a value “1”, is accessed. The compiled code is then searched for optimized code that will be used to encode this bit value. 
   In this example, the optimized code is depicted in the third block  34 . This optimized code is replaced by non-optimized code as depicted in the fourth block  36 . The presence of this non-optimized code encodes a bit value of “1” for the first digit in the watermark. 
   Subsequently, the second digit, “0”, of the signature is then retrieved. The next instance of an unrolled loop would then be detected. In this case the optimized code would not be replaced by non-optimized code thereby encoding the bit value “0” for the second digit of the signature. 
   Thus, the values of the successive bits in the signature would be encoded into the program code as a series of blocks of optimized code and non-optimized code, with presence of optimized code encoding a first bit value and the presence of non-optimized code encoding a second bit value. The program loops until all the bit values in the signature have been encoded as a watermark into the compiled program. 
   The watermarked data can be retrieved by examining the data with a watermarking module that understands the compiler&#39;s optimization algorithm, and outputs the bits related to its non-optimal choices. This process will now be described with reference to  FIG. 4 . 
   Referring to  FIG. 4 , the watermarked code is searched for the presence of optimized code or substituted non-optimized code. If non-optimized code is detected then a first bit value is assigned to current digit of the signature and if optimized code is detected then a second bit value is assigned to the current digit of the signature. The program loops until all the selected optimizations and non-optimizations have been decoded. 
   In the above example, both bit values were encoded by detecting whether a loop unroll had been optimized. Other optimizations, for example constant replacement, can be utilized in the same manner. Alternatively, a combination of optimizations can be utilized to encode the bit values, for example a loop unroll and constant replacements. The presence of the optimized code encodes one bit value and presence of the non-optimized code encodes the other bit value. 
   The invention has now been described with reference to the preferred embodiments. Alternatives and substitutions will now be apparent to persons of ordinary skill in the art. For example, other optimizations than the specific examples described can be utilized to encode the bit values. Additionally, the encoding and decoding processes can be incorporated as part of the compiler or be implemented as independent processes. Accordingly, it is not intended to limit the invention except as provided by the appended claims.